Chapter 13 Local Area Network Tech.

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William Stallings

Data and Computer

Communications

Chapter 13

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LAN Applications (1)

 _{Personal computer LANs}

 _{Low cost}

 _{Limited data rate}

 _{Back end networks and storage area}

networks

 _{Interconnecting large systems}

(mainframes and large storage devices)

 _{High data rate}

 _{High speed interface}  _{Distributed access}  _{Limited distance}

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LAN Applications (2)

 _{High speed office networks}  _{Desktop image processing}  _{High capacity local storage}  _{Backbone LANs}

 _{Interconnect low speed local LANs}  _Reliability

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LAN Architecture

 _{Protocol architecture}  _Topologies

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Protocol Architecture

 _{Lower layers of OSI model}  _{IEEE 802 reference model}  _Physical

 _{Logical link control (LLC)}

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802 Layers -

Physical

 _{Encoding/decoding}

 _{Preamble generation/removal}  _{Bit transmission/reception}

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802 Layers

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802 Layers

-Media Access Control

 _{Assembly of data into frame with address}

and error detection fields

 _{Disassembly of frame}  _{Address recognition}  _{Error detection}

 _{Govern access to transmission medium}

 _{Not found in traditional layer 2 data link}

control

 _{For the same LLC, several MAC options}

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Topologies

 _Tree

 _Bus

 _{Special case of tree}

 _{One trunk, no branches}  _Ring

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Bus and Tree

 _{Multipoint medium}

 _{Transmission propagates throughout medium}  _{Heard by all stations}

 _{Need to identify target station}

 _{Each station has unique address}

 _{Full duplex connection between station and}

tap

 _{Allows for transmission and reception}  _{Need to regulate transmission}

 _{To avoid collisions}  _{To avoid hogging}

 _{Data in small blocks - frames}

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Ring Topology

 _{Repeaters joined by point to point links in}

closed loop

 _{Receive data on one link and retransmit on another}  _{Links unidirectional}

 _{Stations attach to repeaters}

 _{Data in frames}

 _{Circulate past all stations}

 _{Destination recognizes address and copies frame}  _{Frame circulates back to source where it is}

removed

 _{Media access control determines when station}

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Frame

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Star Topology

 _{Each station connected directly to central}

node

 _{Usually via two point to point links}  _{Central node can broadcast}

 _{Physical star, logical bus}

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Media Access Control

 _Where

 _Central

 _{Greater control}

 _{Simple access logic at station}

 _{Avoids problems of co-ordination}  _{Single point of failure}

 _{Potential bottleneck}  _Distributed

 _How

 _Synchronous

 _{Specific capacity dedicated to connection}  _Asynchronous

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Asynchronous Systems

 _{Round robin}

 _{Good if many stations have data to transmit}

over extended period  _Reservation

 _{Good for stream traffic}  _Contention

 _{Good for bursty traffic}

 _{All stations contend for time}  _Distributed

 _{Simple to implement}

 _{Efficient under moderate load}

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MAC Frame Format

 _{MAC layer receives data from LLC layer}  _{MAC control}

 _{Destination MAC address}  _{Source MAC address}

 _LLS

 _CRC

 _{MAC layer detects errors and discards frames}  _{LLC optionally retransmits unsuccessful}

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Logical Link Control

 Transmission of link level PDUs between

two stations

 _{Must support multiaccess, shared medium}  _{Relieved of some link access details by MAC}

layer

 _{Addressing involves specifying source and}

destination LLC users

 _{Referred to as service access points}

(SAP)

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LLC Services

 _{Based on HDLC}

 _{Unacknowledged connectionless service}  _{Connection mode service}

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LLC Protocol

 _{Modeled after HDLC}

 _{Asynchronous balanced mode to support}

connection mode LLC service (type 2 operation)

 _{Unnumbered information PDUs to support}

Acknowledged connectionless service (type 1)

