don montgomery, cse department, southern methodist university – ecs 4344ch2, slide 1 an example...

Don Montgomery, CSE Department, Southern Methodist University – ECS 4344 Ch2, slide 1

an example routing illustration

– using tracert or traceroute– xtraceroute is a graphical version of tracert


layer 1: the physical layer

• node: router or host

• host: general purpose computer

• router: general purpose or custom hardware

• physical layer connection through network adaptor

• network adaptor: NIC (network interface card)


topic 1: transmission media

• guided (see next slide)

• unguided– laser, radio satellites


guided physical media• magnetic media (e.g. tapes & disks)

• twisted pair (e.g. UTP Cat 3, UTP Cat 5)– UTP = Unshielded Twisted Pair; Cat = Category

– (Cat 5 has more twists per cm than Cat 3)

• coaxial cable (e.g. cable TV)– thick

– thin

• optical fiber (e.g. submarine and backbone links)– single mode

– multi mode

• wireless (RF, microwave, infrared)


coaxial cable (“coax”)


principles of fiber optics

(a) light rays from inside a silica fiber impinging on the air/silica boundary at different angles

(b) light trapped by total internal reflection


fiber optic cables

(a) side view of a single fiber

(b) end view of a sheath with three fibers


two types of fiber optic cables

1. single-mode fiber (SMF): core ~8--12mic

2. multi-mode fiber (MMF): core ~50mic


intermission


wired media capacities

cable typical b/w distances

category 5 twisted pair 10 - 100 Mbps 100 mthin-net coax 10 - 100 Mbps 200 mthick-net coax 10 - 100 Mbps 500 mmultimode fiber 100 Mbps 2 kmsingle-mode fiber 100 - 10000 Mbps 40 km


common carrier bandwidths

service bandwidth

DS1 1.544 MbpsDS3 44.736 MbpsSTS-1 51.840 MbpsSTS-3 155.250 MbpsSTS-12 622.080 MbpsSTS-48 2.488320 GbpsSTS-192 9.953280 Gbps


signal characteristics & data rates

nature of signal in medium (air/glass/Cu):-

electromagnetic (EM) waves

speed of EM wave depends on medium

example:

speed of light in vacuum: 3 x 108 meters/sec

speed of electrical signals in copper: 2/3 x speed of light in vacuum


frequency – wavelength relationship

= c' / f

where,

= wavelength of signal

c' = speed of light (EM wave) in given medium

f = frequency of signal


the electromagnetic spectrum

Radio Infrared UVMicrowave

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

Gamma ray


data transmission

q1: what are common problems in signal propagation?

ans: 1. signal attenuation

- and, different frequencies are attenuated differently!

2. delay distortion

- Fourier components travel at different speeds!

3. noise

- thermal noise

- impulse noise

- crosstalk


data transmission

q2: how do you transmit information (bits)?

ans: by modulation

- superimpose electrical signal (corresponding to bits) onto a carrier wave; then, transmit in the medium

- modulation can be of several types (see next slide)


information theory

“the sampling theorem” Harry Nyquist, Claude Shannon, et alia

“exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth

maximum data rate of a finite bandwidth noiseless channel


data transmission: signaling

DC signaling (i.e., baseband signaling): bits encoded & transmitted as square waves (see next

slide) suitable for short links, low speeds

AC signaling (by modulation of a carrier wave) bits first encoded as square waves (see next slide);

then, use these waves to modulate carrier;

finally, transmit modulated carrier suitable for long-haul links, higher speeds


modems: electrical representation of data bits

(a) a binary signal

(b) amplitude modulation

(c) frequency modulation

(d) phase modulation


assertion:

we could use a 2 signal-level system to represent data

i.e., logic 0 = +5v and logic 1 = 0v

any foreseeable problems? yes1. is 0v = logic 0, or is it an idle transmitter?

2. DC bias on line

therefore, we need better encoding strategies

data transmission: bit encoding


data transmission: bit encoding

four types of encoding schemes:

NRZ (Non-Return to Zero) NRZI (Non-Return to Zero Inverted) Manchester 4B/5B (used in conjunction with NRZI)


bit encoding (cont'd.): fig 2.10 (textbook)

Bits

NRZ

Clock

Manchester

NRZI

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

required clock rates?


problems with NRZ?

