chapter 5: link layer & lans

CHAPTER 5: LINK LAYER & LANS

• Flow Control• Error Control• Data Link Protocols• Medium Access Control• IEEE 802 Standards• Multiprotocol Label Switching• Example: Tracing A Web Page Request

FLOW CONTROL

Chapter 5CS 447

frame frameframeframeframeframeframe frame frame frame

NetworkNode

NetworkNode

When a network node transmits frames faster than the next network node can “digest” them, the receiver will usually just discard the

excess frames.

To combat this problem, the Data Link Layer protocol usually contains some kind of “flow control” mechanism.

STOP-AND-WAIT

Chapter 5CS 447

ACK #6ACK #5ACK #4 frame #6frame #5frame #4ACK #3ACK #2ACK #1 frame #3frame #2frame #1

NetworkNode

NetworkNode

One approach to Data Link Layer flow control is for the receiving node to respond with an acknowledgement whenever it has finished

dealing with the previous frame and is ready to receive the next frame.

Variations on this approach include:• “Piggybacking” ACKs on frames going the opposite direction on the

link, in order to reduce line utilization.• Sending negative acknowledgements whenever a received frame is

corrupted or whenever a frame is not received in a timely fashion.• Retransmitting a frame automatically whenever an ACK is not received

in a timely fashion.

SLIDING WINDOWS

Chapter 5CS 447

By increasing the memory on each network node and using a more sophisticated algorithm for keeping track of which frames have and haven’t been accepted, traffic can flow more smoothly between the

nodes.

Sending Node(window size 8)

Receiving Node

F3

F2

F1

F0

F5

F4


Receiving Node

F3

F5

F4ACK(awaiting F3)


Receiving Node

ACK(awaiting F7)

F7

F0

F2

F1


Receiving Node

F4

F3

F5

F7

F6

F0

F2

F1

Sending Node(window size 0) STOP

Receiving Node

F0

F7

F1

F3

F2

F4

F6

F5

Sending Node(window size 0) STOP

Receiving Node

ERROR CONTROL

Chapter 5CS 447

faerm???

frame

Error control at the Data Link Layer is concerned with the detection (and, if possible, the correction) of lost or corrupted frames between

consecutive network nodes.

NetworkNode

NetworkNode

frameNetworkNode

NetworkNode

ERROR DETECTION:PARITY CHECK

Chapter 5CS 447

One way to detect transmission errors is by using parity bits to ensure that each segment of data has an even number of 1’s (even parity) or an odd

number of 1’s (odd parity), depending on which type of parity the protocol is using.

Wants to send message “YO!” in ASCII, using even parity.

ASCII ‘Y’ is 1011001, so tack on a ___ASCII ‘O’ is 1001111, so tack on a ___ASCII ‘!’ is 0010001, so tack on a ___

010

So, the transmitted message is:10110010 10011111 00100010

Received message is:10110010 10011111 00110010

First byte is 10110010, with even parity, so it’s ASCII ‘Y’Second byte is 10011111, with even parity, so it’s ASCII

‘O’Third byte is 00110010, with odd parity, so it’s an

error!!!

One major problem with parity checking: if a segment has an even number of corrupted bits, no error is detected!

101100101001111100100011 NetworkNode

NetworkNode

ERROR DETECTION:CYCLIC REDUNDANCY CHECK

Chapter 5CS 447

For more effective error detection, the cyclic redundancy check was developed.

1. Both stations agree upon a binary “generator”, for example: 110101

2.The sending station tacks len(generator)-1 0’s onto its binary message and does a modulo-2 division by the generator.For example, if the original message is 100011011001000111 with generator 110101, then the division at right is performed:

10001101100100011100000110101 1

110101101100

11

110101110011

111

11010100110100

111001

1101010000110001

11100100001

11010100100110

11100100001001

110101100110

111001000010011

110101100110

1110010000100111

110101100110

11100100001001111

110101100110

111001000010011111

11010110011

3.The sending station transmits its message, with the remainder of the above quotient added as a suffix.

Actual transmission:10001101100100011110011

CRC AT THE RECEIVER

Chapter 5CS 447

4. The receiving station performs a modulo-2 division by the generator on the received message (including the appended CRC suffix).

