advanced computer networks - cs716 power point slides lecture 25

7/27/2019 Advanced Computer Networks - CS716 Power Point Slides Lecture 25

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CS716

Advanced Computer Networks

By Dr. Amir Qayyum


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Review Lecture


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Switched Networks

Two or more nodesconnected by a link

Circular nodes(switches) implementthe network

Squared nodes (hosts)usethe network

A network can be defined recursively as...


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Switched Networks

Two or more networks

connected by one or more

nodes: internetworks

Circular nodes (router or

gateway) interconnectsthe

networks

A clouddenotes any type of

independent network

A network can be defined recursively as...


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Switching Strategies

Circuit switching:Carry bit streams

a. establishes a dedicated

circuitb. links reserved for use

by communicationchannel

c. send/receive bit streamat constant rate

d. example: originaltelephone network

Packet switching:

Store-and-forwardmessages

a. operates on discreteblocks of data

b. utilizes resourcesdynamically according

to traffic demandc. send/receive messages

at variable rate

d. example: Internet


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Multiplexing

Physical links/switches must be shared among users

(Synchronous) Time-Division Multiplexing (TDM)

Frequency-Division Multiplexing (FDM)

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

Multiple flows

on a single link

Do you see any problem with TDM / FDM ?


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Statistical Multiplexing

On-demand time-division, possibly synchronous (ATM)

Schedule link on a per-packet basis

Buffer packets in switches that are contendingfor the link

Packets from different sources interleaved on link

Do you see any problem ?


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Inter-Process Communication

Turn host-to-host connectivity into process-to-processcommunication, making the communication meaningful.

Fill gap between what applications expect and what theunderlying technology provides.

Abstraction for

application-level

communication

Host

Host

Application

Host

Application

Host Host

Channel


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Abstract Channel Functionality

What functionality does a channel provide ?

Smallest set of abstract channel types adequate

for largest number of applications Where the functionality is implemented ?

Network as a simple bit-pipe with all high-level

communication semantics at the hosts

More intelligent switches allowing hosts to bedumb devices (telephone network)


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Performance Metrics

Latency / Delay Time to send message from point A to point B

One-way versus Round-Trip Time (RTT)

Components

Latency = Propagation + Transmit + Queue

Propagation = Distance / c

Transmit = Size / Bandwidth

Note:

No queuing delay in direct (point-to-point) link Bandwidth irrelevant if size = 1 bit

Process-to-process latency includes software processing overhead(dominates over shorter distances)


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Delay x Bandwidth Product

Amount of data in flight or in the pipe

Example: 100ms RTT x 45Mbps BW = 560KB

This much data must be buffered before the senderresponds to slowdown the request

Delay

Bandwidth


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Network Architecture

The challenge is to fill the gap between hardware

capabilities and application expectations, and to do

so while delivering good performance

Designers cope with this complex task by

developing a network archi tectureas a guideline

Layering, protocols, standards


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Layering

Alternative abstractions at each layer

Manageable network components

Modify layers independently

Hardware

Host-to-host connectivity

Application programs

Request/replychannel

Message streamchannel


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Protocols

Building blocks of a network architecture

Each protocol object has two different interfaces

service interface: operations on this protocolpeer-to-peer interface: messages exchanged with peer

Term protocol is overloaded

Specification of peer-to-peer interface

Module that implements this interface

Peer modules are interoperable if both accuratelyfollow the specifications


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Host1 Host2

Service

interface

Peer-to-peer

interface

Protocol Interfaces

High-levelobject

High-levelobject

ProtocolProtocol


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Protocol GraphNetwork Architecture

Collection of protocols and their dependencies Most peer-to-peer communication is indirect

Peer-to-Peer is direct only at hardware level

Host 1 Host 2

File

application

Digitallibrary

application

Video

application

Fileapplication

Digitallibrary

application

Video

application

RRP RRPMSP MSP

HHP HHP

RRP: Request

Reply Protocol

MSP: Message

Stream Protocol

HHP: Host-to-

Host Protocol


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Protocol Machinery

Multiplexing and Demultiplexing (demux key)

Encapsulation (header/body) in peer-to-peerinterfaces

Indirect communication (except at hardware level)

Each protocol adds a header

Part of header includes demultiplexing field (e.g., passup to request/reply or to message stream?)


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Encapsulation

Host 1 Host 2

Applicationprogram

Applicationprogram

Data Data

RRP RRP

RRP Data

HHP HHP

RRP DataHHP

RRP Data


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Standard Architectures

Open System Interconnect (OSI) Architecture

International Standards Organization (ISO)

International Telecommunications Union (ITU),formerly CCITT

X dot series: X.25, X.400, X.500

Primarily a reference model


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OSI Architecture

Application

Presentation

Session

Transport

End host

One or more nodes

within the network

Network

Data link

Physical

Network

Data link

Physical

Network

Data link

Physical

Application

Presentation

Session

Transport

End host

Network

Data link

Physical

Application

Data formatting

Connection management

Process-to-process

communication channel

Host-to-host packet

delivery

Framing of data bits

Transmission of raw bits

User

level

OS

kernel


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Internet Architecture

TCP/IP Architecture

Developed with ARPANET and NSFNET

Internet Engineering Task Force (IETF) Culture: implement, then standardize

OSI culture: standardize, then implement

Became popular with release of Berkeley Software

Distribution (BSD) Unix; i.e. free software Standard suggestions traditionally debated publically

through Request For Comments (RFCs)


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Implementation and design done together

Hourglass Design (bottleneck is IP)

Application vs Application Protocol (FTP, HTTP)

NETnNET2NET1

IP

TCP UDP

FTP HTTP NV TFTP


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Layering is not very strict

Application

TCP UDP

IP

Network


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Networking in the Internet Age


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Network Application Programming

Interface (API) Interface that the OS provides to its networking

subsystem

Most network protocols are implemented in software All systems implement network protocols as part of the OS

Each OS is free to define its own network API

Applications can be ported from one OS to another if APIs

are similar *IF* application program does not interact with other

parts of the OS other than the network (file system, fork

processes, display )


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Protocols and API

Protocols provide a certain set of

services API provides a syntaxby which

those services can be invoked

Implementation is responsible for

mapping API syntax onto protocol

services


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Socket API

Use sockets as abstract endpoints ofcommunication

Issues

Creating & identifying sockets Sending & receiving data

Mechanisms

UNIX system calls and library routines

socket

process


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Protocol-to-Protocol Interface

A protocol interacts with a lower levelprotocol like an application interacts with

underlying network

Why not using available network APIs forPPI ?