1. “baseline wander”

- shift in average (source or perceived) signal strength

2. clock recovery problem

- sender-to-receiver synchronization is lost (see next slide)

instead, why not send clock signal over separate wire?

bit encoding


the clock sync problem(when clock cannot be recovered)


how to extract bits from received signal

separate clock cable too expensiveso, how to recover clock from data signal?

clock recovery depends on lots of transitions

“string of ones” sync problem (NRZI)

“string of zeros” sync problem (Manchester)


bit encoding using 4B/5B: table 2.4 (textbook)

4-Bit Data Symbol 5-Bit Code0000 111100001 010010010 101000011 101010100 010100101 010110110 011100111 011111000 100101001 100111010 101101011 101111100 110101101 110111110 111001111 11101

how to encode in 4B/5B ...

1. first encode data into 5-bit code using table shown to right. (5-bit code ensures that no more than 1 leading zero & no more than 2 trailing zeros)

2. then, encode 5-Bit code using NRZI


does 4B/5B solve clock-sync problem? - long string of zeros? - long string of ones?

what is baud rate? is it the same as bit rate? baud rate of NRZI? baud rate for Manchester encoding? baud rate for 4B/5B?

efficiency of encoding for above schemes? which encoding scheme is most efficient? which one is least efficient?

points to ponder ...


addendum to topic 1: some additional material on the

physical layer

the material in the following slides is not in your textbook, but has been included here

for your study


radio transmission

(a) radio waves in the VLF, LF, and MF bands follow curvature of earth

(b) in the HF band, they bounce off the ionosphere


communication satellites

I. geostationary satellites

II. medium-earth orbit satellites

III. low-earth orbit satellites

IV. satellites versus fiber


communication satellites

communication satellites and some of their properties


communication satellite basics

• launched by rockets

• satellites must maintain orbit stationarity

• satellites follow Kepler's laws of planetary motion

• ability to correct orbital drift

• old age of satellite: orbital decay and death


low-earth orbit satellites -- Iridium

(a) the Iridium satellites form six necklaces around the earth

(b) 1628 moving cells cover the earth


Globalstar

(a) relaying in space (b) relaying on the ground


“unguided” laser transmission

convection currents can interfere with laser communication systems


intermission


public switched telephone system structure of the telephone system

“POTS”

the politics of telephones “PSTN”, “telco”, “telecom”, “Baby Bells” governments own, operate, and/or regulate

the local loop: modems, ADSL and wireless “last mile”, “first mile”

trunks and multiplexing switching


structure of the telephone system

(a) fully-interconnected network“complete graph”, “clique”, “mesh” (context sensitive)

(b) centralized switch(c) two-level hierarchy

squares: “mesh” (context sensitive)


major components of the PSTN

local loops analog twisted pairs going to houses and businesses

trunks digital fiber optics connecting the switching offices

switching offices where calls are moved from one trunk to another


the local loop: modems, ADSL, and wireless


wireless local loops

Architecture of an LMDS system.


DSL: Digital Subscriber Line local loop

Bandwidth versus distance over category 3 UTP for DSL.


the politics of telephones

LATAs, LECs, and IXCs

Circles are LEC switching offices.

Each hexagon belongs to the IXC whose number is on it.


FDM: Frequency Division Multiplexing

(a) original bandwidths(b) bandwidths raised in frequency(b) multiplexed channel


WDM: Wavelength Division Multiplexing

Wavelength division multiplexing.

DWDM, CWDM, ...


TDM: Time Division Multiplexing

T1 framing


sampling for transmission over TDM

Delta modulation.

delta modulation


TDM of multiple data (voice) streams

Multiplexing T1 streams into higher carriers.

U.S. system: Synchronous Optical NETwork (SONET)


TDM framing

Two back-to-back SONET frames.

two SONET frames in sequence


standard TDM line rates

SONET and SDH multiplex rates.


circuit switching vs. packet switching


circuit, message, and packet switching


circuit vs. packet switching

previous figures taken from Computer Networks, 4/e, A.S. Tanenbaum


functions:1. packet framing, bit-stuffing, character stuffing, etc.

