5. data link layer

Lszl Bszrmnyi Computer Networks Link Layer - 1

Computer Networks

5.Data Link Layer


Nodes and Links Hosts and routers are nodes Comm. channels connecting

adjacent nodes are links Wired and wireless links LANs

A 2-PDU (L-PDU) is a frame, encapsulates a datagram

Path over different link protocols over different links Ethernet WLAN Ethernet

link

data-link layer has responsibility of transferring datagram from one node to an adjacent node over a single link


Link Layer Services

Framing, link access Encapsulate datagram into frame, adding header, trailer Channel access if shared medium (MAC)

Media Access Control protocols Physical addresses used in frame headers to identify

source and destination Different from IP address!

Reliable delivery between adjacent nodes Transport layer learned a lot from the link layer (frame

numbering and retransmission if necessary) Seldom used on low bit error link (fiber, some twisted pair) Wireless links: high error rates


Link Layer Services (more) Flow Control

Sliding window protocols (as in the transport layer) Error Detection

Errors caused by signal attenuation, noise. Receiver detects presence of errors:

Signals sender for retransmission or drops frame Error Correction

Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex With half duplex, nodes at both ends of link can

transmit, but not at same time


Adapters Communicating

Link layer implemented in adapter (aka NIC) Ethernet card, PCMCI card,

802.11 card 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

the receiving node Adapter is semi-autonomous Link & physical layers

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol


Error Detection

Error detection not 100% reliable! Protocol may miss some errors, but rarely Larger EDC field yields better detection and correction

EDC= Error Detection and Correction bits (redundancy)

D = Data protected by error checking, may include header fields


Parity CheckingSingle Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0

Even (odd) parity scheme # of 1s in d+1 bits is

always even (odd) Receiver checks this Can detect 1 bit error Insufficient for usual

error-bursts


Cyclic Redundancy Check (CRC) View data bits, D, as a binary number Choose r+1 bit pattern (generator), G, leftmost bit being 1 Goal: choose r CRC bits, R, such that

(d + r) exactly divisible by G (modulo 2, no carries: +,- XOR) D*2r XOR R = n*G D*2r = n*G XOR R R = remainder(D*2r / G) Receiver knows G, divides by G If non-zero remainder: error detected! Can detect all burst errors less than r+1 bits

Widely used in practice (ATM, HDLC); 8,12,16,32 bit G-s


CRC Example

Want:D.2r XOR R = nG

equivalently:D.2r = nG XOR R

equivalently:if we divide D.2r by G, want remainder R

R = remainder[ ]D.2rG

Transmitted d+r (9) bits:101110 | 011


Bit Stuffing in general Special pattern at start and end of frame

Flag: (7EH) (contains 6 consecutive 1s) Data transparency requirement

Data field must be allowed to include flag pattern If the data contains 7EH, change it in a predefined way

Sender (hardware) Adds (stuffs) an additional 0 bit after each 5

consecutive 1s Receiver (hardware)

Sees 5 1 bits, followed by a 0: destuff the 0 bit If no hardware support: use Byte Stuffing in sw.


Bit Stuffing example

7 10

107

5

5


Point to Point Data Link Control

One sender, one receiver, one link Easier than broadcast link No Media Access Control No need for explicit MAC addressing E.g., dialup link, ISDN line

Popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control

Advanced error detection and retransmission mechanisms


PPP Design Requirements [RFC 1557]

Packet framing Encapsulation of any NL (not only IP) datagr. in DL frame Ability to demultiplex upwards

Bit transparency Must carry any bit pattern in the data field May use byte-stuffing instead of bit-stuffing

Error detection (no correction) Multiple types of links Connection aliveness

Detect, signal link failure to network layer Network layer address negotiation

Endpoints can learn/set each others network address


PPP non-requirements

No error correction/recovery No flow control Out of order delivery OK No need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!


PPP Data Frame Flag

Every frame begins and ends with a flag byte (framing) Address and Control

Currently do nothing and can be omitted Protocol

Upper layer protocol (e.g, PPP-Link Control Protocol (LCP), IP, IP Control Protocol (IPCP), AppleTalk, etc)

Default max. length for info: 1500 bytes


Multiple Access Links and Protocols Point-to-point links

E.g. PPP for dial-up access, or point-to-point link between Ethernet switch and host

Broadcast links (shared wire or medium) E.g. traditional Ethernet, or 802.11 wireless LAN (WiFi)


Multiple Access (MAC) Protocols

Single shared broadcast channel Access must be regulated

If two or more nodes transmit simultaneously: interference Only one node can send successfully at a time

Multiple access protocol Distributed algorithm that determines how nodes share

channel, i.e., determine when a certain node can transmit Communication about channel sharing must use the

same channel!


