Datalink Layer

4/14/2008

Admin

Assignment 4

MAC examples survey: GSM/3G Wireless Cable Modem (DOCSIS)

Remaining class schedule

2

3

Recap: The Hourglass Architecture of the Internet

IP

Ethernet FDDIWireless

TCP UDP

Telnet Email FTP WWW

4

Outline

Admin. and recap Link layer overview

5

Link Layer: Introduction

Some terminology: hosts and routers are nodes (bridges and switches too)

communication channels that connect adjacent nodes along a communication path are links wired, wireless dedicated, shared

2-PDU is a frame, encapsulates datagram

“link”

6

Link layer: Context

Data-link layer has responsibility of transferring datagram from one node to another node over a link

Datagram transferred by different link protocols over different links, e.g., Ethernet on first link, frame relay on

intermediate links 802.11 on last link

transportation analogy

trip from New Haven to San Francisco taxi: home to union

station train: union station

to JFK plane: JFK to San

Francisco airport shuttle: airport to

hotel

7

Link Layer Services Framing, encapsulate datagram into frame,

adding header, trailer, e.g., multiplexing/demultiplexing ‘physical addresses’ used in frame headers to

identify src, dest • different from IP address !

Error detection: detect errors caused by signal attenuation, noise.

Error correction: receiver improves link qualify by identifying and

correcting bit error(s) without resorting to retransmission

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Link Layer Services (more)

Link access: channel access if shared medium

Flow control: pacing between adjacent sending and receiving nodes

Reliable delivery between adjacent nodes we learned how to do this already ! seldom used on low bit error link (fiber, some twisted

pair) common for wireless links: high error rates

9

Adaptors Communicating

link layer implemented in “adaptor” (aka NIC) Ethernet card,

modem, 802.11 card

adapter is semi-autonomous, implementing link & physical layers

sending side: encapsulates datagram

in a frame adds error checking bits,

rdt, flow control, etc.

receiving side looks for errors, rdt, flow

control, etc extracts datagram,

passes to receiving node

sendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

10

LAN/MAC/Physical Address

Each adapter has a unique link layer address (also called MAC address)

• used as address in datalink frames to identify the interface

• 48 bit MAC address (for most types of LANs) burned in the adapter ROM

• MAC address allocation administered by IEEE;manufacturer buys portion of MAC address space (to assure uniqueness)

11

Comparison of IP address and MAC Address

IP address is hierarchical for routing scalability

IP address needs to be globally unique (if no NAT)

IP address depends on IP network to which an interface is attached NOT portable

MAC address is flat

MAC address does not need to be globally unique, but the current assignment ensures uniqueness

MAC address is assigned to a device portable

12

Recall Earlier Routing Discussion

Starting at A, given IP datagram addressed to E:

look up net. address of E, find C

link layer sends datagram to C inside link-layer frame; the dest. address should be C’s MAC address

C’s MACaddr

A’s MACaddr

A’s IPaddr

E’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

C

Question: how to determine MAC address of C knowing C’s IP address?

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ARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL> TTL (Time To Live): time

after which address mapping will be forgotten (typically 20 min)

[yry3@cicada yry3]$ /sbin/arpAddress HWtype HWaddress Flags Mask Ifacezoo-gatew.cs.yale.edu ether AA:00:04:00:20:D4 C eth0artemis.zoo.cs.yale.edu ether 00:06:5B:3F:6E:21 C eth0lab.zoo.cs.yale.edu ether 00:B0:D0:F3:C7:A5 C eth0

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ARP Protocol

ARP is “plug-and-play”: nodes create their ARP tables without

intervention from net administrator

A broadcast protocol: A broadcasts query frame, containing queried

IP address • all machines on LAN receive ARP query

destination D receives ARP frame, replies• frame sent to A’s MAC address (unicast)

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Outline

Admin. and recap Link layer service Error detection

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Error Detection

D = Data protected by error checking, may include header fieldsED = Error Detection bits (redundancy)

• Error detection not 100% reliable!• a good error detector may miss some errors, but rarely• larger ED field generally yields better detection

‘

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Error Detection: Simple Techniques Parity Checking: add a parity bit for every d data bits so that the number of ones of each d+1 data and parity bits is even (odd is also OK, so long consistent)

Internet checksum

10111 0001 1010 1011

1010 1010 0101 0101

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Cyclic Redundancy Check: Background Widely used in practice, e.g.,

Ethernet, DOCSIS (Cable Modem), FDDI, PKZIP, WinZip, PNG

For a given data D, consider it as a polynomial D(x) consider the string of 0 and 1 as the

coefficients of a polynomial• e.g. consider string 10011 as x4+x+1

addition and subtraction are modular 2, thus the same as xor

Choose generator polynomial G(x) with r+1 bits, where r is called the degree of G(x)

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Cyclic Redundancy Check: Encode Given data G(x) and D(x), choose R(x)

with r bits, such that D(x)xr+R(x) is exactly divisible by G(x)