Inefficiencies built into the socket interface

Application programmer tolerate them to

simplify their task

inefficiency at one level

Protocol implementers do not tolerate them

inefficiencies at several layers of protocols


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Protocol-to-Protocol Interface Issues

Configure Multiple Layers

Static vs Extensible

Process Model

Avoid context switches

Buffer Model

Avoid data copies


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Process Model

(a) (b)Process-per-Protocol Process-per-Message

inter-process

communication

procedure

call


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Buffer Model

Buffer Copy Buffer Copy

Application Process

Topmost Protocol

send() deliver()


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Network Programming

Things to Learn

Internet protocols (IP, TCP, UDP, )

Sockets API (Application Programming Interface)

Why IP and Sockets

Allows a common name space across most of

Internet

IP (Internet Protocol) is standard Reduces number of translations, which incur

overhead

Sockets: reasonably simple and elegant Unix

interface (most servers run Unix)


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Socket Programming

Reading: Stevens 2nd edition, Chapter 1-6

Sockets API: A transport layer service interface

Introduced in 1981 by BSD 4.1

Implemented as library and/orsystem calls

Similar interfaces to TCP and UDP

Can also serve as interface to IP (for super-user)

known as raw sockets Linux also provides interface to MAC layer (for super-

user) known as data-link sockets


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Client-Server Model

Asymmetric relationship

Server/Daemon

Well-known name Waits for contact

Process requests, sends

replies

Client Initiates contact

Waits for response

Server

Client Client

Client


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Client-Server Model

Bidirectional communication channel

Service models

Sequential: server processes only one clients requests at

a time Concurrent: server processes multiple clients requests

simultaneously

Hybrid: server maintains multiple connections, butprocesses requests sequentially

Server and client categories not disjoint

Server can be client of another server

Server as client of its own client (peer-to-peerarchitecture)


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TCP Connections

TCP connection setup via 3-way handshake

J and K are sequence numbers for messages

Client Server

SYN J

SYN K

ACK J+1

ACK K+1 Hmmm

RTT is

important!


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TCP Connections

TCP connection teardown (4 steps) (either client

or server can initiate connection teardown)

Client Server

FIN J

FIN K

ACK K+1

ACK J+1

active close

passive close

closes connection

Hmmm

Latency

matters!


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UDP - Aspects of Services

Unit of transfer is a datagram (variable

length packet)

Unreliable, drops packets silently

No ordering guarantees

No flow control

16-bit port space (distinct from TCP ports)

allows multiple recipients on a single host


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Addresses and Data

Internet domain names: human readable

Mnemonic

Variable Length

e.g. www.case.edu.pk, www.carepvtltd.com

(FQDN)

IP addresses: easily handled by routers/computers

Fixed Length

Tied (loosely) to geography

e.g. 131.126.143.82 or 212.0.0.1


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Endianness

Machines on Internet have different

endianness

Little-endian (Intel, DEC): leastsignificant byte of word stored in lowest

memory address

Big-endian (Sun, SGI, HP): mostsignificant byte...


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Socket Address Structures

Socket address structures (all fields in network byte orderexcept sin_family)

IP address

struct in_addr {in_addr_t s_addr; /* 32-bit IP address */

};

TCP or UDP address

struct sockaddr_in {

short sin_family; /* e.g., AF_INET */

ushort sin_port; /* TCP / UDP port */

struct in_addr; /* IP address */

};


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Address Conversion

All binary values used and returned by these functionsare network byte ordered

struct hostent* gethostbyname (const char* hostname);

translates English host name to IP address (uses DNS)

struct hostent* gethostbyaddr (const char* addr, size_tlen, int family);

translates IP address to English host name (not secure)

int gethostname (char* name, size_t namelen);

reads hosts name (use with gethostbyname to find localIP)


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Address Conversion

in_addr_t inet_addr (const char* strptr);

translate dotted-decimal notation to IP address; returns -1on failure, thus cannot handle broadcast value255.255.255.255

int inet_aton (const char* strptr, struct in_addr inaddr);

translate dotted-decimal notation to IP address; returns 1 onsuccess, 0 on failure

char* inet_ntoa (struct in_addr inaddr);

translate IP address to ASCII dotted-decimal notation (e.g.,128.32.36.37); not thread-safe


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Socket API

Creating a socket

int socket(int domain, int type, int

protocol)

domain (family) = AF_INET, PF_UNIX,

AF_OSI

type = SOCK_STREAM, SOCK_DGRAMprotocol = TCP, UDP, UNSPEC

return value is a handlefor the newly

created socket


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Sockets (cont)

Passive Open (on server)

int bind(int socket, struct sockaddr *addr, intaddr_len)

int listen(int socket, int backlog)

int accept(int socket, struct sockaddr *addr, intaddr_len)

Active Open (on client)

int connect(int socket, struct sockaddr *addr,

int addr_len)


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Sockets (cont)

Sending Messages

int send(int socket, char *msg, int mlen, int flags)

Receiving Messages

int recv(int socket, char *buf, int blen, int flags)


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Point-to-Point Links

Reading: Peterson and Davie, Ch. 2

OutlineHardware building blocksEncoding

Framing

Error Detection

Reliable transmission Sliding Window Algorithm


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Direct Link Issues in the OSI and

Hardware/Software Contexts

Transport

Network

Data Link

Physical

Session

Presentation

Application

user-level software

kernel software

(device drivers)

reliability

framing, error detection, MAC

encodinghardware

(network adapter)


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Hardware Building Blocks

Nodes

Hosts: general-purpose computers

Switches: typically special-purpose hardwareRouters (connecting networks): varies

Links

Copperwire with electronic signalingGlass fiber with optical signaling

Wireless with electromagnetic (radio, infrared,

microwave) signaling


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Links

Physical Media

Twisted pair cable

Coaxial cable

Optical fiber

Space

Media is used to propagate signals Signals are electromagnetic waves of certain

frequency, traveling at speed of light


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Signals Over a Link

Signal is modulated for transmission

Varying frequency/amplitude/phase to

receive distinguishable signals

Binary data (0s and 1s) is encoded in

a signal

Make it understandable by the receiving

host


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Bits Over a Link

Bit streams may be transmitted both

ways at a time on a point-to-point link

Full Duplex

Sometimes two nodes must alternate

link usage

Half Duplex


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Encoding

Signals propagate over a physical medium

Modulate electromagnetic waves

e.g. vary voltage Encode binary data onto signals that propagate

Signalling component

Signal

Bits

Node NodeAdaptor Adaptor


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Encoding

Problems with signal transmission

Attenuation: signal power absorbed by medium

Dispersion: a discrete signal spreads in space

Noise: random background signals

modulator demodulatora string

of signals

Digital data

(a string of

symbols)

Digital data

(a string of

symbols)


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RS-232(-C)

Communication between computer and

modem

Uses two voltage levels (+15V, -15V), abinary voltage encoding

Data rate limited to 19.2 kbps (RS-232-C)

raised in later standards


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Binary Voltage Encoding

NRZ (Non-Return to Zero)