5. If the remainder of this quotient is non-zero, then a transmission error has occurred. Otherwise, we’re reasonably certain that there’s been no error!

100011011001000111100111101011

110101101100

11

110101110011

111

11010100110100

111001

1101010000110001

11100100001

11010100100111

11100100001001

110101100100

111001000010011

110101100010

1110010000100111

110101101111

11100100001001111

110101110101

111001000010011111

11010100000

FRAME FORMATS

Chapter 5CS 447

When formatting the Data Link Layer’s frames for transmission on the Physical Layer, it’s necessary to mark the frame with a header so the receiving network node will recognize the beginning of the frame.

Network Layer PacketSpecial Data Link Header

To enable the receiver to recognize the end of the frame, several options exist.


Include a size field in the header


Use a standard size for all frames


Include a special trailer coded with a bit sequence that’s guaranteed not to occur in the rest of the frame

Special Trailer

SPECIFIC DATA LINK PROTOCOLS

Chapter 5CS 447

A variety of Data Link Layer framing formats have been developed.

HDLCDeveloped by the OSI folks, the High-Level

Data Link Control protocol is commonly used in traditional packet-switching networks, like

X.25.

SLIPDeveloped by the TCP/IP

folks, the Serial Line Internet Protocol and the Point-To-Point Protocol are used to send IP datagrams

across slow serial lines.

PPP

ATMAsynchronous Transfer Mode was

developed to address the transition of communication data from voice and text

to multimedia.

HIGH-LEVEL DATA LINK CONTROL

Chapter 5CS 447

01111110 Address Control Data CRC Checksum 01111110Address Control Data CRC Checksum01111110 01111110

Delimiting fields to mark the beginning and ending of the frame. Require bit stuffing!

Address field to identify the specific node with which communication is occurring in a multipoint line. (Not used in a meaningful way in a point-to-point line.)

Control field to identify the type of frame being transmitted:• Information frames start with a zero, followed by a 3- or 7-bit sequence number

(for sliding window purposes), a bit to indicate whether this is a polling frame (from the multipoint primary station) or a final frame in a sequence (from a multipoint secondary station), and a 3- or 7-bit ACK sequence number.

• Supervisory frames start with a 10, followed by a 2-bit type (ACK, NAK/Go-Back-N, ACK/Halt, or NAK/S-R), a Poll/Final bit, and an ACK sequence number.

• Unnumbered frames start with a 11, followed by a 1-bit Poll/Final bit, and a 5-bit command (Frame Reject, Disconnect, Set Up Link w/Asynchronous Response Mode, Set Up Link w/Normal Response Mode, etc.)

Data field containing the Network Layer bits that were handed down to it.Checksum field to perform the good ol’ Cyclic Redundancy Check!

SERIAL LINE INTERNET PROTOCOL

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11000000 “Stuffed” IP Datagram 11000000

Delimiting fields at the beginning and ending of the frame. Require byte stuffing!

The Network Layer’s data (i.e., IP’s datagram), stuffed to ensure that the delimiter never occurs: whenever 11000000 occurs in the data, it’s replaced with 11011011 11011100, and whenever 11011011 occurs in the data, it’s replaced with 11011011 11011101.

“Stuffed” IP Datagram11000000 11000000

Note that SLIP has several problems:• It only works with IP; no other Network Layer protocol is

supported.• It does no error checking, leaving such problems to the higher

layers.• Addresses must be known in advance by both communicating

nodes, since no address fields are available.• It isn’t an approved IP standard, so numerous, incompatible

versions exist.Why is SLIP popular in spite of these problems?

• There are free versions readily available, all working with the ubiquitous EIA-232D modem interface!

POINT-TO-POINT PROTOCOL

Chapter 5CS 447

Delimiting fields at the beginning and ending of the frame. Just require bit stuffing!Address field always uses the value 11111111, signifying that every transmission is a broadcast!Control field always uses the value 00000011, signifying that every transmission is unnumbered (i.e., sliding windows are not supported!).

Payload field containing the Network Layer bits that were handed down to it, or the link control or network control message being relayed.

Checksum field to again perform our old friend, the Cyclic Redundancy Check!

