1

• Hardware Building Blocks• Five Issues

– Encoding– Framing– Error Detection– Reliable delivery– Access Mediation

Direct Link Networks

2

Building Blocks

• Nodes: PC, special-purpose hardware…– hosts– switches

• Links (Transmission Media): – Guided: twisted pair, coax cable, optical fiber

– Unguided: radio, microwaves, infrared, lasers

3

Radio Infrared UVMicrowave Gamma ray

f (Hz)

FM

Coax

Satellite

TV

AM Terrestrial microwave

Fiber optics

X ray

100

104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016

102 106 108 1010 1012 1014 1016 1018 1020 1022 1024104

The Electromagnetic Spectrum

4

Properties of A Link• Frequency• Encoding

– Lower layers• modulation

– Upper layers• “high” signal• “low” signal

• Multiple-access links– How many bit streams can be encoded on it at a given time.

• Point-to-point links– Full-duplex– Half-duplex

• How can I get one?

5

Common types of cables and fibers available for local links

Cable Typical Bandwidths Distance

Category 5 twisted pair (Cat-5)

10 – 100 Mbps 100m

Thin-net Coax 10 – 100 Mbps 200m

Thick-net coax 10 – 100 Mbps 500m

Multimode fiber 100 Mbps 2km

Single-mode fiber 100-2400Mbps 40km

6

Common bandwidths available from the carriers

Services Bandwidth

DS1 1.544 Mbps

DS3 44.736 Mbps

STS-1 51.840 Mbps

STS-3 155.520Mbps

STS-N N * 51.840 Mbps

7

Common services available to connect your home

Services Bandwidth

POTS 28.8 – 56 Kbps

ISDN 64 – 128 Kbps

ADSL 16Kbps – 8.448 Mbps

VDSL 12.96 - 55.2 Mbps

CATV 20 – 40 Mbps

Last-Mile Links

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Centraloffice

Subscriberpremises

1.554– 8.448 Mbps

16– 640 Kbps

Local loop

Centraloffice

Neighborhood opticalnetwork unit

STS-N

over fiber

Subscriberpremises

VDSL at 12.96– 55.2 Mbps

over 1000– 4500 feet of copper

ADSL (asymmetric digital subscriber line)

VDSL

9

ADSL versus Cable

• Bandwidth

• The number of subscriber

• Availability

• Security

• Reliability

10

Encoding

• Signals propagate over a physical medium– modulate electromagnetic waves

– e.g., vary voltage

• Encode binary data onto signals– e.g., 0 as low signal and 1 as high signal

– 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

11

Problem: Consecutive 1s or 0s

• Long strings of 1s or 0s cause:– baseline wander

– Unable to recover clock (clock drift)

• Low signal (0) may be interpreted as no signal

12

Alternative Encodings

• Non-return to Zero Inverted (NRZI)– make a transition from current signal to encode a one;

stay at current signal to encode a zero– solves the problem of consecutive ones

• Manchester– 0 low-to-high transition– 1 high-to-low transition– transmit XOR of the NRZ encoded data and the clock– only 50% efficient (the bit rate = the half baud rate)

13

Encodings (cont)

• 4B/5B (FDDI)– every 4 bits of data encoded in a 5-bit code– 5-bit codes selected to have no more than one leading 0

and no more than two trailing 0s – thus, never get more than three consecutive 0s– resulting 5-bit codes are transmitted using NRZI – achieves 80% efficiency

14

Encodings (cont)

Bits

NRZ

Clock

Manchester

NRZI

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

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Framing

• Break sequence of bits into a frame• Typically implemented by network adaptor

– Determining where the frame begins and ends

Frames

BitsAdaptor Adaptor Node BNode A

16

Sentinel Based Approach

• Byte-Oriented Protocols– e.g. BISYNC, PPP

– problem: special character appears in the data portion

– solution: character stuffing• Preceding the ETX with a DLE (data-link-escape) character

SY

N

Header Body

8 8 8 8 168

SY

N

SO

H

ST

X

ET

X

CRC

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• Bit-Oriented Protocols – delineate frame with special pattern: 01111110– e.g., HDLC, SDLC

– problem: special pattern appears in the payload– solution: bit stuffing

• sender: insert 0 after five consecutive 1s• receiver: delete 0 that follows five consecutive 1s

Header Body

8 16 16 8

CRCBeginningsequence

Endingsequence

18

Counter-based Approaches

• Counter-based– include payload length in header– e.g., DDCMP

– problem: count field corrupted– solution: catch when CRC fails

SY

N

Header Body

8 8 4214 168

SY

N

Cla

ss CRCCount

19

Clock-based Approaches

• SONET– e.g., SONET: Synchronous Optical Network– each frame is 125-s long– STS-n (STS-1 = 51.84 Mbps)

Overhead Payload

90 columns

9 rows

STS-1Hdr STS-1Hdr STS-1Hdr

STS-3cHdr

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•Encoding in SONET– NRZ– sender: XOR of the data by the use of a well- known bit pattern (127 bits long).–Receiver: ???