NRZI (NRZ Inverted) Manchester (used by IEEE 802.3,

10 Mbps Ethernet)

4B/5B (8B/10B) in Fast Ethernet


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Non-Return to Zero (NRZ)

Encode binary data onto signals

e.g. 0 as low signal and 1 as high signal

Voltage does not return to zero between bits

Known as Non-Return to Zero (NRZ)

Bits

NRZ

0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0


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Problem: Consecutive 1s or 0s

Low signal (0) may be interpreted as no signal

High signal (1) leads to baseline wander

Unable to recover clock Senders and receivers clock have to be precisely

synchronized

Receiver resynchronizes on each signal transition

Clock Drift in long periods without transition

Senders clock

Receivers clock


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Alternative Encodings

Non-Return to Zero Inverted (NRZI)

Make a transition from current signal

(switch voltage level) to encode/transmit a

one

Stay at current signal (maintain voltage

level) to encode/transmit a zero

Solves the problem of consecutive ones

(shifts to 0s)


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Alternative Encodings

Manchester (in IEEE 802.310 Mbps

Ethernet)

Split cycle into two parts

Send high--lowfor 1, low--highfor 0

Transmit XOR of NRZ encoded data and the

clock

Only 50% efficient (1/2 bit per transition)


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4B/5B Encoding

Every 4 consecutive bits of data encoded in a 5-bitcode (symbol)

4-bit pattern is translated to a 5-bit pattern (not addition)

5-bit codes selected to have no more than oneleading 0 and no more than two trailing 0s

00xxx (8 symbols) and xx000 (4 symbols) are illegal

5 free symbols (non-data)

Thus, never gets more than three consecutive 0s Resulting 5-bit codes are transmitted using NRZI

Achieves 80% efficiency


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Binary Voltage Encoding

Problem: wide frequency range required, implying

Significant dispersion

Uneven attenuation

Prefer to use narrow frequency band (carrierfrequency)

Types of modulation

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase/Phase Shift

Combination of these (e.g. QAM)


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Phase Modulation Algorithm

Send carrier frequency for one period

Perform phase shift

Shift value encodes symbol Value in range [0, 360] Multiple values for multiple symbols

Represent as circle1350 450

2250 3150

1800 00

900

2700

8-symbol

example


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Constellation Pattern for V.32 QAM

For a given symbol:

1. Perform phase shift

2. Change to new

amplitude 450

150

Points in constellation

diagram

Chosen to maximize errordetection

Process called trellis coding


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Bit Rate and Baud Rate

Bit rate is bits per second

Baud rate is symbols per second

If each symbol contains 4 bits then

data rate is 4 times the baud rate


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What Limits Baud Rate ?

Baud rates are typically limited by

electrical signaling properties

No matter how small the voltage or how

short the wire, changing voltages takes

time

Electronics are slow as compared to optics


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Summary of Encoding

Problems: attenuation, dispersion, noise

Digital transmission allows periodic regeneration

Variety of binary voltage encodingsHigh frequency components limit to short range

More voltage levels provide higher data rate

Carrier frequency and modulationAmplitude, frequency, phase, and combination (QAM)

Nyquist (noiseless) and Shannon (noisy) limits on

data rates


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Framing

Breaks continuous stream/sequence of bits into a

frame and demarcates units of transfer

Typically implemented by network adaptor

Adaptor fetches/deposits frames out of or into host memory

Frames

BitsAdaptor Adaptor Node BNode A


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Advantages of Framing

Synchronization recovery

Consider continuous stream of unframed bytes

Recall RS-232 start and stop bits Multiplexing of link

Multiple hosts on shared medium

Simplifies multiplexing of logical channels

Efficient error detection

Frame serves as unit of detection (valid or invalid)

Error detection overhead scales as log N


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Approaches

Organized by end of frame detectionmethod

Approaches to framingSentinel (marker, like C strings)

Length-based (like Pascal strings)

Clock-based


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Approaches

Other aspects of a particularapproach

Bit-oriented or byte-oriented

Fixed or variable length

Data-dependent or data-independentlength


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Framing with Sentinels

End of frame: special byte or bit pattern

Choice of end of frame marker

Valid data byte or bit sequence e.g. 01111110

Physical signal not used by valid data symbol

8 16 16 8

Beginning

sequenceHeader Body CRC

Ending

sequence


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Sentinel Based Approach

Problem: equal size frames are not possible

Frame length is data-dependent

Sentinel based framing examples

High-Level Data Link Control (HDLC)

protocol

Point-to-Point Protocol (PPP)ARPANET IMP-IMP protocol

IEEE 802.4 (token bus)


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Length-based Framing

Include payload length in header

e.g., DDCMP (byte-oriented, variable-length)

e.g. RS-232 (bit-oriented, implicit fixed length)

Problem: count field corrupted

Solution: catch when CRC fails

8 148

SYN SYN Class Length

8 42

Header

16

Body CRC


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Clock-based Framing

Continuous stream offixed-length frames

Each frame is 125s long (all STS formats) (why?)

Clocks must remain synchronized

e.g. SONET: Synchronous Optical NETwork

Dominated standard for long distance transmission

Multiplexing of low-speed links onto one high-speed link

Byte-interleaved multiplexing

Payload bytes are scrambled (data XOR 127 bit-pattern)

STS-n (STS1 = 51.84 Mbps)


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SONET Frame Format (STS-1)

Overhead Pay load

90 columns

9 rows


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Clock-based Framing

Problem: how to recover frame

synchronization

2-byte synchronization pattern starts eachframe (unlikely to occur in data)

Wait until pattern appears in same place

repeatedly


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Clock-based Framing

Problem: how to maintain clock

synchronization

NRZ encoding, data scrambled (XORd)with 127-bit pattern

Creates transitions

Also reduces chance of finding false sync

pattern


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Error Detection

Validates correctness of each frame

Errors checked at many levels

Demodulation of signals into symbols(analog)

Bit error detection/correction (digital)

our main focusWithin network adapter (CRC check)

Within IP layer (IP checksum)

Possibly within application as well


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Error Detection and Correction

Possible binary voltage

encoding symbol

Neighborhoods and erasure

region

+15

-15

voltage

0

1

? (erasure)

Possible QAM symbol

Neighborhoods in green

All other space results in erasure

Input to digital level: valid symbols or erasures


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Error Detection: How ?

How to detect error ?

Add redundant information to a

frame to determine errors Transmit two complete copies of

data

n redundant bits forn-bitmessage

Error at the same position in two

copies go undetected


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Error Detection: How ?

