Data Communications

Local Area NetworkTechnology

LAN Applications (1)

Personal computer LANsLow costLimited data rate

Back end networks and storage area networksInterconnecting large systems (mainframes and

large storage devices)High data rateHigh speed interfaceDistributed accessLimited distanceLimited number of devices

LAN Applications (2)

High speed office networksApplications include desktop image processing,

andHigh capacity local storage

Backbone LANsUsed to interconnect low speed local LANsDisadvantages include:

Reliability – backbone failure can be catastrophic

Capacity – has to support a large capacityCost – can be high

LAN Architecture

Four basic components when describing a particular LAN:Protocol architectureTopologiesMedia access controlLogical Link Control

Let’s examine each of these components in more detail

Protocol Architecture

LANs involve the lower layers of OSI modelIEEE 802 reference model is standardPhysical layerMedia access control (MAC) sublayerLogical link control (LLC) sublayer

IEEE 802 v OSI

802 Layers - The Physical Layer

What functions are provided by the physical layer?Encoding/decoding of signalsPreamble generation/removalBit transmission/receptionTransmission medium and topology

802 Layers -Media Access Control Sublayer

Assembly of data into frame with address and error detection fields

Disassembly of frameAddress recognitionError detection

Govern access to transmission mediumNot found in traditional layer 2 data link control

For the same LLC, several MAC options may be available

MAC Frame Format

MAC layer receives data from LLC layerMAC layer adds control field, destination

MAC address, source MAC address, and CRC to LLC data unit

MAC layer detects errors and discards frames

(LLC optionally retransmits unsuccessful frames)

Typical Frame Format

Logical Link Control SublayerLLC provides the interface to higher levelsIt provides transmission of link level PDUs

between two stationsUnlike other link control protocols, LLC must

support multi-access, shared mediumAnd it is relieved of some link access details

by MAC layerAddressing involves specifying source and

destination LLC usersReferred to as service access points (SAP)Typically the user is a higher level protocol

LLC Services

Based on HDLCUnacknowledged connectionless service – a

datagram service - leave the error and flow control to a higher layer, such as TCP

Connection mode service – a logical connection is set up between 2 users and error and flow control are provided

Acknowledged connectionless service – a cross between the first two – datagrams are acked but no prior logical connection is set up

LAN Protocols in Context

Five Basic LAN Topologies

Bus Original, but no new installations

TreeSpecial case of bus – multiple branches

RingPretty much dead on LANs, more on MANs

Star-wired busWireless

LAN Topologies

Star-wired Bus LANsUse unshielded twisted pair wire

Minimal installation cost

Attach to a central active hubTwo links

Transmit and receive

Hub repeats incoming signal on all outgoing lines

Link lengths limited to about 100mFiber optic - up to 500m

Logical bus - with collisions

Hubs and Switches

Shared medium hubCentral hubHub retransmits incoming signal to all outgoing linesOnly one station can transmit at a timeWith a 10Mbps LAN, total capacity is 10Mbps

SwitchHub acts as switchIncoming frame switches to appropriate outgoing lineUnused lines can also be used to switch other trafficWith two pairs of lines in use, overall capacity is now

20Mbps

SwitchNo change to software or hardware of

workstations / devicesEach device has dedicated capacity!Scales wellStore and forward switch

Accept input, buffer it briefly, then output

Cut through switchTake advantage of the destination address being

at the start of the frameBegin repeating incoming frame onto output line

as soon as address recognizedMay propagate some bad frames

Hubs and Switches

Wireless LANs

MobilityFlexibilityHard to wire areasReduced cost of wireless systemsImproved performance of wireless systems

Wireless LAN Applications

LAN Extension - large buildings, hard to connect locations

Cross building interconnectionNomadic accessAd hoc networks

Example Applications

Factory floor workers can access part and process specs

College students can connect to campus net from almost any location

Medical professionals can access patient data bedside or on location

Office workers can move laptops from cubicle to meeting room to …

Retail sales handhelds, warehouse transactions, stock market uses, and many, many more

Basic Components

Backbone wired LAN that wireless workstation is going to connect to

Control module (CM) - the interface device between a wireless workstation and the wired LAN; contains either bridge or router functionality plus access logic such as CSMA, polling or token-passing; aka access point

