Data Link Layer

Local Area Networks

Dr. Ramana

I.I.T Rajasthan

Dr. Ramana ( I.I.T Rajasthan ) Local Area Networks 1 / 27

Outline of the Lectures

1 IntroductionLAN TopologiesFrame Transmissions on Various Topologies

2 LAN Protocol ArchitechureLogical Link Control - IEEE 802.2Medium Access Control - MACContention based MACFrame Formats - IEEE 802.3 and Ethernet-IIToken based MACLAN DevicesSpanning Tree AlgorithmShared vs Switched Ethernets


Introduction

Introduction

LAN consists of a shared transmission medium and a set ofhardware and software devices for interfacing devices to themedium.A medium access mechanism is needed to allow an orderlyaccess to the shared medium.Size is restricted to few kilometers.Owned by a single organization.Used mainly for carrying data traffic.Data rates are higher and ranges from 100 Mbps to 10 Gbps.Key elements:

Topology - bus,ring,tree,star,Transmission medium - coax,twistedpair,optical fiber, wireless,Medium access control - round-robin, reservation, contention


Introduction

WAN

Router

Layer 3switch

Layer 3switch

Layer 2switch

10/100Mbps

10/100Mbps

Typical Premises Network Configuration

11Mbps

1Gbps

1Gbps

1Gbps

1Gbps

Laptop withwireless connection

Layer 2switch

Layer 2switch


Introduction LAN Topologies

(d) Star

Central Hub, Switch,or Repeater

LAN Topologies

(a) Bus

Terminatingresistance

Tap RepeaterFlow of data

(c) Ring

(b) Tree

Headend


Introduction Frame Transmissions on Various Topologies

A

A

C transmits frame addressed to A

Frame is not addressed to B; B ignores it

A copies frame as it goes by

A

A

Frame Transmission on a Bus LAN

B C

A B C

A B C


Introduction Frame Transmissions on Various Topologies

C

A

B A

(a) C transmits frame addressed to A

(b) Frame is not addressed to B; B ignores it

(c) A copies frame as it goes by

(d) C absorbs returning frame

Frame Transmission on a Ring LAN

C

A

B

A

C

A

B

A

C

A

B

A


LAN Protocol Architechure

Sublayers of Data Link Layer

LLC - provide an interface to higher layers and perform flow anderror contolMAC - framing, frame transmission and reception, medium accesscontrol

Physical

Data Link

Medium

Network

Transport

Session

Presentation

Application

OSI ReferenceModel

Physical

Medium AccessControl

Medium

Logical Link Control( ) ( ) ( )

UpperLayer

ProtocolsLLC ServiceAccess Point

(LSAP)

Scopeof

IEEE 802Standards

IEEE 802 Protocol Layers Compared to OSI Model

IEEE 802Reference

Model


LAN Protocol Architechure

TCP segment

IP datagram

LLC protocol data unit

MAC frame

Application data

TCPheader

IPheader

LLCheader

MACheader

MACtrailer

LAN Protocols in Context

Application Layer

TCP Layer

IP Layer

LLC Layer

MAC Layer


LAN Protocol Architechure Logical Link Control - IEEE 802.2

LLC Details - IEEE 802.2

Derived from HDLC and provides 3 types of services to the usersType 1 - Unacknowledged connectionless

Simple service, no flow and error controlsDelivery of data is not guaranteed (higher layers may take care ofthe reliability of data)Uses unnumbered frame to transfer the users data

Type 2 - Connection modelogical connection will be setup between the two usersuses Asynchronous balanced mode of operations and other modesare not supportedflow and error control are provided via S-frameswould be useful when no reliability is provided at higher layers

Type 3 - Acknowledged connectionlessno connection setup, but each transmitted frame is ackedto guard against lost frames, 1-bit sequence number is used.would be useful when each transmission to be received by multiplestations


LAN Protocol Architechure Medium Access Control - MAC

MAC - Overview

Governs access to the shared mediumInaddtion, interface to physical layer, Sending/receiving frames,Frame synchronization, and Error detectionMedium access mechanisms could be

either centralized or distributedand synchronous or asynchronous

Synchronous: FDM, synchronous TDM, but not well usedAsynchronous: Round Robin, Reservation, Contention

Centralized vs. Distributed access controlAdvantages of centralized control

Easier to provide centralized control with priorities, etc.Individual node logic is simpleAvoids problem of group coordination

DisadvantagesLess reliableMay become bottleneck and reduce efficiencyOverheads may be higher if propagation delay is high


LAN Protocol Architechure Medium Access Control - MAC

(Cont.)