We want only kredundant bits for an

n-bitmessage, where k < < n

In Ethernet, 32-bit CRC for 12,000 bits(1500 bytes)

kbits are derived from the original

message

Both the sender and receiver know the

algorithm


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Hamming Distance (1950 Paper)

Minimum number of bit flips betweencode words

2 flips for parity3 flips for voting

n-bit error detection

No code word changed into another codeword

Requires Hamming distance of n+1


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Hamming Distance (1950 Paper)

n-bit error correction

N-bit neighborhood: all code words

within n bit flipsNo overlap between n-bit

neighborhoods

Requires Hamming distance of 2n+1


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Digital Error Detection Techniques

Two-dimensional parity

Detects up to 3-bit errors

Good for burst errors

Internet checksum (used as backup to CRC)Simple addition

Simple in software

Cyclic redundancy check (CRC)Powerful mathematics

Tricky in software, simple in hardware

Used in network adapter


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Two-Dimensional Parity

Adding one extra bit to a 7-bit

code to balance 1s

extra parity byte for the entire

frame

Catches all 1, 2 and 3 bit errors

and most 4 bit errors

14 redundant bits for a 42-bit

message, in the example

1011110 1

1101001 0

0101001 1

1011111 0

0110100 1

0001110 1

1111011 0

Parity

bits

Parity

byte

Data


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Internet Checksum Algorithm

Not used at the link level but provides same sort offunctionality as CRC and parity

Idea:

Add up all words (16-bit integers) that are transmitted Transmit the result (checksum) of that sum

Receiver performs the same calculation on received dataand compares the result with the received checksum

If the results do not match, an error is detected 16 redundant bits for a message of any length

Weak protection, accepted as a last line of defense


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Cyclic Redundancy Check

Theory

Based on finite-field (binary-valued)

arithmetic

Bit string represented as polynomial

Coefficients are binary-valued

Divide bit string polynomial by generatorpolynomial to generate CRC

Practice

Bitwise XORs


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Add kbits of redundant data to an n-bit message

Want k


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Transmit polynomialP(x) that is evenly divisibleby C(x)

Shift left kbits, i.e.M(x)xk

Add remainder ofM(x)xk

/ C(x) intoM(x)xk

Receiver receives polynomial P(x) + E(x)

E(x) = 0 implies no errors

Receiver divides (P(x) +E(x)) by C(x); remainder

will be zero ONLY if:E(x) was zero (no error), or

E(x) is exactly divisible by C(x)


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Reliable Transmission

Error-correcting codes are not advanced

enough to handle the range of bit and burst

errors

Corrupt frames generally must be discarded

A reliable link-level protocol must recover

from discarded frames

Goals for reliable transmissionMake channel appear reliable

Maintain packet order (usually)

Impose low overhead / allow full use of link


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Reliability accomplished using

acknowledgments and timeouts

ACK is a small control frame

confirming reception of an earlier frame

Having no ACK, sender retransmits after

a timeout


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Automatic Repeat reQuest (ARQ)

algorithms

Stop-and-wait

Concurrent logical channels

Sliding window

Go-back-n, or selective repeat Alternative: Forward Error Correction

(FEC)


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Automatic Repeat reQuest

Acknowledgement (ACK)Receiver tells sender when frame received

Cumulative ACK(used by TCP): have

received specified frame and all previous

Selective ACK (SACK): specifies set of

frames received

Negative ACK (NACKor NAK): receiverrefuses to accept frame now, e. g. when

out of buffer space


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Automatic Repeat reQuest

Timeout: sender decides that frame waslost and tries again

ARQ also called

Positive Acknowledgement with

Retransmission (PAR)


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Stop-and-Wait

Send a single frame

Wait for ACK or timeout

If ACK received, continue with next frame

If timeout occurred, send again (and wait) Frame lost in transit; or corrupted and discarded

Sender Receiver

Frame 0

Frame1

ACK0

ACK1


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Stop-and-Wait

Frames delivered reliably and in order Is that enough ?

No, we need performance, too.

Problem: keeping the pipe full ?

Example

1.5Mbps linkx 45ms RTT = 67.5Kb (~8KB)

1KB frames implies 182 Kbps (1/8th link utilization)

Want the sender to transmit 8 frames before waitingfor ACK

Throughput remains 182 Kbps regardless of the linkbandwidth !!

Concurrent Logical Channels


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g

Multiplex several logical channels over a single p-to-p

physical link (include channel ID in header) Use stop-and-wait for each logical channel

Maintain three bits ofstate for each logical channel:

Boolean saying whether channel is currently busySequence number for frames sent on this channel

Next sequence number to expect on this channel

ARPANET IMP-IMP supported 8 logical channels

over each ground link (16 over each satellite link)


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Concurrent Logical Channels

Header for each frame include 3-bit

channel number and 1-bit sequence

number

Same number of bits (4) as the sliding

window requires to support up to 8

outstanding frames on the link

Slidi Wi d


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Sliding Window

Allow sender to transmit multiple frames beforereceiving an ACK, thereby keeping the pipe full

Upper bound on outstanding un-ACKed frames

Also used at the transport layer (by TCP)

Sender Receiver

Time


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Sliding Window Concepts

Considerordered stream of data

Broken into frames

Stop-and-Wait

Window of one frame

Slides along stream over time

Sliding window algorithms generalize this notion

Multiple-frame send window

Multiple-frame receive window

time


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Sliding Window - Sender

Assign sequence number to each frame (SeqNum) Maintain three state variables:

Send Window Size (SWS)

Last Acknowledgment Received (LAR)

Last Frame Sent (LFS) Maintain invariant: LFSLARSWS

Advance LARwhen ACK arrives

Buffer up to SWS frames and associate timeouts

time

14 1512 1311 19 2017 1816

LAR=13 LFS=18 SWS


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Sliding Window - Receiver

Maintain three state variables Receive Window Size (RWS)

Largest Frame Acceptable (LFA)

Next Frame Expected (NFE)

Maintain invariant: LFANFE+1RWS

Frame SeqNumarrives: IfNFESeqNumLFA accept

IfSeqNumNFE orSeqNum> LFA discarded

Send cumulative ACKs

time

14 1512 1311 19 2017 1816

NFE=13 LFA=17 RWS

Sliding Window Issues


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When a timeout occurs, data in transit decreases

Pipe is no longer full when packet losses occur

Problem aggravates with delay in packet loss

detection

Early detection of packet losses improvesperformance:

Negative Acknowledgements (NACKs)

Duplicate AcknowledgementsSelective Acknowledgements (SACKs)

Adds complexity but helps keeping the pipe full


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Sliding Window Classification

Stop-and-wait: SWS=1, RWS=1

Go-back-N: SWS=N, RWS=1

Selective repeat: SWS=N, RWS=M

(usually M = N)

Selective Repeat

Go-back-NStop-and-Wait


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Sequence Number Space

SeqNumfield is finite; sequence numbers wraparound

Sequence number space must be larger thannumber of outstanding frames (SWS)

SWS


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Required Sequence Number Space ?