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Bus LANs

 _{Signal balancing}

 _{Signal must be strong enough to meet}

receiver’s minimum signal strength requirements

 _{Give adequate signal to noise ration}

 _{Not so strong that it overloads transmitter}  _{Must satisfy these for all combinations of}

sending and receiving station on bus

 _{Usual to divide network into small}

segments

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Transmission Media

 _{Twisted pair}

 _{Not practical in shared bus at higher data rates}

 _{Baseband coaxial cable}

 _{Used by Ethernet}

 _{Broadband coaxial cable}

 _{Included in 802.3 specification but no longer made}

 _{Optical fiber}

 _Expensive

 _{Difficulty with availability}  _{Not used}

 _{Few new installations}

 _{Replaced by star based twisted pair and optical}

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Baseband Coaxial Cable

 _{Uses digital signaling}

 _{Manchester or Differential Manchester}

encoding

 _{Entire frequency spectrum of cable used}  _{Single channel on cable}

 _{Bi-directional}

 _{Few kilometer range}

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10Base5

 _{Ethernet and 802.3 originally used 0.4}

inch diameter cable at 10Mbps

 _{Max cable length 500m}

 _{Distance between taps a multiple of 2.5m}  _{Ensures that reflections from taps do}

not add in phase

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10Base2

 _Cheapernet

 _{0.25 inch cable}  _{More flexible}

 _{Easier to bring to workstation}  _{Cheaper electronics}

 _{Greater attenuation}

 _{Lower noise resistance}  _{Fewer taps (30)}

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Repeaters

 _{Transmits in both directions}  _{Joins two segments of cable}  _{No buffering}

 _{No logical isolation of segments}

 _{If two stations on different segments send}

at the same time, packets will collide

 _{Only one path of segments and repeaters}

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Ring LANs

 _{Each repeater connects to two others via}

unidirectional transmission links

 _{Single closed path}

 _{Data transferred bit by bit from one repeater to}

the next

 _{Repeater regenerates and retransmits each bit}  _{Repeater performs data insertion, data}

reception, data removal

 _{Repeater acts as attachment point}

 _{Packet removed by transmitter after one trip}

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Listen State Functions

 _{Scan passing bit stream for patterns}  _{Address of attached station}

 _{Token permission to transmit}

 _{Copy incoming bit and send to attached}

station

 _{Whilst forwarding each bit}  _{Modify bit as it passes}

 _{e.g. to indicate a packet has been}

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Transmit State Functions

 _{Station has data}

 _{Repeater has permission}  _{May receive incoming bits}

 _{If ring bit length shorter than packet}  _{Pass back to station for checking}

(ACK)

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Bypass State

 _{Signals propagate past repeater with no}

delay (other than propagation delay)

 _{Partial solution to reliability problem (see}

later)

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Ring Media

 _{Twisted pair}

 _{Baseband coaxial}  _{Fiber optic}

 _{Not broadband coaxial}

 _{Would have to receive and transmit on}

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Timing Jitter

 _{Clocking included with signal}

 _{e.g. differential Manchester encoding}  _{Clock recovered by repeaters}

 _{To know when to sample signal and recover bits}  _{Use clocking for retransmission}

 _{Clock recovery deviates from midbit transmission}

randomly

 _Noise

 _{Imperfections in circuitry}

 _{Retransmission without distortion but with}

timing error

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Solving Timing Jitter Limitations

 _{Repeater uses phase locked loop}

 _{Minimize deviation from one bit to the}

next

 _{Use buffer at one or more repeaters}  _{Hold a certain number of bits}

 _{Expand and contract to keep bit length}

of ring constant

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Potential Ring Problems

 _{Break in any link disables network}  _{Repeater failure disables network}

 _{Installation of new repeater to attach new}

station requires identification of two topologically adjacent repeaters

 _{Timing jitter}

 _{Method of removing circulating packets}

required

 _{With backup in case of errors}

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Star Ring Architecture

 _{Feed all inter-repeater links to single site}

 _Concentrator

 _{Provides central access to signal on every}

link

 _{Easier to find faults}

 _{Can launch message into ring and see how}

far it gets

 _{Faulty segment can be disconnected and}

repaired later

 _{New repeater can be added easily}

 _{Bypass relay can be moved to concentrator}

 _{Can lead to long cable runs}

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Star LANs

 _{Use unshielded twisted pair wire (telephone)}  _{Minimal installation cost}

 _{May already be an installed base}

 _{All locations in building covered by existing}

installation

 _{Attach to a central active hub}  _{Two links}

 _{Transmit and receive}

 _{Hub repeats incoming signal on all outgoing}

lines

 _{Link lengths limited to about 100m}  _{Fiber optic - up to 500m}

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Hubs and Switches

 _{Shared medium hub}

 _{Central hub}

 _{Hub retransmits incoming signal to all outgoing lines}  _{Only one station can transmit at a time}