01111110 Address Control Payload CRC Checksum 01111110ProtocolAddress Control Payload CRC Checksum01111110 01111110Protocol

Protocol field to identify the type of data in the Payload field:• Protocol 00000000 00100001 means the payload is an IP datagram.• Protocol 11000000 00100001 means the payload is link control data, used to

establish, configure, and test the particular link being used.• Protocol 10000000 00100001 means the payload is network control data, used

to identify the Network Layer protocol being used (e.g., IP, AppleTalk, OSI, DECnet).

ASYNCHRONOUS TRANSFER MODE

Chapter 5CS 447

The Virtual Path Identifier (VPI) specifies the number of a particular path that several virtual circuits take through the network node; by hierarchically identifying such routes, individual nodes may just use this prefix to forward a cell, rather than having to examine the entire path/circuit sequence.The Virtual Circuit Identifier (VCI) completes the identification of the particular virtual circuit being used.

The Payload Type Identifier (PTI) signifies the sort of data being transmitted:• 000 and 001 signify uncongested user data cells (with a Final bit on the end).• 010 and 011 signify congested user data cells (with a Final bit on the end).• 100 and 101 signify maintenance info (locally or end-to-end).• 110 signifies the cell is relaying end-to-end congestion info.

The Header Error Check (HEC) byte is a CRC remainder for just the header.The 48-byte Payload includes the data and any AAL (ATM Adaptation Layer) headers that might have been added at the protocol layer above ATM.

VPI VCI PTI HEC PayloadCLP

The Cell Loss Priority (CLP) bit is used to distinguish high- and low-priority traffic.

VPI VCI PTI HEC PayloadCLP

MEDIUM ACCESS CONTROL

Chapter 5CS 447

Various algorithms have been formulated for providing access to a shared transmission channel to multiple independent stations.

frame frame frame

Contention Systems

Carrier Sense Systems

Reservation Systems

framereserv.

00:00:0500:00:0400:00:0300:00:0500:00:0400:00:0300:00:0200:00:0500:00:0400:00:0300:00:0200:00:0100:00:00

frame w/reserv.

CSMA/CD

Chapter 5CS 447

I haven’t heard anything for a while, so I’m sending!

110100010101001010101010

Everything I’ve heard so far is exactly what I’ve been sending!

0011100100001010111100101001011011110100010101001010101010

I haven’t heard anything for a while, so I’m sending!

111000111110100111100

That’s not what I sent!COLLISION!!!

That’s not what I sent!COLLISION!!!

100010101110100101010101101001011010101000010110101010101010101001010111010101000110111001010

IEEE 802 STANDARDS

Chapter 5CS 447

The IEEE 802 Local and Metropolitan Area Network Standards Committee has the basic charter to create, maintain, and

encourage the use of IEEE/ANSI and equivalent IEC/ISO JTC 1 standards primarily within layers 1 and 2 of the OSI (Open

System Interconnection) Reference Model.

The committee was formed in February 1980 and met at least three times per year as a Plenary body ever since that time.

An explicit objective since inception has been the goal of establishing international standards in JTC 1.

The IEEE series of standards are known as IEEE 802.xxx and the JTC 1 series of equivalent standards are known as ISO

8802-nnn.In the IEEE 802 context, "local" means campus and

"metropolitan" means intra-city.

IEEE 802.1:HIGHER-LAYER LAN PROTOCOLS

Chapter 5CS 447

Internetworking standards for

bridging different LAN and MAN

protocols.

IEEE 802.2:LOGICAL LINK CONTROL

Chapter 5CS 447

The LLC sits on top of the Medium Access Control sublayer of the Data Link Layer, and

is responsible for:• Framing Network Layer packets • Link synchronization• Message acknowledgement• Error detection and possible recovery• Flow control

Physical Layer

Data Link Layer

Network Layer

Medium Access Control Sublayer

Logical Link Control Sublayer

Transport Layer

Session Layer

Presentation Layer

Application Layer

Current Status: INACTIVE

IEEE 802.3:ETHERNET

Chapter 5CS 447

10Base-T twisted pair

Ethernet connection with

RJ-45 jack

10Base-2 coax Ethernet connection with T-junction tap

10Base-5 coax Ethernet cable,

capable of 10Mbps

10Base-F fiber optics

Ethernet hub

ETHERNET CONFIGURATIONS

Chapter 5CS 447

ETHERNET FRAME FORMAT

Chapter 5CS 447

Preamble: Seven Manchester-encoded 10101010-bytes to enable synchronization.