Assume SWS=RWS (simplest, and typical)

Sender transmits full SWS

Two extreme cases at receiver

None received (waiting for 0SWS 1) All received (waiting for SWS2 SWS1)

All possible packets must have unique SeqNum

SWS < (MaxSeqNum+1)/2 or

SWS+RWS


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Shared Media: Problems

Problem: demands can conflict, e. g.two hosts send simultaneously

STDM does not address this problem -

centralizedSolution is a medium access control

(MAC) algorithm

Sh d M di S l ti


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Shared Media: Solutions

Three solutions (out of many)Carrier Sense Multiple Access with

Collision Detection (CSMA / CD)

Send only if medium is idle Stop sending immediately if collision

detected

Token Ring/FDDI pass a token around aring; only token holder sends

Radio / wireless (IEEE 802.11)


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History of Ethernet

Developed by Xerox PARC in mid-1970s

Roots in Aloha packet-radio network

Standardized by Xerox/DEC/Intel in 1978 Similar to IEEE 802.3 standard

IEEE 802.3u standard defines Fast

Ethernet (100 Mbps)

New switched Ethernet now popular


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EthernetAlternative Technologies

Can be constructed from a thinner cable (10Base2)

rather than 50-ohm coax cable (10Base5)

Newer technology uses 10BaseT (twisted pair)

Several point-to-point segments coming out of amultiway repeater called hub

HubHub


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EthernetMultiple Segments

Repeaters forward thebroadcast signal on all out

going segments (10Base5)

Maximum of 4 repeaters (2500m), 1024 hosts

Repeater

Host


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Ethernet Packet Frame

Preamble allows the receiver to synchronize withsignal

Frame must contain at least 46 bytes to detect

collision 802.3 standard substitutes length with type field

Type field (demux key) is the first thing in data portion

A device can accept both frames: type > 1500

Destaddr

64 48 32

CRCPreambleSrcaddr

Type Body

1648


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Ethernet MACCSMA/CD

Multiple access

Nodes send and receive frames over a

shared link Carrier sense

Nodes can distinguish between an idle

and busy link Collision detection

A node listens as it transmits to detectcollision


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CSMA/CD MAC Algorithm

If line is idle (no carrier sensed)

Send immediately

Upper bound message size of ~1500

bytes

Must wait 9.6s between back-to-back frames


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CSMA/CD MAC Algorithm

If line is busy (carrier sensed)

Wait until the line becomes idle and

then transmit immediatelyCalled 1-persistent (special case ofp-

persistent)

Ifcollision detectedStop sending data and jam signal

Try again later


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Constraints on Collision Detection

In our example, consider

my-machines message reaches your-machine

at Tyour-machines message reaches my-machine

at 2T

Thus, my-machine must still betransmitting at 2T

h i i


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Ethernet Min. Frame Size

RTT on a maximally configured Ethernet of

2500m, with 4 repeaters is about 51.2 s

2500m / 2 x 108 m/s = 12.5 s

2 x 12.5 = 25 us + repeater delays

51.2 s on 10 Mbps corresponds to 512 bits

(64 bytes)

Therefore, the minimum frame length for

Ethernet is 64 bytes (header + 46 bytes data)

R t Aft th C lli i


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Retry After the Collision

How long should a host wait toretry after a collision ?Binary exponential backoff

Maximum backoff doubles witheach failure (exponential)

After N failures, pick an N-bit

number 2N discrete possibilities from 0 tomaximum

E h F R i


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Ethernet Frame Reception

Sender handles all access control

Receiver simply pulls frames from

network

Ethernet controller/card

Sees all framesSelectively passes frames to host

processor

Experience With Ethernet


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Experience With Ethernet

Number ofhosts limited to 200 inpractice, standard allows 1024

Range much shorter than 2.5 kmlimit in standard

Round-trip time is typically 5 or 10

s, not 50s

T k Ri O i


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Token Ring Overview

Token Ring network was a candidate to replace

Ethernet; used in some MAN backbones

16Mbps IEEE 802.5 (based on earlier 4Mbps IBM ring)

100Mbps Fiber Distributed Data Interface (FDDI)

IBM T k Ri IEEE 802 5


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IBM Token RingIEEE 802.5

Ring is viewed as a single sharedmedium

Each node is allowed to transmit accordingto some distributed algorithm for mediumaccess

All nodes see all frames; destination saves

a copy of frame as it flows past

The term token indicates the way theaccess to shared channel is managed

T k i T k Ri


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Token in a Token Ring

Token is a special bit pattern that rotates

around the ring

A node must capture token before

transmitting

A node releases token after done transmitting

Immediate release-token follows last frame(FDDI)

Delayed releaseafter last frame returns

to sender

T k i T k Ri


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Token in a Token Ring

Remove your frame when it comes

back around

Transmit another frame or re-insert

the token

Stations get round-robin serviceas the token circulates around the

ring

Ph i l P ti


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Physical Properties

Data rate can be 4 Mbps or16 Mbps

Encoding of bits uses differential

Manchester

Ring may have up to 250 (802.5) or

260 (IBM) nodes

Physical medium is twisted pair (IBM

Token Ring)

T k Ri MAC


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Token Ring MAC

Network adaptor contains receiver, transmitter

and some storage of bits between them

Token circulates if no station has anything to

send

Ring must have enough capacity to store entire

token

At least 24 stations with 1-bit storage for 24-bitlong token (if propagation delay is negligible)

This situation is avoided by designating a

monitor

T k Ri MAC


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Token Ring MAC

Any station that has a data to send can

seize token

In 802.5, simply 1 bit in second byte

token is modified

First two bytes ofmodified tokenbecome preamble for the next frame

F F t


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Frame Format

Illegal Manchester codes in the start andend delimiters

Frame priority and reservation bits in

access control byte Demux key in frame control byte

A and C bits for reliable delivery, in statusbyte

Body CRCSrcaddr

Variable48

Destaddr

48 32

Enddelimiter

8

Framestatus

8

Framecontrol

8

Accesscontrol

8

Startdelimiter

8

Ti d T k Al ith


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Timed Token Algorithm

Token Holding Time (THT)

Upper limit on how long a station can

hold the tokenA node checks before putting each frameon ring that its transmit time would notcause THT to exceed

Long THT achieves better utilizationwith few senders

Short THT helps when multiple nodes

have data to send

R li bl D li


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Reliable Delivery

The A and C bit in the packet

trailer for reliability

Both bits are initially set to 0

Destination sets A bit if it sees

the frame and sets C bit if it

copies the frame into its adaptor

Token Ring Packet Priorities


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Token Ring Packet Priorities

A station willing to send priority n

packet can set reservation bits to n, if

this makes it lower in valueIt captures the token when the current

sender releases it with priority set to n

Strictpriority scheme: no lower-priority packets get sent when higher

priority packets are waiting

Token Maintenance


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Token Maintenance

Token rings have a designated

monitor node

Any station can become the monitoraccording to a well defined procedure

Monitor is elected when the ring is

first connected, or when the current

monitor fails

Token Maintenance


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Monitor periodically announces itspresence