The bits correspond to D(x)xr+R(x) are sent to the receiver

+x

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Cyclic Redundancy Check: Decode

Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x) if non-zero remainder: error detected! if zero remainder, assumes no error

Encode:CRC(G)

DT = D(x)xr+R(x) T’

check

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CRC: Steps and an Example

Suppose the degree of G(x) is r

Append r zero to D(x), i.e. consider D(x)xr

Divide D(x)xr by G(x). Let R(x) denote the reminder

Send <D, R> to the receiver

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The Power of CRC Let T(x) denote D(x)xr+R(x), and E(x) the polynomial of the

error bits the received signal is T’(x) = T(x)+E(x)

Since T(x) is divisible by G(x), we only need to consider if E(x) is divisible by G(x)

Encode:CRC(G)

DT = D(x)xr+R(x) T’

check

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The Power of CRC

Detect a single-bit error: E(x) = xi

if G(x) contains two or more terms, E(x) is not divisible by G(x)

Detect an odd number of errors: E(x) has an odd number of terms: lemma: if E(x) has an odd number of terms, E(x) cannot

be divisible by (x+1)• suppose E(x) = (x+1)F(x), let x=1, the left hand will be 1, while

the right hand will be 0 thus if G(x) contains x+1 as a factor, E(x) will not be

divided by G(x)

Many more errors can be detected by designing the right G(x)

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Example G(x)

16 bits CRC: CRC-16: x16+x15+x2+1,

CRC-CCITT: x16+x12+x5+1 both can catch

• all single or double bit errors• all odd number of bit errors• all burst errors of length 16

or less• >99.99% of the 17 or 18 bits

burst errors

CRC-16 hardware implementationUsing shift and XOR registers

http://en.wikipedia.org/wiki/CRC-32#Implementation

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Example G(x) 32 bits CRC:

CRC32: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

used by Ethernet, FDDI, PKZIP, WinZip, and PNG GSM phones

For more details see the link below and further links it contains: http://en.wikipedia.org/wiki/Cyclic_redundancy_check

                      .

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Error Correcting Code (ECC) A lower bound:

assume the objective is to correct all single-bit errors

assume m message bits assume r check bits (thus total transmitted bits

n = m+r) since the check bits need to be able to identify

all of the (m+r+1) cases, therefore m+r+1 <= 2r

Examples suppose m = 7, what is the lower bound value

on r? how about m = 1000?

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Hamming Code (1950)

Achieve the lower bound The bits of the code word are numbered

consecutively, starting from 1 The bits that are powers of 2 (1, 2, 4, 8,

…) are check bits For the k-th bit, rewrite k as a sum of

powers of 2, e.g. 11 = 1+2+8, 29 = 1+4+8+16; let Sk denote the powers of 2 in the sum for k

For the check bit 2i, calculate its value as the parity check of those k if 2i in Sk

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Hamming Code: Example

Message is 7 bits Consider 1001000 We then need 4 check bits 1+2 1+4 2+4 1+2+4 1+8 2+8 1+2+8

1 2 3 4 5 6 7 8 9 10 11

c c m c m m m c m m m 1 0 0 1 0 0 0 0 0 1 0

If one bit is error, then the related check bits do not check, we find the flipped bit by summing the indices of all the check bits which failed the parity check

bit#

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Other Codes

Many other types of codes are commonly used in networks, e.g., Reed-Solomon codes (used in DSL, WiMax) convolutional codes tornado codes

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Outline

Admin. and recap Link layer overview Error detection and correction Media access control (MAC) protocols

overview

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Multiple Access Links and Protocols

Two types of “links”: point-to-point

e.g., a leased dedicated line, PPP for dial-up access

broadcast (shared wire or medium) traditional Ethernet 802.11 wireless LAN satellite

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Multiple Access Protocols Single shared broadcast channel

thus, if two or more simultaneous transmissions by nodes, due to interference, only one node can send successfully at a time (see CDMA later for an exception)

multiple access protocol Protocol that determines how nodes share

channel, i.e., determines when nodes can transmit Communication about channel sharing must use

channel itself !

Discussion: properties of an ideal multiple access protocol.