 _{With a 10Mbps LAN, total capacity is 10Mbps}

 _{Switched LAN hub}

 _{Hub acts as switch}

 _{Incoming frame switches to appropriate outgoing line}  _{Unused lines can also be used to switch other traffic}  _{With two pairs of lines in use, overall capacity is now}

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Switched Hubs

 _{No change to software or hardware of}

devices

 _{Each device has dedicated capacity}  _{Scales well}

 _{Store and forward switch}

 _{Accept input, buffer it briefly, then output}  _{Cut through switch}

 _{Take advantage of the destination address being}

at the start of the frame

 _{Begin repeating incoming frame onto output}

line as soon as address recognized

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Hubs and

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Wireless LANs

 _Mobility

 _Flexibility

 _{Hard to wire areas}

 _{Reduced cost of wireless systems}

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Wireless LAN Applications

 _{LAN Extension}

 _{Cross building interconnection}  _{Nomadic access}

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LAN Extension

 _{Buildings with large open areas}  _{Manufacturing plants}

 _Warehouses

 _{Historical buildings}  _{Small offices}

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Cross Building Interconnection

 _{Point to point wireless link between}

buildings

 _{Typically connecting bridges or routers}

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Nomadic Access

 _{Mobile data terminal}

 _{e.g. laptop}

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Ad Hoc Networking

 _{Peer to peer}

 _Temporary

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Wireless LAN Requirements

 _Throughput

 _{Number of nodes}

 _{Connection to backbone}  _{Service area}

 _{Battery power consumption}

 _{Transmission robustness and security}  _{Collocated network operation}

 _{License free operation}  _{Handoff/roaming}

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Wireless LAN Technology

 _{Infrared (IR) LANs}

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Bridges

 _{Ability to expand beyond single LAN}

 _{Provide interconnection to other LANs/WANs}

 _{Use Bridge or router}

 _{Bridge is simpler}

 _{Connects similar LANs}

 _{Identical protocols for physical and link layers}

 _{Minimal processing}

 _{Router more general purpose}

 _{Interconnect various LANs and WANs}

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Why Bridge?

 _Reliability

 _Performance  _Security

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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

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Bridge Design Aspects

 _{No modification to content or format of frame}  _{No encapsulation}

 _{Exact bitwise copy of frame}

 _{Minimal buffering to meet peak demand}  _{Contains routing and address intelligence}

 _{Must be able to tell which frames to pass}  _{May be more than one bridge to cross}

 _{May connect more than two LANs}  _{Bridging is transparent to stations}

 _{Appears to all stations on multiple LANs as if they}

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Bridge Protocol Architecture

 _{IEEE 802.1D}  _{MAC level}

 _{Station address is at this level}

 _{Bridge does not need LLC layer}  _{It is relaying MAC frames}

 _{Can pass frame over external comms system}  _{e.g. WAN link}

 _{Capture frame}  _{Encapsulate it}

 _{Forward it across link}

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Fixed Routing

 _{Complex large LANs need alternative routes}

 _{Load balancing}

 _{Fault tolerance}

 _{Bridge must decide whether to forward frame}  _{Bridge must decide which LAN to forward}

frame on

 _{Routing selected for each source-destination}

pair of LANs

 _{Done in configuration}

 _{Usually least hop route}

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Spanning Tree

 _{Bridge automatically develops routing}

table

 _{Automatically update in response to}

changes

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Frame forwarding

 _{Maintain forwarding database for each port}

 _{List 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 X

 _{If address not found, forward to all ports except}

X

 _{If address listed for port Y, check port Y for}

blocking or forwarding state

 _{Blocking prevents port from receiving or}

transmitting

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Address Learning

 Can preload forwarding database  _{Can be learned}

 _{When frame arrives at port X, it has come}

form the LAN attached to port X

 _{Use the source address to update}

forwarding database for port X to include that address

 _{Timer on each entry in database}

 _{Each time frame arrives, source address}

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Spanning Tree Algorithm

 _{Address learning works for tree layout}  _{i.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 identifier}  _{Exchange between bridges to establish}

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Required Reading

 _{Stallings chapter 13}