Start: One Manchester-encoded 10101011-byte to delimit the start of the frame.

Destination Address: 2- or 6-byte Ethernet card address, burned into the card.

Data: The actual data handed down from the Network Layer.

Checksum: 4-byte Cyclic Redundancy Check.

Length: 2-byte length of the data field (range: 0-1500 bytes).

Preamble Start DestinationAddress Data Padding ChecksumSource

Address Length

Source Address: 2- or 6-byte Ethernet card address, burned into the card.

Padding: 0-46 bytes of dummy info, to ensure a 64-byte minimum frame length.

Preamble Start DestinationAddress Data Padding ChecksumSource

Address Length

BINARY EXPONENTIAL BACKOFF ALGORITHM

Chapter 5CS 447

When a collision does occur on Ethernet, each station must retransmit, but they’d like to avoid another collision, so they independently generate random wait times before

their attempted retransmissions.

00:00:00 00:00:0100:00:0000:00:0100:00:00 00:00:0200:00:0100:00:0000:00:0500:00:0400:00:0300:00:0200:00:0100:00:00 00:00:0300:00:0200:00:0100:00:00

frame frameframe frameframe frameframe

COLLISION!Pick a wait time

between 0 and 1:0


between 0 and 1:1


between 0 and 3:1


between 0 and 3:2


between 0 and 7:5


between 0 and 7:3

IEEE 802.4:TOKEN BUS

Chapter 5CS 447

P=4;S=7

P=7;S=6

P=4;S=7P=2;S=9

P=1;S=3

P=5;S=4

P=8;S=5

P=9;S=1

P=3;S=8

P=6;S=2

To preserve the simplicity of the bus structure, while eliminating the unpleasantness of collisions, 802.4 passes a special “token” from

station to station, using a prearranged predecessor/successor numbering system.

When a station receives the token, it has “permission” to transmit normally across the bus. It is allowed to transmit for a certain

length of time, then it must pass the token to its successor.

Current Status: DISBANDED

IEEE 802.5:TOKEN RING

Chapter 5CS 447


IEEE 802.6:DISTRIBUTED QUEUE/DUAL BUS

Chapter 5CS 447


Two buses are maintained, with data flowing in opposite directions, and every networked machine connected to both buses.

Each bus has a head-end that generates 53-byte cells, which can be filled by the machines that are passed according to an access

protocol.

Access Node

Access Node

Access Node

Access Node

Access Node

Access Node

Slot Generator

Bus Terminator

Slot Generator

Bus Terminator

empty slot full slot empty slot full slotempty slot full slot

empty slotfull slot empty slot full slotempty slot full slot

MORE DISBANDED GROUPS

Chapter 5CS 447

• IEEE 802.7: Broadband• Implementing broadband on LANs using coaxial cable

• IEEE 802.8: Fiber Optics• Physical Layer interfaces and MAC sublayer protocols

• IEEE 802.9 Integrated Data And Voice• Bundling ISDN and Ethernet onto a single cable

• IEEE 802.10: Interoperable LAN Security• Security functions that could be used in LANs and MANs

• IEEE 802.12: Demand Priority Access Method• Effort to combine benefits of Ethernet and Token Ring

• IEEE 802.13: The Group That Shall Not Be Named• Triskaidekaphobia

• IEEE 802.14: Cable Modems• Cable operators ultimately set up their own standard


IEEE 802.11:WIRELESS LANS

Chapter 5CS 447

This “Wi-Fi” protocol divide the world into three regions for the purpose of frequency allocation.Region 1 (includes Europe and Africa)

• Digital European Cordless Telecommunications• High Performance European Radio LAN• Groupe Speciale Mobile

Region 2 (includes United States)• FCC-governed Spread Spectrum• Personal Communication Services

Region 3 (includes Japan and Australia)

The standard includes specs on:• MAC protocol (no collision detection)• Encryption algorithm• Minimal battery capabilities• Vendor licensing

IEEE 802.15:WIRELESS PANS

Chapter 5CS 447

Personal area networks include Bluetooth’s

“piconets”, small localized networks of devices that

communicate with each other by perpetually hopping

between frequencies in a manner that prevents mutual

interference as well as external eavesdropping.

