Data Link Layer IS250 Spring 2010 chuang@ischool.berkeley.edu

Data Link Layer

IS250Spring 2010

chuang@ischool.berkeley.edu

2

Network Technologies

802.3 Ethernet 802.11 WiFi 802.16 WiMax DSL Cable modem Cellular (3G, 2.5G, …)

SONET, STS, OC …

LAN: local area network

MAN: metropolitan area network

WAN: wide area network

Backbone network Access network The “last mile” aka “first mile” aka “local loop”

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Link Layer Functionalities

Framing Error detection or correction Media access control

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Framing

Message transmitted over link as string of 0’s and 1’s

Sender and receiver has to agree where is the beginning and end of a message framing

A frame is a link layer message unit - The prelude and/or postlude are special characters or character sequences that help establish the beginning and end of the frame

- The header contains control information that is used by the network (e.g., network address; error detection)

- The payload contains data that is meaningful only for the sender and receiver

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Dealing with Errors

Data can be corrupted during transmission- Bits lost- Bit values changed

Frame includes additional information to help detect or correct errors- Set by sender; checked by receiver

Statistical guarantee

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Error Detection

Message M H = f(M) Transmitted T = M || H

Received R = M’ || H’

If H’ = f(M’) then no error

M

H

f

M H

M’ H’

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Error Detection Schemes

Parity- Send an additional parity bit (H) per character

- Even parity: if # of 1’s in character is odd, H = 1; else H = 0

- Odd parity: if # of 1’s in character is odd, H = 0; else H = 1

- Cannot detect even numbers of bit errors Checksum

- Treat data as sequence of integers- Compute and send arithmetic sum (H)- Handles multiple bit errors, but not all errors

Cyclic Redundancy Check (CRC)

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Checksum Examples

Checksum computed over data Checksum appended to frame

2nd bit reversed in each item, but checksum is the same

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Cyclic Redundancy Check

CRC is a mathematical function of data, computed as the remainder from a division of two binary numbers, one representing the message M, and the other a fixed divisor P.

Example: CRC-CCITT:- P(X) = x16 + x12 + x5 + 1; or- P = 10001000000100001

CRC-CCITT can detect:- all single/double bit errors- All odd-numbered bit errors- 100% of burst errors <= 16 bits- 99.997% of burst errors = 17 bits- 99.998% of burst errors >= 18 bits

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CRC Algorithm CRC can be computed and verified using binary long division

Numerical example- Use CRC scheme with P(x) = x5 + x4 + x + 1- P: 110011 (6 bits)- Message M: 11100011 (8 bits)- Divide M by P; use the remainder as the CRC (what we call ‘H’ on slide 6) [Note: remainder one bit shorter than P]

- Transmitted message T = M || H- Receiver performs CRC verification on received message T’: divide T’ by P will produce zero remainder if no error

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Numerical Example              10110110 110011 ) 1110001100000          110011            101111            110011             111000             110011               101100               110011                111110                110011                11010 = HT = M || H = 1110001111010

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Verification              10110110 110011 ) 1110001111010          110011            101111            110011             111001             110011               101010               110011                110011                110011                00000No error since remainder is 0

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Media Access Control

How do multiple, independent computers coordinate access to a shared communication medium?

Ethernet

Wi-Fi

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Local Area Networks

LAN characteristics:- High throughput- Relatively low cost

- Distance limitations

- Often rely on shared media

Different topologies

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Ethernet

Most widely deployed LAN technology

IEEE 802.3 standard Several generations

- Same frame format- Different data rates (10Mbps, 100Mbps, 1Gbps, 10Gbps)

- Different media (coax, twisted pair, fiber)

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Shared Medium

Shared medium used for all transmissions

Only one station transmit at any time

Stations take turns using medium

Media access control (MAC) policy ensures fairness

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Data Transmission in Ethernet

Only one station transmit at any time Signal propagates across entire cable All stations receive transmission CSMA/CD media access scheme

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CSMA/CD

Multiple access (MA)- Multiple computers attach to shared media

- Each uses same access algorithm Carrier Sense (CS)

- Wait until medium is idle- Begin to transmit frame

Simultaneous transmission possible

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CSMA/CD (2)

Two simultaneous transmissions- Interfere with one another- Called a collision

CSMA plus collision detection (CD)- Listen to medium during transmission- Detect whether another station’s signal interferes

- Back off from interference and try again

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Example

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Back-off after Collision

When collision occurs- Wait random time t1, 0 <= t1 <= d- Use CSMA and try again

If second collision occurs- Wait random time t2, 0 <= t2 <= 2d

Double range for each successive collision

Called exponential backoff

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Wireless Ethernet (Wi-Fi)

Uses unlicensed spectrum (ISM band)

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WLAN Media Access

Limited range- Not all stations receive all transmissions- Cannot use CSMA/CD

Example- Maximum transmission distance is d- Stations 1 and 3 do not receive each other’s transmissions

Known as the “hidden terminal” problem

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CSMA/CA

Collision avoidance (CA)- upon sensing idle channel, waits for a random backoff duration before attempting to transmit

RTS/CTS Mechanism- Handshake before data transmission- Request to Send (RTS): “X is about to send to Y”

- Clear to Send (CTS): “Y is about to receive from X”

- Data frame sent from X to Y- Collisions of control messages possible; but control messages are much shorter than data frames

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Addressing in Shared Medium

All stations on shared media receive all transmissions

Each frame contains address of intended recipient

Stations discard any frame addressed to another station

Shared media provide no confidentiality- Network analyzers can run in promiscuous mode- Designed for testing/debugging- Allows network interface to accept all packets

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Ethernet Addressing & Frame Format

Each station assigned unique 48-bit (6 byte) address- Known as Ethernet address, MAC address, or Physical address

Address assigned when network interface card (NIC) manufactured

Ethernet frame format:

/Size

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Ethernet Evolution

Original Ethernet: bus topology

Modern Ethernet: star topology

Ethernet hub: - Propagates each incoming

signal to all connections- noise and collisions also

propagated Ethernet switch:

- Operates on frames- Does not forward noise or

collisions- Understands addresses- Only forwards when necessary- Allows independent

transmission on different segments

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Layer 2 is not limited to LANs

Backbone operators (e.g., AT&T) deploy and operate long-haul copper-based or fiber-based digital circuits

SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) standards support framing, multiplexing, synchronization.

OC-192 9.953Gbps

OC-768 39.813Gbps