Claim token sent by a station seeing

no monitor

If the sender receives back the claim

token, it becomes monitor

If another station is also contending for

monitor, some rule defines the monitor

Fiber Distributed Data Interface


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Fiber Distributed Data Interface

Similar to 802.5/IBM token rings but runs on fiber Consists of a dual ring: two independent rings that

transmit data in opposite directions at 100Mbps

Tolerates a single link break or node failure (self-

healing ring)

(a) (b)

FDDI Physical Properties


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FDDIPhysical Properties

Variable size buffer (980 bits)

between input and output

interfaces (10ns bit time)Not required to fill buffer before

starting transmission

Maximum 500 stations, maximum

2 km distance between any pair of

stations

FDDI Physical Properties


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FDDIPhysical Properties

Total 200 km fiber: dual nature

implies 100 km cable

connecting all stations

Physical media can be coax or

twisted pair cable

Uses 4B/5B encoding



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Token Holding Time (THT)

Upper limit on how long a station can hold

the token

Configured to some suitable value

Token Rotation Time (TRT)

How long it takes the token to traverse thering (time since a host released the token)

TRT


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Target Token Rotation Time

(TTRT)

agreed-upon or negotiated

upper bound on TRT

MAC Algorithm


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MAC Algorithm

Each node measures TRT

between successive token

arrivals

Ifmeasured-TRT > TTRT

Token is late

Can not send data

FDDI Traffic Classes


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FDDI Traffic Classes

Synchronous traffic

Latency sensitiveGets higher priority

Can always send data

Bounded Priority Traffic


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Bounded Priority Traffic

If a node has large amount of

synchronous data

It will send regardless of measured TRTTTRT will become meaningless !!!

Therefore, total synchronous data

during one token rotation is bounded

by TTRT

Token Maintenance


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The procedure when a nodeJoins the ring (startup)

Suspects a failure

Claim frame is used in order toGenerate a new Token

Agree on TTRT (so that an application

can meet its timing constraints) A node can send a claim frame without

holding the token

Frame Format


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4B/5B control symbols for start and end of frame

Control Field

1st bit: asynchronous (0) versus synchronous (1) data

2nd bit: 16-bit (0) versus 48-bit (1) addresses

Last 6 bits: demux key (includes reserved patterns fortoken and claim frame)

Status Field

From receiver back to sender; error in frame

Recognized address; accepted frame (flow control)

Body CRCSrcaddr

Variable48

Destaddr

48 32

End offrame

8

Status

24

Control

8

Start offrame

8

Wireless LANs


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IEEE 802.11 standard

Designed for use in a small area (offices,

campuses)

Bandwidth: 1, 2 or 11 MbpsUp to 56Mbps in newer 802.11a standard

Targets three physical media

Two spread spectrum radio (2.4GHz freq)One diffused infrared (10m range, 850 nm

band)

802.11 MAC: CSMA/CA


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802.11 MAC: CSMA/CA

Similar to Ethernet

Defer the transmission until the link

becomes idleTake back offif collision occurs

Is it sufficient ?

All nodes are not always within

reach of (to hear) each other

Hidden and Exposed Nodes


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p

Hidden nodes

Sender thinks its OK to send when its not (false +ve)

A-C and B-D are hidden nodes in the figure below

Exposed nodes

Sender does not send when its OK to send (falseve) B and C are exposed nodes in the figure below

A B C D

Multiple Access with Collision

A id (MACA)


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Avoidance (MACA)

Sender transmits RequestToSend

(RTS) frame

Contains intended time to hold the

medium

Receiver replies withClearToSend(CTS) frame

MACA for Wireless (MACAW)


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MACA for Wireless (MACAW)

Collision detection

No active collision detection

Known only if CTS or ACK is

not received

Binary exponential back off(BEB) is used in case of

collision, like in Ethernet

802.11 - Distribution System


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y

Nodes roam freely but operatewithin a structure

Tethered by wired network

infrastructure (Ethernet ?)

Each Access Point (AP) services

nodes in some regionEach mobile node associates itself

with an AP

Managing Connectivity/Roaming


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Managing Connectivity/Roaming

How wireless nodes select Access Point ?

Scanning (active search for an AP)

Node sends Probe frame All APs within reach reply with Probe

Response frame

Node selects one AP; sends itAssociate

Request frame

AP replies withAssociation Response

New AP informs old AP via wired backbone

Managing Connectivity


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Managing Connectivity

Active scanning: when a node join or move Passive scanning: AP periodically sendsBeacon frame, advertising its capabilities

B

H

A

F

G

D

AP-2

AP-3AP-1

EC

C

Distribution sy stem

Frame Format


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Frame Format

Control field contains three subfields:

6-bit Type field (data, RTS, CTS, scanning);

1-bit ToDS; and

1-bit FromDS

A single frame contains up to 2312 bytes of data

Addr4 Pay loadSeqCtrlAddr3Addr2Addr1 CRC

0 18,4964816 32484848

Duration

16

Control

16

ToDS=0, FromDS=0 C A

ToDS=1, FromDS=1 E AP-3 AP-1 A

Overview


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Overview

Also called network interface card (NIC)

Components (high-level overview)

Options for use

Data motion

Event notification

Potential performance bottlenecks

Programming device drivers

Typical Workstation Architecture


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Typical Workstation Architecture

CPU

Cache $

MemoryI/O bus

Network

Adaptormemory

bus

Communication ?

To

Network

Typically where data linkfunctionality is implemented

Components of a Network Adaptor


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Components of a Network Adaptor

Bus interface communicates with a specific host Bus defines protocol for CPU-adaptor communication

Link interface speaks correct protocol on network

Implemented by a chip set, in software or on FPGA

Buffering between different speed bus and link

Host

I/Ob

us

Network Adaptor

Bus

Interface

Link

Interface network

Host Perspective


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Host Perspective

Adaptor is ultimately

programmed by CPU

Adaptor exports a Control

Status Register (CSR)

CSR is readable and writablefrom CPU at some memory

address

Data Motion Options for Network

Ad U


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Adaptor Use

Transfer frames between adaptorand host memory

Programmed input/output (PIO)Processor manages itself each

access (loads/stores)

Faster than DMA forsmall amountsof data

Data Motion Options for Network

Ad t U


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Adaptor Use

Direct memory access (DMA)

Adaptor gets buffer descriptor lists

by host for read/writeProcessor is not involved: free to do

other things

Can be faster than memory copythrough CPU

Start-up cost

Data Motion


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CPU

Cache $

MemoryI/O bus

Network

Adaptormemory

bus

To

Network

Data movement

path using PIO

Data movement

path using DMA

Network Adaptor: Event Notification


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Hardware interruptsProcessor free to do other things