By sharing “slave” devices across piconets, “master” devices can form larger

“scatternets”.

IEEE 802.16:BROADBAND WIRELESS ACCESS

Chapter 5CS 447

WirelessHUMAN (Wireless High-Speed Unlicensed Metropolitan Network) Systems

Local Multipoint Distribution

System

With new high-speed wireless techniques being developed, it’s possible to advance from the small wireless LAN system to a more sophisticated wireless MAN system, potentially supplying powerful, upgradeable communications systems to residential and industry

customers.

IEEE 802.17:RESILIENT PACKET RING

Chapter 5CS 447

A ring-based protocol with prioritization and

fairness built in, it eliminates the token used in FDDI and the

need for master nodes of DQDB.

• TDM channels are established dynamically.

• Channel bandwidths are allowed to change dynamically.

• Multicasting is built in to the protocol.

• Simplex channels are used, supporting asymmetric communication and high bandwidth utilization simultaneously.

AND THE REST...

Chapter 5CS 447

IEEE 802.18: Radio Regulatory Technical Advisory Group

Monitor and actively participate in ongoing radio regulatory activities, at the national and international levels.

IEEE 802.20: Mobile Broadband Wireless Access

Develop the specification for an efficient packet-based air interface that is

optimized for the transport of IP-based services. The goal is to enable worldwide

deployment of affordable, ubiquitous, always-on and interoperable multi-vendor mobile broadband wireless

access networks that meet the needs of business and residential end user

markets.

IEEE 802.19: Coexistence Technical Advisory Group

Develop and maintain policies defining the responsibilities of 802 standards

developers to address issues of coexistence with existing standards and other standards under development.

IEEE 802.21: Media Independent Handoff

Develop and supports algorithms enabling seamless handover between networks of the same type as well as handover between different network types (e.g., cellular, mobile, packet

radio, wireless LAN, and wireless PAN).

IEEE 802.22: Wireless Regional Area Network

Develop a standard for a cognitive radio-based PHY/MAC/air interface for use by

license-exempt devices on a non-interfering basis in spectrum that is

allocated to the TV Broadcast Service.

IEEE 802.23: Emergency Service Working Group

Define a media-independent framework to provide consistent access and data that facilitate compliance to applicable

civil authority requirements for transferring data required by an

emergency services request.

VIRTUAL LANS

Chapter 5CS 447

There are occasions when it is beneficial to give a collection of endstations the characteristics of a LAN, in spite of their lack of

physical proximity.The Benefits of the VLAN Approach:• VLANs improve security by isolating groups. High-security users can be grouped into

a VLAN, possibly on the same physical segment, and no users outside that VLAN can communicate with them.

• VLANs facilitate broadcast control by allowing stations to be grouped by functionality instead of physical location.

• VLANs also facilitate network management by allowing configuration changes to take place without recabling.

VLANSwitch

VLANSwitch

Router

VLAN 1

VLAN 2

VLAN 3

VLAN 4

VLAN 1

VLAN 2

VLAN 3

VLAN 1 VLAN 2

VLAN 1VLAN 2VLAN 4

MULTIPROTOCOL LABEL SWITCHING

Chapter 5CS 447

MPLS was originally designed as a mechanism for speeding up the routing

of IP packets over ATM networks.

Advances in switching hardware have transformed the main

advantage of MPLS into its ability to support multiple service

models and to perform traffic management.

MPLS LABEL STACK

Chapter 5CS 447

Internal MPLS routers examine only the top label in a packet’s label stack, disregarding all lower level

information (ATM, PPP, Frame Relay, etc.).Label TC BS TTLLabel TC BS TTL

Label: Label lookup ID number, which is swapped as the packet progresses from router to router within the MPLS network.

Traffic Class: Signifies quality of service priority and explicit congestion notification.

Bottom-of-Stack Flag: When set, indicates the last label in the packet’s label stack.

Time-to-Live: 8-bit hop countdown before packet is discarded as undeliverable.

The label stack essentially establishes a hierarchical virtual private network (VPN) that

provides traffic isolation and differentiation without substantial overhead.

TRACING A WEB PAGE REQUEST

Chapter 5CS 447

Having examined the protocol stack from the Application Layer, through the Transport and Network Layers, and all the way down to the Data Link Layer, let’s pull it all together and trace what happens as a student attaches a laptop to the campus

network and accesses Google...