2. error detection, error correction

- generate and process acknowledgements

- “checksumming” of packets

3. fragmentation (and reassembly) of packets;

sequence numbering, etc.

4. flow control

5. channel access control (on broadcast/shared media)

the data link layer


relationship between packets and frames


why do we frame packets?

because:

1. data packets must be separated from one another

- inserting time gaps between packets does not work

because: network may squeeze out time gaps

2. error checking for individual frames is easy/convenient

1. framing


the four main ways to frame packets ...

1. using character counts

2. using framing characters, with character stuffing

3. using start and end flags, with bit stuffing

4. clock-based framing (on SONET)

framing methods


idea:

field in data link header specifies frame size in characters

framing method 1: using character counts

130 .. .. .. .. <checksum>

character count

a frame of 130 characters

• any foreseeable problems?


problems in using character counts

(a) without errors (b) with one bad error


idea: special characters (sentinels (delimiters)) tell receiver when frames begin or end:

framing method 2:using framing characters, with character stuffing

STX <Header> A C B E F <Trailer> ETX

STX = start of transmission, ETX = end of transmission

example frame with payload 'ACBEF' provided by network layer:

• any foreseeable problems?


what if 'STX' or 'ETX' occur accidentally in data?

solution: use stuffing character DLE!

DLE = Data Link Escape (term “escaped character” ring a bell?)

example:

suppose sender's network layer sends following data to its DL layer:

A C B ETX M STX B G

then, sender's DL layer transmits the following over the physical layer:

STX <Hdr> A C B DLE ETX M DLE STX B G <Trlr> ETX

and, receiver's DL layer destuffs & sends following data to its network layer:

STX <Hdr> A C B DLE ETX M DLE STX B G <Trlr> ETX

-what if DLE occurs in payload?

glitch in character stuffing


what if 'DLE' itself occurs accidentally in data?

solution: then stuff DLE, as shown before!

example:suppose sender's network layer sends following data to its DL layer:

A C DLE M STX Gthen, sender's DL layer transmits the following over the physical layer:

STX <Hdr> A C DLE DLE M DLE STX G <Trlr> ETX

and, receiver's DL layer destuffs & sends following to its network layer:

STX <Hdr> A C DLE DLE M DLE STX G <Trlr> ETX


any disadvantages of character-based framing?

• DLE, STX, ETX are all ASCII characters

• some hardware explicitly supports ASCII, some does not


idea: special bit patterns (“sentinels”, “flags”, “delimiters”) tell receiver when frames begin or end:

framing method 3:using framing bits (flags), with bit stuffing

01111110 101110..100010111<Trailer> 01111110<Header>

01111110 = flag indicating Start of Frame01111110 = flag indicating End of Frame

example:

resulting DL frame

but, what if framing flags occur accidentally in data?


solution: prevent this as follows ... after every five consecutive 1s, automatically stuff a 0!

example:source network layer sends following to DL layer ...

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

then, source DLL sends following over PHY layer ...

01111110 1 1 0 1 1 1 1 1 0 1 0 1 1 1 1 1 0 1 1 1 1 1 0 01111110

... sink DLL destuffs & sends following to its network layer:

01111110 1 1 0 1 1 1 1 1 0 1 0 1 1 1 1 1 0 1 1 1 1 1 0 01111110


what if error(s) occur(s) in transmission?

consider an earlier example:

01111110 1 1 0 1 1 1 1 1 0 1 0 1 1 1 1 1 0 1 1 1 1 1 0 01111110

case 1: error is in any position except framing flags

case 2: error is in framing flag itself

can receiver recover from error(s)?

what about from character stuffing errors in DLE, ETX, STX characters?


idea: in a SONET frame, the first 2 bytes contain a flag; frames are periodic; but, no bit stuffing! why? ...

framing method 4:clock-based framing (SONET)

Overhead Payload

90 columns

9 rows

STS-1 frame


errors in transmission

wave-guided digital transmission

copper

fiber

analog transmission

wireless

single-bit errors

burst errors


error control

error detection

error correction (FEC)

ACKs, NACKs

timers, timeouts

sequence numbers


• basic idea: add redundant information1. naïve method

• k redundant bits per n-bit message, for k << n• the k bits: “error detecting code”

2. parity3. 2-dimensional parity4. checksumming5. cyclic redundancy check (CRC)

error detection


error detection: 2-D parity

1011110 1

1101001 0

0101001 1

1011111 0

0110100 1

0001110 1

1111011 0

Paritybits

Paritybyte

Data

“odd parity” or “even parity”?