Access Control MechanismsRound-Robin

Each node, in turn, is given opportunity to transmit. Either a centralcontroller polls a node to permit to go, or nodes can coordinateamong themselves. “Token” is passed. Simple but overhead maybe high if traffic is high

ReservationPartition channel so each node gets a slice of the bandwidthNode wishing to transmit makes “reservations” for time slots inadvance. Central or distributed.

Contention (Random Access)No control on who tries; If “collision” occurs, retransmission afterrandom timeout is attempted.


LAN Protocol Architechure Contention based MAC

Contention based MAC - Overview

All the nodes contend for the medium. No control is exercised todetermine whose turn it is.Simple to implment and suitable for bursty traffic in low andmoderate traffic volumes.Performance may collapse under heavy loads.Example: Aloha, Slotted Aloha, CSMA, 1 persistent, p-persistent,non-persistent CSMA, CSMA/CD, CSMA/CACSMA-Carrier Sense Multiple Access, CD - Collision Detection,CA - Collision Avoidance



Variants of CSMA

Channel Busy

CSMA Persistence and Backoff

Ready

1-Persistent: •Transmit as soon as channel goes idle •If collision, back off

Nonpersistent: •Transmit if idle •If busy, wait random time and repeat process •If collision, back off

Constant or variable delay

P-Persistent: •Transmit as soon as channel goes idle with probability P •Otherwise, delay one time slot and repeat process •If collision, back off

time



CSMA/CD

B

CSMA/CD Operation

A DC

A's transmission

C's transmission

Signal on bus

TIME t0

A's transmission

C's transmission

Signal on bus

TIME t1

A's transmission

C's transmission

Signal on bus

TIME t2

A's transmission

C's transmission

Signal on bus

TIME t3



CSMA/CD Flow chart

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

Jam channel b=CalcBackoff(); wait(b);attempts++;

No

Yes

attempts < 16

attempts == 16



Binary Exponential Backoff (BEB) Algorithm

Adaptors detect collision based on voltage differencesUp on detecting collision, all transmitting nodes send a highvoltage signal, called JAM signal and execute BEB algorithm.BEB-Algorithm for a frame under transmission, F

1 Increment number of collisions experience from frame F asC ← C + 1

2 If C < 161 Derive a random number K , which is number of timeslots, from a

contention window (cw) = {0, min(2C-1,1023)}.2 Defer the transmission of F by time T = K× slotlength, where

slotlength = 51.2µs3 Else

1 Discard frame F and reset C ← 0, proceed with the next frametrasnmission



On Slot Time

Slotlength = 51.2µs, which is minimum frame duration ortransmission time. It is equal to the transmission time of 64-byteframe at date rate 10Mbps.It is a function of datarate (10 Mbps), number of LAN segments(5), length of each segment (500 meters), delay introduced byeach repeater (3 µs)LAN Length (L) = 500 x 5 = 2500 metersRound Trip Distance = 5000 metersTotal propagation time (tprop) = 5000

2×108 = 25µsDelay added by each repeater (trep) = 3µs x 2 (Bi-Direction) x 4Repeaters = 24µsRound Trip Time (RTT) = tprop + trep = 25 + 24 = 49µs.The nearest frame size (in power of 2) which corresponds to thisminimum frame transmission time is 64 bytes. So the slottime @10 Mbps data rate is 51.2µs.


LAN Protocol Architechure Frame Formats - IEEE 802.3 and Ethernet-II

IEEE 802.3 and Ethernet-II Frame Format7 1 6 6 2 46 – 1500 4

PreambleSOF

Destination Address

Source Address

Length 802.2 Header and Data FCS

a) IEEE 802.3 frame

8 6 6 2 46 – 1500 4

PreambleDestination

Address Source

Address Type Data FCS

b) Ethernet – II frame – proposed by DIX (Digital, Intel, Xerox) consortium Note: Field sizes are given in Bytes

• Preamble: Alternating pattern of ones and zeros tells receiving stations that a frame is coming (Ethernet or IEEE 802.3). Ethernet frame includes an additional byte that is the equivalent of the Start-of-Frame field specified in the IEEE 802.3 frame.