Events delivered immediatelyState (register) save/restore

expensive

Context switches more

expensive

Network Adaptor: Event Notification


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p

Event polling

Processor must periodically

check

Events wait until next check

No extra state changes

Device Drivers


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Operating system routines anchoring

protocol stack to network hardware

Initialize device, transmit frames,field interrupts

Code contains device specific detailsDifficult to read but simple in logic

Performance Bottlenecks


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Link capacity

Processor computing power I/O bus bandwidth

Overhead involved in each bustransfer

Performance Bottlenecks


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Memory bus bandwidth

Memory hierarchy with cachelevels

Memory accesses results in

multiple memory copies in

different buffers

Packet Switches


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A multi-input multi-outputdevice

Local star topology

Performance independent of

connectivity (e.g. adding new host) if switch is

designed with enough aggregatecapacity

Maximumdegree < physical network limit

Forwarding


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g

Packets arrive at one of the several inputs and

have to be forwarded/switchedto one of the

available outputs

Connectionless and connection-oriented approach todetermine the correct output

Which way should it go ?

First challenge:forwarding

Routing


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g

Forwarding requires information

Second

challenge:

routing

How to maintain forwarding information ?

Contention and Congestion


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g

If arrival rate for a certain output is greater than

the output capacity, then contention occurs

If arrival rate of packets is too high to cause buffer

overflow, then congestion occurs

Who

goesfirst ?

Any one is

dropped ?

Network Layers and Switches


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One or more nodes

within the network

User

level

OS

kernel

host

switch

switch

betweendifferent

physical

layers

Transport

Network

Data Link

Physical

Session

Presentation

Application

Network

Data Link

Physical

Packet Switching / Forwarding


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Three approaches

Datagram or connectionless

approachVirtual circuit or connection-oriented

approach

Source routing

Important notion: unique global

dd h t

Datagram Switching / Forwarding


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Every packet contains enough

information

Enables switch to decide how to forward it

Switch translates global address to

output port

Maintains forwarding table for translations Each packet forwarded and travels

independently

Datagram Switching


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Managing tables in large, complex networks withdynamically changing topologies is a realchallenge for the routingprotocol

01

3

2

0

1

3

2

0

13

2

Switch 3

Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host E

Host F

Host G

Host H

At switch 1:Dest Port#/Interface

A 2

B 1

C 3

D 0

E 1

Datagram Model


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No round trip time delay waiting forconnection setup

Host can send data anywhere, anytime as soon as it

is readySource has no way of knowing if the network is

capable of delivering a packet or if the destination

host is even up

Packets are treated independently

Possible to route around link and node failures

dynamically

Virtual Circuit Switching


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Explicit connection setup (and tear-

down) phase from source to destination:

connection-orientedmodelSubsequence packets follow established

circuit

Supporting connections in networklayer may be useful for service notions

VC Tables in VC Switching


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Setup message in signaling process(to create VC table) is forwarded

like a datagram Acknowledgment of connection

setup to downstream neighbors to

complete signalingData transfer phase can start after

ACK is received

Signaling in VC Switching


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Setup message isforwarded fromHost A to Host B

On connectionrequest, eachswitch creates anentry in VC tablewith a VCI for theconnection

0

13

2

2

1

3

0

0

13

2

Switch 3

Host B

Switch 2

Switch 1

Host A

I/F VCI I/F VCI

in in out out

setup B

setup B

setup B

setup B

2 5 1

I/F VCI I/F VCI

in in out out

2 7 3

I/F VCI I/F VCI

in in out out

3 9 0

Virtual Circuit Model


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Typically wait full RTT forconnection setup before sending

first data packetCan not avoid failures dynamically,

must re-establish connection (old

one is torn down to free storagespace)

Source Routing


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Packet header contains sequence

of address/ports on path from

source to destinationOne direction per switch: port, next

switch (absolute)

Switches read, use, and then discard

directions

Data Transfer in Source Routing


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Analogous to

following

directions0

13

2

2

1

3

0

0

13

2

Switch 3

Host B

Switch 2

Switch 1

Host A

data 0 1 3

data 3 0 1

data 1 3 0

data 3 0 1

data 1 0 3

data 2 3 0 1

Source Routing Model


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Source host needs to know the

correct and complete topology of

the networkChanges must propagate to all hosts

Packet headers may be large and

variable in size: the length is

unpredictable

Implementation and Performance


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Packet arriving at interface 1 has to go on interface 2

Point of contention for packets: I/O and memory bus

CPU

Main memory

I/O bus

Interface 1

Interface 2

Interface 3

Building Extended LANs


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Traditional LANShared medium (e.g. Ethernet)

Cheap, easy to administer

Supports broadcast traffic Problem

Want to scale LAN concept

Larger geographic area (Greater than O(1 km))

More hosts (Greater than O(100))

But retain LAN-like functionality

Solution: bridges

Bridges


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Connect two or more LANs with a bridgeTransparently extends a LAN over multiple

networks

Accept & forward strategy (in promiscuous

mode)Level 2 connection (does not add packet header)

A

Bridge

B C

X Y Z

Port 1

Port 2

Learning Bridges


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Learn table entries based on source addressTimeout entries to allow movement of hosts

Table is an optimization need not be complete

Always forward broadcast frames Uses datagram or connectionless forwarding

A

Bridge

B C

X Y Z

Port 1

Port 2

Host Port

A 1

B 1C 1

X 2

Y 2

Z 2

Learning BridgesA


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Problem

Redundancy (desirable to handle failures, but )

Makes extended LAN structure cyclicFrames may cycle forever

Solution: spanning tree

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

Spanning Tree


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Subset of forwarding possibilities All LANs reachable, but

Acyclic

Bridges run a distributed algorithm tocalculate the spanning tree

Select which bridge actively forward

Developed by Radia Perlman of DEC

Now IEEE 802.1 specification

Reconfigurable algorithm

Spanning Tree Algorithm


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All designated bridges forward frames

On all designated ports

On preferred port (path leading to root)

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

B2

LAN

Designated port

Preferred port

Designated bridge

Distributed Spanning Tree Algorithm


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Bridges exchange configuration

messages

ID for bridge sending the message

ID for what the sending bridge

believes to be root bridge

Distance (hops) from sending bridge

to root bridge

Limitations of Bridges


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Do not scale

Spanning tree algorithm does not

scaleBroadcast does not scale

Do not accommodate heterogeneityOnly supports networks with same

address formats

ATM (Asynchronous Transfer Mode)


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Common in WANs, can also be usedin LANs

Competing technology with Ethernet, but

areas of application only partially overlap

Connection-oriented packet-

switched network

Virtual-circuit routing

Typically implemented on SONET

( th h i l l ibl )

ATM Signaling


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Connection setup called signaling(standard Q.2931)

Route discovery, resource resv, QoS, ...

