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?