User module (UM) - wireless workstation or device

Single Cell Wireless LAN

Multi Cell Wireless LAN

Cross Building Interconnection

Point to point wireless link between buildings

Typically connecting bridges or routersUsed where cable connection not possible

e.g. across a street

Nomadic Access

Mobile data terminale.g. laptop

Transfer of data from laptop to serverCampus or cluster of buildings

Ad Hoc Networking

Peer to peerTemporarye.g. conference

Wireless LAN Configurations

Wireless LAN RequirementsThroughput - should be reasonably highNumber of nodes - may need to support

100s of nodes throughout the LANConnection to backbone - most require a CM

type of connection to backboneService area - should typically be 100 to 300

m diameterBattery power consumption - should be low,

so MAC sublayer cannot be in constant communication with access point

Wireless LAN Requirements

Transmission robustness and security - have to avoid interference and eavesdropping

Collocated network operation - might have to support two or more wireless LANs in the same area

License free operationHandoff/roamingDynamic configuration of workstations

Wireless LAN Technology

All current wireless LAN products fall into one of the following categories:Infrared (IR) LANs - limited to a single roomSpread spectrum LANs - no FCC licensing

requiredNarrow band microwave

Protocols You Should Know

IEEE 802.11IEEE 802.11aIEEE 802.11bIEEE 802.11gHiperLAN

IEEE 802.11 LANs

Basic service set (BSS, or cell)Set of stations using same MAC protocolCompeting to access shared mediumMay be isolated or may connect to backbone

via access point (bridge)

Extended service setTwo or more BSSs connected by distributed

systemAppears as single logic LAN to LLC level

Types of 802.11 Stations

No transitionStationary or moves within direct

communication range of single BSS

BSS transitionMoves between two BSSs within a single ESS

ESS transitionFrom a BSS in one ESS to a BSS in another ESSDisruption of service likely

802.11 Physical Types

Infrared1Mbps and 2MbpsWavelength 850-950nm

Direct sequence spread spectrum2.4GHz ISM bandUp to 7 channelsEach 1Mbps or 2Mbps

Frequency hopping spread spectrum2.4GHz ISM band1Mbps or 2Mbps

802.11 MAC Layer

What kind of access protocol should a wireless network use?CSMA-type makes sense for ad-hoc networks

where there is no central authorityA centralized access protocol makes sense for

systems employing a base station / access point; especially useful for high priority or time sensitive data

802.11 MAC Layer

Protocol created : Distributed wireless foundation MAC (DWFMAC)A distributed access control protocol with an

optional centralized control built on top of that

The MAC layer is divided into two sublayers: DCF and PCFDCF uses contention-based accessPCF uses a centralized MAC algorithm

802.11 MAC Layer

Distributed coordination function (DCF)The lower sublayerCSMANo collision detection (cell may be too wide and

each workstation may not hear all other workstations)

Also includes a set of delays which essentially provides a set of priority levels

DCF Priority Scheme

If medium is idle, station waits to see if medium remains idle for a time equal to IFS (interframe space). If still idle, transmit

If medium is busy (either initially found busy or becomes busy during IFS), station continues to listen

When medium becomes idle, station delays another IFS. If still idle after IFS, station chooses a random backoff factor. When backoff counter reaches zero, transmit packet

DCF Priority Scheme

Where is priority scheme?Short IFS (SIFS) - used for all immediate

response actions, eg ACKs, CTSs, poll responses

Midlength IFS (PIFS) - used by the centralized controller in the PCF scheme when issuing polls

Long IFS (DIFS) - used as a minimum delay for ordinary async frames contending for access

802.11 MAC Layer

Point coordination function (PCF)Optional and implemented on top of DCFPolling performed by central masterPolling performed round robin fashionPolled station may respond with SIFS

IEEE 802.11b

First modification to the 802.11 standardUses the 2.4 GHz bandTransmits data up to 11 Mbps (theoretically,

in practice – more like 6 Mbps)

IEEE 802.11a

Higher speed protocolTransmissions in the 5 GHz bandUses a modulation technique called

orthogonal frequency division multiplexingCan run at several data rates, up to 54 MbpsFirst products shipped in 2002.