21

Error Detection

• Two-Dimensional Parity• Cyclic-Redundancy Check• Internet Checksum Algorithm

22

1011110 1

1101001 0

0101001 1

1011111 0

0110100 1

0001110 1

1111011 0

Paritybits

Paritybyte

Data

Two-Dimensional Parity

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Cyclic Redundancy Check

• Add k bits of redundant data to an n-bit message– want k << n

– e.g., k = 32 and n = 12,000 (1500 bytes)

• Represent n-bit message as n-1 degree polynomial– e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1

• Let k be the degree of some divisor polynomial– e.g., C(x) = x3 + x2 + 1

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CRC (cont)

• Transmit polynomial P(x) that is evenly divisible by C(x) – shift left k bits, i.e., M(x)xk

– subtract remainder of M(x)xk / C(x) from M(x)xk

• Receiver polynomial P(x) + E(x)– E(x) = 0 implies no errors

• Divide (P(x) + E(x)) by C(x); remainder zero if:– E(x) was zero (no error), or– E(x) is exactly divisible by C(x)

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Selecting C(x)

• All single-bit errors, as long as the xk and x0 terms have non-zero coefficients.

• All double-bit errors, as long as C(x) contains a factor with at least three terms

• Any odd number of errors, as long as C(x) contains the factor (x + 1)

• Any ‘burst’ error (i.e., sequence of consecutive error bits) for which the length of the burst is less than k bits.

• Most burst errors of larger than k bits can also be detected

• See Table 2.6 on page 102 for common C(x)

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Internet Checksum Algorithm

• View message as a sequence of 16-bit integers; sum using 16-bit ones-complement arithmetic; take ones-complement of the result.

u_shortcksum(u_short *buf, int count){ register u_long sum = 0; while (count--) { sum += *buf++; if (sum & 0xFFFF0000) { /* carry occurred, so wrap around */ sum &= 0xFFFF; sum++; } } return ~(sum & 0xFFFF);}

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Acknowledgements & TimeoutsSender Receiver

Frame

ACK

Tim

eout

Tim

e

Sender Receiver

Frame

ACK

Tim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

ACKTim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

Tim

eout

Frame

ACKTim

eout

(a) (c)

(b) (d)

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Stop-and-Wait

• Problem: keeping the pipe full• Example

– 1.5Mbps link x 45ms RTT = 67.5Kb (8KB)

– 1KB frames implies 1/8th link utilization

Sender Receiver

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Sliding Window• Allow multiple outstanding (un-ACKed) frames• Upper bound on un-ACKed frames, called window

Sender Receiver

Tim

e

……

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SW: 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: LFS - LAR <= SWS

• Advance LAR when ACK arrives • Buffer up to SWS frames

SWS

LAR LFS

… …

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SW: Receiver• Maintain three state variables

– receive window size (RWS)– largest acceptable frame (LAF)– last frame received (LFR)

• Maintain invariant: LAF - LFR <= RWS

• Frame SeqNum arrives:– if LFR < SeqNum < = LAF accept– if SeqNum < = LFR or SeqNum > LAF discarded

• Send cumulative ACKs

RWS

LFR LAF

… …

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Sequence Number Space• SeqNum field is finite; sequence numbers wrap around• Sequence number space must be larger then number of

outstanding frames• SWS <= MaxSeqNum-1 is not sufficient

– suppose 3-bit SeqNum field (0..7)– SWS=RWS=7– sender transmit frames 0..6– arrive successfully, but ACKs lost– sender retransmits 0..6– receiver expecting 7, 0..5, but receives second incarnation of 0..5

• SWS < (MaxSeqNum+1)/2 is correct rule• Intuitively, SeqNum “slides” between two halves of sequence

number space

33

Concurrent Logical Channels• Multiplex 8 logical channels over a single link• Run stop-and-wait on each logical channel• Maintain three state bits per channel

– channel busy– current sequence number out– next sequence number in

• Header: 3-bit channel num, 1-bit sequence num– 4-bits total– same as sliding window protocol

• Separates reliability from order