Spring 2004EE4272

Direct Link Networks (I)

Hardware Building Blocks (nodes & links) 5 Issues before Exchanging Packets

EncodingFramingError DetectionReliable TransmissionMedial Access Control

Spring 2004EE4272

Hardware Building Blocks: Nodes Assume general-purpose (programmable) computers;

e.g., . Sometimes replaced with special- purpose hardware.

Finite memory (implies limited buffer space) Connects to network via a . Fast processor, slow memory (bottleneck of computer)

Spring 2004EE4272

Category 5 twisted pair 10-100Mbps, 100m50-ohm coax (ThinNet) 10-100Mbps, 200m75-ohm coax (ThickNet) 10-100Mbps, 500mMultimode fiber 100Mbps, 2kmSingle-mode fiber 100-2400Mbps, 40km

Service to ask for Bandwidth you getISDN 64 KbpsT1 1.544 MbpsT3 44.736 MbpsSTS-1 51.840 MbpsSTS-3 155.250 MbpsSTS-12 622.080 MbpsSTS-24 1.244160 GbpsSTS-48 2.488320 Gbps

Hardware Building Blocks: Links Sometimes you install your own

Sometimes leased from the phone company

Reading Assignment: P67-P75

Spring 2004EE4272

Encoding propagate over a physical medium

modulate electromagnetic waves e.g., vary voltage

Spring 2004EE4272

Encoding (Cont): NRZ Encode binary data onto signals

e.g., 0 as low signal and 1 as high signal known as Non-Return to zero (NRZ)

Problem of NRZ code: . Low signal (0) may be interpreted as no signal High signal (1) leads to . Unable to recover .

Bits

NRZ

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

Spring 2004EE4272

Alternatives: NRZI and Manchester Non-return to Zero Inverted (NRZI):

Make a transition from the current signal to encode a 1, and stay at the current signal to encode a 0; .

Manchester: Transmits the XOR of the encoded data and the clock; only 50%

efficient (baud rate=modulation rate).

Bits

NRZ

Clock

Manchester

NRZI

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

Spring 2004EE4272

4B/5B Encoding every 4 bits of data encoded in a 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 .

resulting 5-bit codes are transmitted using .

achieves 80% efficiency

Spring 2004EE4272

Frames

Bits

Node A Node BAdaptor Adaptor

Framing Break sequence of bits into a . Typically implemented by network .

Spring 2004EE4272

Header Body

8 16 16 8

CRCBeginningsequence

Endingsequence

Framing Approaches: determine where the frame begins & ends

Sentinel-baseddelineate frame with special pattern: 01111110e.g., HDLC, SDLC, PPP

problem: special pattern appears in the payloadsolution: .

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

if next bits are 10: end-of-frame marker if next bits are 11: error

Spring 2004EE4272

SY

N

Header Body

8 8 4214 168

SY

N

Cla

ss CRCCount

Framing Approaches (cont) Counter-based

include payload length in headere.g., DDCMP

problem: .solution: catch when CRC fails

Spring 2004EE4272

Overhead Payload

90 columns

9 rows

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

STS-3cHdr

Framing Approaches (cont) Clock-based

each frame is longe.g., SONET: Synchronous Optical NetworkSTS-n (STS-1 = )

Spring 2004EE4272

Error Detection:Two-Dimensional Parity

Spring 2004EE4272

Error Detection: Internet Checksum Algorithm

Basic Idea: Add up all the words that are transmitted and then

transmit the result of that with the data. The receiver the same way and the result with the received data.

One of the implementations: view message as a . Add these

integers together using 16-bit , and then take the ones complement of the result. That 16-bit number is the .

Spring 2004EE4272

Cyclic Redundancy Check Theoretical Foundation: a branch of mathematics

called .

Add k bits of redundant data to an n-bit . want k << n e.g., k = 32 and n = 12,000 (1500 bytes)

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

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

Spring 2004EE4272

CRC (cont) Transmit polynomial P(x) that is by

C(x) shift left k bits ( k is the degree of C(x) ), i.e., M(x).k subtract remainder of M(x).k/C(x) from M(x).k

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

Divide (P(x) + E(x)) by C(x); remainder zero if: E(x) was ,or E(x) is exactly by C(x)

Spring 2004EE4272

Selecting C(x) All single-bit errors, as long as the xk and x0 terms have .

All , as long as C(x) contains a factor with at least .

Any , as long as C(x) contains the factor (x + 1)

Any 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 2.5 on page 96 for common C(x)

Spring 2004EE4272

Reliable Transmission Recover from Corrupt Frames

;also called Forward Error Correction (FEC)

and Timeouts; also called Automatic Repeat Request (ARQ)

Sender Receiver

Tim

eout

Tim

e

Sender Receiver

Tim

eout

Tim

eout

Sender Receiver

Tim

eout

Tim

eout

Sender Receiver

Tim

eout

Tim

eout

(a) (c)

(b) (d)

Spring 2004EE4272

Stop-and-Wait

Problem: only one at a time (one frame per RTT) can not keep the pipe full

Example 1.5Mbps link x 45ms RTT = 67.5Kb (8KB) (delay X bandwidth

product) 1KB frames implies 1/8th link utilization

Sender Receiver

Spring 2004EE4272

Sliding Window

Sender Receiver

Tim

e

……

123

…

N

Basic Idea: Allow sender to transmit before

receiving an , thereby keeping the pipe full. There is an upper limit (called ) on the number of (un-ACKed) frames allowed.

Spring 2004EE4272

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

send window size (SWS) sequence # of last acknowledgment received (LAR) sequence # of last frame sent (LFS)

Maintain invariant: LFS - LAR <= SWS at all time

Advance/update LAR when ACK arrives to allow a new frame be sent

Buffer up to SWS frames for retransmission if needed

SWS

LAR LFS

… …

Spring 2004EE4272

SW: Receiver Maintain three state variables

(RWS): upper bound on the # of out-of-order frames

sequence # of (LAF) sequence # of (LFR)

Maintain invariant: LAF - LFR <= RWS

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

Mechanism of Sending cumulative . LFR = SeqNumtoAck LAF = LFR + RWS

RWS

LFR LAF

… …

Spring 2004EE4272

SeqNum field is ; sequence numbers wrap around (reuse)

Sequence number space must be than 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 when .

Sequence Number Space