IEEE 802.11g

Modification on 802.11bExtends the 2.4 GHz technology to 54 MbpsProducts now appearing but no final

standard yet exists (as of 5/19/03)Since same frequency range as 802.11b,

can use same layout – just need to replace NICs and access points

HiperLAN/2

Drafted by the European Telecommunications Standards Institute

Like 802.11a, promises up to 54 Mbps data rates in the 5 GHz band

Some consider HiperLAN/2 to be technically superior to the IEEE standard

Interesting Facts

Higher power requirements to transmit the 5 GHz signals may make it difficult for laptops

802.11b interface cards dropping from $150 to $75

802.11a cards may start at around $200 and drop to $150 ??

802.11b access points $500 - $600?802.11a access points may start around

$1000 and drop to $650 - $750?

Interesting Facts

The range of a 5.4 GHz radio is less than that of a 2.4 GHz radio

Rough rule - 802.11b has a range of approx 250 - 300 feet; 802.11a will have a range of approx 90 feet

Shorter wavelength of 5.4 GHz has trouble going through walls, floors, furniture, etc.

You will need roughly 4 times as many 5 GHz access points as 2.4 GHz access points

Bridges

Ability to expand beyond single LANProvide interconnection to other LANs/WANsUse Bridge (switch) or routerBridge is simpler

Connects similar LANsIdentical protocols for physical and link layersMinimal processing

Router more general purposeInterconnect various LANs and WANs

Why Bridge?

ReliabilityPerformanceSecurityGeography

Functions of a Bridge

Read all frames transmitted on one LAN and accept those address to any station on the other LAN

Using MAC protocol for second LAN, retransmit each frame

Do the same the other way round

Bridge Operation

Bridge Design AspectsNo modification to content or format of frameNo encapsulationExact bitwise copy of frameMinimal buffering to meet peak demandContains routing and address intelligence

Must be able to tell which frames to passMay be more than one bridge to cross

May connect more than two LANsBridging is transparent to stations

Appears to all stations on multiple LANs as if they are on one single LAN

Address Learning

Can preload forwarding databaseCan be learnedWhen frame arrives at port X, it has come

form the LAN attached to port XUse the source address to update

forwarding database for port X to include that address

Timer on each entry in databaseEach time frame arrives, source address

checked against forwarding database

Frame forwarding

Maintain forwarding database for each portList station addresses reached through each port

For a frame arriving on port X:Search forwarding database to see if MAC

address is listed for any port except XIf address not found, forward to all ports except XIf address listed for port Y, check port Y for

blocking or forwarding stateBlocking prevents port from receiving or

transmitting

If not blocked, transmit frame through port Y

Bridge Protocol Architecture

IEEE 802.1DMAC level

Station address is at this level

Bridge does not need LLC layerIt is relaying MAC frames

Can pass frame over external WANCapture frameEncapsulate itForward it across linkRemove encapsulation and forward over LAN link

Connection of Two LANs

Fixed RoutingComplex large LANs (see next figure) need

alternative routesLoad balancingFault tolerance

Bridge must decide whether to forward frameBridge must decide which LAN to forward frame onRouting selected for each source-destination pair of

LANsUsually least hop routeOnly changed when topology changes, thus fixed routing

Spanning tree routing (algorithm) most common

Multiple LANs

Loop of Bridges

Spanning Tree Approach

Bridge automatically develops routing tableAutomatically updates in response to

configuration changesFrame forwardingAddress learningLoop resolution

Spanning Tree Algorithm

Address learning works for tree layouti.e. no closed loops

For any connected graph there is a spanning tree that maintains connectivity but contains no closed loops

Each bridge assigned unique identifierExchange between bridges to establish

spanning tree

Review Questions1. What are the functions of the LLC and MAC?2. What is a star-wired bus?3. Why is a 100 Mbps hub slower than a 100 Mbps

switch?4. What are the different wireless LAN applications?5. What is the difference between 802.11, 802.11a,

802.11b, and 802.11g?6. How does the DCF Priority Scheme work?7. How does a bridge compare to a hub and a switch?8. What is the function of the spanning tree algorithm?