Charter network 142.45.0.0/15

Google’s network 64.233.160.0/19 64.233.169.105

web server

DNSserver

SIUE network148.63.13.0/11

browser

web page

router(runs DHCP)

STUDENT CONNECTS TO INTERNET

Chapter 5CS 447

1. The connecting laptop needs to get its own IP address, the address of a first-hop router, and the address of a DNS server: use DHCP (Dynamic Host Configuration Protocol)

DHCPUDPIP

EthPhy

DHCP

DHCPDHCPDHCP

DHCP

DHCPUDPIP

EthPhy

DHCPDHCPDHCP

DHCPDHCP

• The DHCP request is encapsulated in UDP, then in IP, and ultimately in Ethernet

• An Ethernet frame is broadcast (with destination FFFFFFFFFFFF) on the LAN, and is received at the router running the DHCP server

• The Ethernet payload is demultiplexed to IP, then to UDP, and finally to DHCP

router(runs DHCP)

STUDENT CONNECTS TO INTERNET (continued)

Chapter 5CS 447

2. The DHCP server formulates a DHCP ACK , containing the client’s IP address, the IP address of the first-hop router for client, and the name and IP address of the DNS server

DHCPUDPIP

EthPhy

DHCPDHCPDHCP

DHCP

DHCPUDPIP

EthPhy

DHCPDHCPDHCP

DHCP

DHCP

• The DHCP server encapsulates the ACK, that frame is forwarded through the LAN, back to the student’s machine, where it is demultiplexed

• The DHCP client (i.e., the student’s laptop) has now received the DHCP ACK reply, with its own IP address, the IP address of its first-hop router, and the name and address of the DNS server

RETRIEVING GOOGLE’S ADDRESS

Chapter 5CS 447

3. Before sending its HTTP request, the student’s laptop needs the IP address of www.google.com: Use DNS

DNSUDPIP

EthPhy

DNSDNSDNS

• A DNS query is created and encapsulated in UDP, IP, and Ethernet. In order to send the frame to the router, the MAC address of the router interface is needed: use ARP

• An ARP query is broadcast and received by the router, which replies with an ARP reply giving the MAC address of the router interface

• The client now knows the MAC address of its first-hop router, so it can now send a frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

DNSUDPIP

EthPhy

DNSDNS

DNSUDPIP

EthPhy

DNS

DNS server

Charter network 142.45.0.0/15

RETRIEVING GOOGLE’S ADDRESS (continued)

Chapter 5CS 447

DNS

DNS

• An IP datagram containing the DNS query is forwarded via the LAN switch from the client to its first-hop router

• The IP datagram is forwarded from the campus network into the Charter network, and then routed (via tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to the DNS server

• The datagram is demultiplexed to the DNS server, which replies to the client with the IP address of www.google.com

DNSUDPIP

EthPhy

DNSDNSDNS

DNS

64.233.169.105web server

ESTABLISHING TCP CONNECTION

Chapter 5CS 447

HTTPTCPIP

EthPhy

HTTP• To send its HTTP

request, the client first opens a TCP socket to the web server

• A TCP SYN segment (step 1 in the 3-way handshake) is inter-domain routed to the web server

SYN

SYNSYN

SYN

TCPIP

EthPhy

SYNSYN

SYN

• The TCP connection is now established

SYNACKSYNACK

SYNACK

SYNACKSYNACK

SYNACK

SYNACK

• The web server responds with a TCP SYNACK (step 2 in the 3-way handshake)

64.233.169.105web server

INVOKING THE APPLICATION

Chapter 5CS 447

HTTPTCPIP

EthPhy

HTTP • The HTTP request is sent into the TCP socket

• An IP datagram containing the HTTP request routed to Goggle’s web server

HTTPTCPIP

EthPhy

HTTPHTTPHTTP

• That web server responds with the HTTP reply (containing the Google web page)

HTTP

HTTPHTTPHTTP

HTTP

• An IP datagram containing the HTTP reply is routed back to the client

HTTPHTTP

HTTP

HTTP

HTTP

• The Google web page is displayed

chapter 5: link layer & lans

Documents

received frame

parity checkpage

parity bits

previous frame

parity checking

s odd parity

data link layer flow

data link layer protocol