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Ideal Mulitple Access Protocol

Broadcast channel of rate R bps- Efficiency: when one node wants to transmit, it

can send at rate R

- Fairness: when N nodes want to transmit, each can send at average rate R/N

- Decentralized: no special node to coordinate transmissions no synchronization of clocks

- Simple

34

MAC Protocols: a Taxonomy

Goals efficient, fair, decentralized, simple

Three broad classes: channel partitioning

divide channel into smaller “pieces” (time slot, frequency, code)

Non-partitioning random access

• allow collisions “taking-turns”

• a token coordinates shared access to avoid collisions

35

Outline

Admin. and recap Link layer overview Error detection and correction Media access control (MAC) protocols

channel partitioning

36

Channel Partitioning: TDMA

TDMA: time division multiple access Access to channel in "rounds" Each station gets fixed length slot (length =

pkt trans time) in each round Unused slots go idle Example: 6-station LAN, 1,3,4 have pkt, slots

2,5,6 idle

37

Channel Partitioning: FDMA

FDMA: frequency division multiple access Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go

idle Example: 6-station LAN, 1,3,4 have pkt,

frequency bands 2,5,6 idle

frequ

ency

bands time

5

1

4

3

2

6

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TDMA and FDMA

Often combined in practice, for example, in cellular phone networks:

GSM uses 200 KHz channels divided into eight time slots. A single handset uses one slot in two channels for sending and receiving.

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1 2 3 4 5 6 7 8

935-960 MHz124 channels (200 kHz)downlink

890-915 MHz124 channels (200 kHz)uplink

frequ

ency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 µs577 µs

tail user data TrainingSguardspace S user data tail

guardspace

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA

S: indicates data or control

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Channel Partitioning: CDMA

CDMA (Code Division Multiple Access) Used mostly in wireless broadcast channels

(cellular, satellite, etc) A spread-spectrum technique

Example: Sprint , Verizon 3G802.11

History: http://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/hedy/lemarr.htm

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CDMA: Encoding

All users share same frequency, but each user m has its own unique “chipping” sequence (i.e., code) cm to encode data, i.e., code set partitioning e.g. cm = 1 1 1 -1 1 -1 -1 -1

Assume original data are represented by 1 and -1

Encoded signal = (original data) modulated by (chipping sequence) assume cm = 1 1 1 -1 1 -1 -1 -1

if data is d, send d cm, • if data d is 1, send cm

• if data d is -1 send -cm

CDMA: Encoding

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user data d(t)

chipping sequence c(t)

resultingsignal

1 -1

-1 1 1 -1 1 -1 1 -11 -1 -1 1 11

X

=

tb

tc

tb: bit periodtc: chip period

-1 1 1 -1 -1 1 -1 11 -1 1 -1 -11

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CDMA: Decoding

Inner-product (summation of bit-by-bit product) of encoded signal and chipping sequence if inner-product > 0, the data is 1; else -1

44

CDMA Encode/Decode

Code of user m cm: 1 1 1 -1 1 -1 -1 -1

- The number of bitsof each chipping sequence is M

Encode

Decode

45

CDMA: Deal with Multiple-User Interference

Two codes Ci and Cj are orthogonal, if , where we use “.” to denote inner

product, e.g.

If codes are orthogonal, multiple users can “coexist” and transmit simultaneously with minimal interference:

iiij

jj cdccd )(

0 ij cc

C1: 1 1 1 -1 1 -1 -1 -1 C2: 1 -1 1 1 1 -1 1 1-----------------------------------------C1 . C2 = 1 +(-1) + 1 + (-1) +1 + 1+ (-1)+(-1)=0

Analogy: Speak in different languages!

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CDMA: Two-Sender Interference

Code 1: 1 1 1 -1 1 -1 -1 -1Code 2: 1 -1 1 1 1 -1 1 1

Discussions

Advantages of channel partitioning

Problems of channel partitioning

47

48

Outline

Admin. and recap Link layer overview Error detection and correction Media access control (MAC) protocols

o channel partitioning random access

49

Random Access Protocols

When a node has packets to send transmit at full channel data rate R. no a priori coordination among nodes

Two or more transmitting nodes -> “collision”

Random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions) Examples of random access MAC protocols:

slotted ALOHA and pure ALOHA CSMA and CSMA/CD, CSMA/CA

Backup

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Forward Error Correction Code: Block Erasure Code

Basic idea: encode the k-bit data to n bits so that if the receiver receives any k bits, it can recover the original

x

y

52

FEC/Block: Example Implementation

Suppose data signal is x, and the encoded signal y = Gx, where G is the generator matrix; assume receives only a subset of y

The y-bits in yellow are received; other y-bits are dropped (corrupted)

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FEC: An Example using Vandermonde Matrix Suppose gij are the entries after the

identify matrix gij= ai

j-1, where ai are distinct numbers

Suppose k=3, and n=5 (typical reed-solomon k=223, n =255)

3

2

1

222

211

5

4

3

2

1

1

1

100

010

001

x

x

x

aa

aa

y

y

y

y

y

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FEC: An Example using Vandermonde Matrix

Suppose y2 and y3 are dropped, then we have y1, y4, and y5. Given the relationship (we know they are y1, y4, y5)

Since the matrix is not singular, we can recover x1, x2, and x3 from y1, y4, y5

3

2

1

222

211

5

4

3

2

1

1

1

100

010

001

x

x

x

aa

aa

y

y

y

y

y

3

2

1

222

211

5

4

1

1

1

001

x

x

x

aa

aa

y

y

y