Ideal Multiple Access Protocol

Assume a broadcast channel of rate R bps The ideal requirement is

1. When 1 node transmits, it can send at rate R2. When M nodes want to transmit, each can send

at an average rate R/M (fairness)3. Fully decentralized

No special node to coordinate transmissions No synchronization of clocks, no time-slots

4. Simple


General MAC Protocol Classes

1.Channel Partitioning Divide channel into smaller pieces

(time slots, frequency or code division) Allocate piece to node for exclusive use

2.Random Access Channel not divided, allow collisions Recover from collisions

3. Taking turns Tightly coordinate shared access to avoid collisions


Channel Partitioning MAC protocols: TDMA

TDM (Time Division Multiplexing) Channel divided into N time slots, one per user;

TDMA (Time Division Multiple Access ) Access to channel in "rounds" Each station gets fixed length slot

(length = frame transmission time) in each round Unused slots go idle Example: 6-station LAN, 1,3,4 have frame, slots 2,5,6 idle


Channel Partitioning MAC protocols: 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

freq

uenc

y ba

nds


Channel Partitioning MAC protocols: CDMA

CDMA (Code Division Multiple Access) Senders may send simultaneously, coding helps to

separate different sources All users share same frequency, but each user has own

chipping sequence (i.e., code) to encode data Allows code set partitioning (if codes are orthogonal) Used mostly in wireless broadcast channels (cellular,

satellite, etc) The code needs high frequency (chipping frequency) Encoded signal = (original data) X (chipping sequence) Decoding: inner-product of encoded signal and chipping

sequence Multiple users can coexist with minimal interference


CDMA Encode/Decode

di: ith data bit cm: mth bit of the

code Zi,m : mth bit of

the encoded data bit

Makes no sense with one single sender-receiver pair


CDMA: two-sender interference

Additive signals E.g if 4 senders

send 1,1,1,-1: The received

signal is 2 Receiver gets

the sum of all channels: Z*

Each receiver can recover its channel, if the codes are chosen orthogonally


Random Access Protocols When node has packet 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:

ALOHA and slotted ALOHA CSMA, CSMA/CD (Ethernet), CSMA/CA


Slotted ALOHA

Assumptions All frames same size (L) Time is divided into

equal size slots (L/R), time to transmit 1 frame

Nodes start to transmit frames only at beginning of slots

Nodes are synchronized If 2 or more nodes

transmit in the same slot, all nodes detect collision

Operation When node obtains fresh

frame, it transmits in next slot

No collision: node can send new frame in next slot

If collision, node retransmits frame in each subsequent slot with probability p until success

Toss a coin, probability of head: p, tails: 1-p

If head: retransmit If tails: skip the slot


Slotted ALOHA

Pros Single active node can

continuously transmit at full rate of channel

Highly decentralized: only slots in nodes need to be in sync

Simple

Cons Collisions and wasting

slots Empty (idle) slots Nodes may be able to

detect collision in less than time to transmit packet

Slot evenets


Slotted Aloha efficiency

Suppose N nodes with many frames to send, each transmits in a given slot with probability p

Probability that others do not transmit: (1-p)N-1

Probability of success of one node in a slot = p(1-p)N-1

Probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes, find p that maximizes Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1 as Ngoes to infinity

Gives: 1/e = .37(e: natural logarithm)

Efficiency is the long-run fraction of successful slots when theres many nodes, each with many frames to send

At best: channelused for useful transmissions 37%of time!


Pure (unslotted) ALOHA Unslotted Aloha: simpler, no synchronization When frame first arrives

Transmit immediately Collision probability increases

Frame sent at t0 may collide with other frames sent in [t0-1,t0+1] Efficiency: half of that of slotted ALOHA: 18%


CSMA (Carrier Sense Multiple Access)

CSMA If channel sensed idle:

transmit entire frame If channel sensed busy:

defer transmission Collisions can still

occur Two nodes may not

hear each others transmission due to propagation delay

Entire transmission time wasted

spatial layout of nodes


CSMA/CD (CSMA+Collision Detection)

CSMA/CD Collisions detected fast Colliding transmissions

aborted Wasted time shorter

Collision detection: Easy in wired LANs

Compare transmitted and received signals

Difficult in wireless LANs


Taking Turns MAC protocols

Channel partitioning MAC protocols: Share channel efficiently and fairly at high load Inefficient at low (resp. bursty) load:

Delay in channel access 1/N bandwidth allocated even if only 1 active node!

Random access MAC protocols Efficient at low (resp. bursty) load

Single node can fully utilize channel High load

Collision overhead Taking turns protocols

Look for best of both worlds!


Taking Turns MAC protocols

Polling: Master node

invites slave nodes to transmit in turn

Concerns: polling overhead latency single point of failure

(master)

Token passing: Control token passed from one

node to next sequentially Token owner may send

(for a limited time) Token message Concerns:

token overhead latency token may get lost: must be

regenerated, but never duplicated


Summary of MAC protocols What do we do with a shared media? Channel Partitioning, by time, frequency or code

Time Division, Frequency Division, Code Division Random partitioning (dynamic),

ALOHA, S-ALOHA, CSMA, CSMA/CD Carrier sensing: easy in some technologies (wire), hard

in others (wireless) CSMA/CD used in Ethernet

Taking Turns polling from a central site token passing
Computer NetworksNodes and LinksLink Layer ServicesLink Layer Services (more)Adapters CommunicatingError DetectionParity CheckingCyclic Redundancy Check (CRC)CRC ExampleBit Stuffing in generalBit Stuffing examplePoint to Point Data Link ControlPPP Design Requirements [RFC 1557]PPP non-requirementsPPP Data FrameMultiple Access Links and ProtocolsMultiple Access (MAC) ProtocolsIdeal Multiple Access ProtocolGeneral MAC Protocol ClassesChannel Partitioning MAC protocols: TDMAChannel Partitioning MAC protocols: FDMAChannel Partitioning MAC protocols: CDMACDMA Encode/DecodeCDMA: two-sender interferenceRandom Access ProtocolsSlotted ALOHASlotted ALOHASlotted Aloha efficiencyPure (unslotted) ALOHACSMA (Carrier Sense Multiple Access)CSMA/CD (CSMA+Collision Detection)Taking Turns MAC protocolsTaking Turns MAC protocols Summary of MAC protocols

5. data link layer

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