2-D parity

does this catch all errors? ..

how about:

1-bit errors?

2-bit errors?

3-bit errors?

4-bit errors?


error detection: checksumming

source:1. add up all the data words to find checksum2. transmit checksum along with original data

sink:1. extract checksum from frame2. compute its own checksum on received data3. computed checksum == extracted checksum?4. if comparison fails, then reject frame


checksumming

advantage: easy, fast, efficientdisadvantage: weak protection (link layer not)

implementation:

• add using 1's complement arithmeticwhy? ... because

1. easy to implement in hardware2. fast in software3. implementation is big-endian/little-endian neutral


checksumming

one's complement arithmetic

additive inverse == bitwise complement

not(x), x', x-bar

zero: 00...00; -zero: 11...11

one’s complement of x = x’

0 + x = x’; -0 + x = x’

to subtract by x, add x’


checksumming

use 4-bit numbers in 1's complement for ...

(pay particular attention to carry and overflow rules)

3 + 4

5 + (-2)

5 + (-6)

(-5) + 6

(-3) + (-4)

(-5) + (-6)


checksumming

IP packet header checksum:

1's-complement of

16b 1's complement sum of

all 16b words in header

check at receiver:

1's complement sum over all 16b words

including checksum word

if result == -0 (FF), then header is valid


error detection: CRC

how to use CRC?

Data

Data CRC bits

Compute CRC

Add other headers, framing bits; then transmit


CRC

idea:

1. think of an (n+1) bit message as an 'n' degree polynomial; call this M(x)

e.g., 10011010 = x7+x4+x3+x1

2. select suitable 'divisor' polynomial (aka generator polynomial) of degree k ( k < n)

e.g., G(x) = x3+x2+1

(here, note that k = 3)


CRC

idea (cont'd.):

3. calculate CRC bits using modulo-2 arithmetic; append these bits at the end of data bits; call all of this T(x):

Data CRC bits

T(x)

goal: make T(x) exactly divisible by G(x) before transmision; receiver then computes T(x) / G(x)

if T(x) is not exactly divisible by G(x), then received frame is bad


CRC

• how to make T(x) exactly divisible by G(x)?• here's how: N / D = (Q, R)

• therefore, (N-R) / D = (Q, 0)

1. calculate xk * M(x):

Data 0 ... 0

P(x)

M(x)k zeros


CRC

(how to make T(x) exactly divisible by G(x)? (contd.))

2. compute P(x) / G(x) using modulo-2 division

3. subtract remainder R(x) from P(x)

then, we have:

T(x) = P(x) – R(x) ... see figure below

thus, T(x) is perfectly divisible by G(x)

Data CRC

T(x)

M(x)k bits


CRC

rules of polynomial arithmetic modulo 2:

• if deg(B(x)) > deg(C(x)), then B(x) is divisible by C(x)

• if deg(B(x)) = deg(C(x)), then B(x) is divisible by C(x) once

• B(x)/C(x) = B(x) – C(x)

• B(x) – C(x) = B(x) XOR C(x)


CRC

example from text (page 99):

given:

G(x) = x3+x2+1

M(x) = x7+x4+x3+x1

what is the value of P(x)?

(i.e., what bits are actually transmitted by the sender?)


CRC


CRC• how could an error escape the CRC method?• let E(x) be the errors “added” to T(x)• so, T(x) + E(x) is the incorrectly transmitted

frame

• one-bit error?• two-bit error?• error with odd number of bits?• burst error?

1500B (12000b) Ethernet frame: 32b CRC


[needs work] Error Detection and Correction

Hamming distance (h) Error detection d H >= d+1 Hamming code

Burst error detection using hamming code Product codes

don montgomery, cse department, southern methodist university – ecs 4344ch2, slide 1 an example...

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