Send through networkRequest setup circuit

Send setup frame on setup circuit

Establish locally

No intermediate switch involvement

Requires pre-established virtual path

Cell Switching (ATM)


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Fixed length (53 bytes) frames arecalled cells

5-byte (header + 1byte CRC8) +

48-byte payload

Standard defines 3 layers (5 sublayers)

Layers interface to physical media andto higher layers (e.g. encapsulating

variable-length frames)

Cell Switching (ATM)


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2-level connection hierarchyVirtual circuits

Virtual pathsBundles of virtual circuits

Travel along common route

Reduces forwarding

information

ATM Cell Format

User Network Interface (UNI)


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User-Network Interface (UNI)

Host-to-switch format

GFC: Generic Flow Control (still being defined)

VCI/VPI: Virtual Circuit/Path Identifier Type: management, congestion control, AAL5 (later)

CLP: Cell Loss Priority

HEC: Header Error Check (CRC-8)

Network-Network Interface (NNI) Switch-to-switch format

GFC becomes part of VPI field

GFC VPI VCI Type CLP HEC(CRC-8) payload

4 16 3 18 384 (48 bytes)8

Segmentation and Reassembly

ATM Adaptation Layer (AAL)


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ATM Adaptation Layer (AAL) Application to ATM cell mapping

AAL header contains information for reassembly

AAL1, AAL2 for applications needing guaranteed rate

AAL3/4 designed for variable-length packet data

AAL5 is an alternative standard for packet data

AAL

ATM

AAL

ATM

ATM Layers ATM Adaptation Layer (AAL)


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Convergence Sublayer (CS) supportsdifferent application service models

Segmentation and Reassembly (SAR)supports variable-length frames

ATM Layer

Handles virtual circuits, cell headergeneration, flow control

Physical layer

Transmission Convergence (TC)handles error detection, framing

Physical medium dependent (PMD)sublayer handles encoding

ATM

AALCS

SAR

PHY

TC

PMD

AAL 3/4


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Provides information to allow variable size packetsto be sent in fixed-size ATM cells

Convergence Sublayer Protocol Data Unit (CS-PDU)

CPI: Common Part Indicator (version field)

Btag/Etag:beginning and ending tags (same)

BAsize: hint on reassembly buffer space to allocate Length: size of whole PDU

Segmented into cells: header/trailer + 44-byte data

CPI Btag BAsize payload Pad 0 Etag Length

8 16 0-24 88 < 64 KB 8 16

ATM Cell Format for AAL 3/440 4 352 (44 bytes) 62 10 16


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Type (is-start? and is-end? bits)

BOM (10): Beginning Of Message

COM (00): Continuation Of Message EOM (01): End Of message

SSM (11): Single-Segment Message

SEQ: Sequence Number (forcell loss/reordering)

MID: multiplexing ID (mux onto virtual circuits)

Length: number of bytes of PDU in this cell

ATM header type seq MID payload length CRC-10

Encapsulation and Segmentation for

AAL3/4


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44 bytes 44 bytes 44 bytes


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CS-PDU Format

Pad so trailer always falls at the end of ATM cell

Length: size of PDU (data only)

CRC-32 (detects missing or misordered cells)

Cell Format

End-of-PDU bit in Type field of ATM header

0 - 47 2< 64 KB 2 32

data pad reserved length CRC-32

Encapsulation and Segmentation for

AAL 5


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User data

48 bytes 48 bytes 48 bytes

ATM header Cell payload

Padding

CS-PDUtrailer

Virtual Paths with ATM


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Two level hierarchy of virtual connection: 8-bitVPI and 16-bit VCI

Switches in the public network use 8-bit VPI

Corporate sites use full 24-bit address (VPI + VCI)

Much less connection-state info in switches

Virtual path: fat pipe with bundle of virtual circuits

Public network

Network BNetwork A

ATM as a LAN Backbone


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Different from traditional LANs, no native supportfor broadcast or multicast

E1

H5

H6

H7

H1E3

H2

H4

H3E2

ATM linksEthernet link

Ethernet switch

ATM switchATM-attachedhost

Shared Ethernet Emulation with LANE


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All hosts think they are on the same Ethernet

LANE / Ethernet

Adaptor Card

H

H

H

HH

Ethernet

Switch

ATM Switch

LANE / EthernetAdaptor Card

H

H

H

H

H

EthernetSwitch

ATM Switch

ATM / LANE Protocol Layers


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Higher-layer

protocols

(IP, ARP, . . .)

Signalling

+ LANE

AAL5

ATM

PHY

ATM

PHY PHY

Higher-layer

protocols

(IP, ARP, . . .)

Signalling

+ LANE

AAL5

ATM

Host Switch Host

PHY

Ethernet-like

interface

Clients and Servers in LANE


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LAN Emulation Client (LEC)

Host, bridge, router or switch

LAN Emulation Server (LES)

Maintains clients MAC and

ATM addressesMaintains ATM address of BUS



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LAN Emulation Configuration

Server (LECS)

High-level network managementwhen LEC starts up

Reachable by preset VC (recall

known server port#)Maintains mapping of ATM address

to LANE type



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Broadcast and Unknown Server (BUS) Emulates broadcast and multicast; critical to LANE

Uses point-to-multipoint VC with all clients

Servers physically located in one or more devices

H2H1

BUSLESATM network

Point-to-point VC

Point-to-multipoint VCLECS

LANE Registration


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1. Client contacts LECS on predefinedVC, and sends ATM address to it

2. LECS returns LAN type, MTU andATM address of LES

3. Client signals connection to LES, andregisters MAC and ATM addresseswith LES

4. LES returns ATM address of BUS

5. Client signals connection to BUS

6. Bus adds client to point-to-multipointVC

ATM Network

LECS

LES BUS

H1 H2

H3

LANE Circuit Setup


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1. Client (H1) knows destinationMAC address of receiver (H2)

2. Client (H1) sends 1st packet toBUS

3. BUS sends address resolutionrequest to LES

4. LES returns ATM address toclient (H1)

5. Client (H1) signals connectionto H2 for subsequent packets

ATM Network

LECS

LES BUS

H1 H2

H3

Contention in Switches Some packets destined for same output


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Some packets destined forsame output

One goes first

Others delayed or dropped

Delaying packets requires buffering

Finite capacity, some packets must still drop

At inputs

Increases/adds false contention

Sometimes necessary

At outputs

Can also exert backpressure

Output BufferingCustomer


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1x6 Switch

x

a

Standard check-in lines

Customer

service

trying to check-inyou Mr. X

writing

complaintletter

Mr. A

waiting to

claim refund

of Rs.100

Input Buffering: Head-of-line

advanced computer networks - cs716 power point slides lecture 25

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