• Start-of-Frame (SOF): The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet.

• Destination and Source Addresses: (MAC or Hardware address) Source address is always a unicast (single-node) address and destination address can be unicast, multicast (group), or broadcast (all nodes).

• Type (Ethernet): The type specifies the upper-layer protocol to receive the data after Ethernet processing is completed. • Length (IEEE 802.3): The length indicates the number of bytes of data that follows this field. Ex. IP – 0x0800, ARP - 0x0806 • Data (Ethernet): After physical-layer and link-layer processing is complete, the data contained in the frame is sent to an

upper-layer protocol, which is identified in the Type field. Although Ethernet-II does not specify any padding (in contrast to IEEE 802.3), Ethernet expects at least 46 bytes of data. While in IEEE 802.3 padding bytes are inserted to ensure at least a 64-byte frame.


LAN Protocol Architechure Frame Formats - IEEE 802.3 and Ethernet-II

LLC PDU in a Generic MAC Frame Format

MACFrame

LLCAddress FieldsI/G

I/G = Individual/GroupC/R = Command/Response

DSAP value C/R SSAP value

MACControl

DestinationMAC Address

SourceMAC Address LLC PDU CRC

LLCPDU DSAP

1 octet 1 1 or 2 variable

SSAP LLC Control Information


LAN Protocol Architechure Token based MAC

Token based MAC


LAN Protocol Architechure LAN Devices

Bridges and Switches

• • •

• • •

LAN A

LAN B

Bridge

Bridge Operation

Frames withaddresses 11 through20 are accepted andrepeated on LAN B

Frames withaddresses 1 through10 are accepted andrepeated on LAN A

Station 1 Station 2 Station 10

Station 11 Station 12 Station 20


LAN Protocol Architechure LAN Devices

(Cont.)

PhysicalPhysical

MAC

MAC-H LLC-H MAC-TUser Data

LLC-H User Data

User Data

LLC

Usert1

t3, t4, t5, t6

t2, t7

t1, t8

t2t3 t4 t5 t6

t7

t8

Physical

MAC

LLC

User

MAC

(a) Architecture

(b) Operation

Connection of Two LANs by a Bridge

PhysicalLAN LAN


LAN Protocol Architechure Spanning Tree Algorithm

Dealing with Loops - Spanning Tree Algorithm

LAN A

LAN B LAN C

LAN D LAN E LAN GLAN F

Bridge101

Bridge107

Bridge102

Bridge103

Bridge104

Bridge105

Bridge106

Station 1

Station 4 Station 5 Station 6 Station 7

Station 2 Station 3

Configuration of Bridges and LANs, with Alternate Routes


LAN Protocol Architechure Spanning Tree Algorithm

Impact of Loops

LAN Y

LAN X

Bridgea

Bridgeb

t2t1

t0 t0

Station B

Station A

Loop of Bridges


LAN Protocol Architechure Shared vs Switched Ethernets

HUB

Station Station Station Station

Station

HHUB

Two-Level Star Topology

IHUBIHUB

Two cables(twisted pair or

optical fiber)

Transmit

Receive


LAN Protocol Architechure Shared vs Switched Ethernets

Hubs vs Switches

Shared Bus - 10 Mbps

10 Mb

ps

10 Mb

ps

10 Mb

ps

10 Mb

ps

A B C D

(a) Shared medium bus

A B C D

(b) Shared medium hub

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacityup to 10 Mbps

A B C D

(c) Layer 2 switch

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacityN ¥ 10 Mbps

LAN Hubs and SwitchesDr. Ramana ( I.I.T Rajasthan ) Local Area Networks 27 / 27

Data Link Layer

Documents

frame frame transmission

local area networksdr

ab frame

byaa frame transmission

t rajasthandr

ita copies frame

shared transmission

protocol layers