William Stallings Data and Computer Communications Chapter 13

William Stallings Data and Computer Communications Chapter 13 Local Area Network Technology

LAN Applications (1) z Personal computer LANs y. Low cost y. Limited data rate z Back end networks and storage area networks y. Interconnecting large systems (mainframes and large storage devices) x. High data rate x. High speed interface x. Distributed access x. Limited distance x. Limited number of devices

LAN Applications (2) z High speed office networks y. Desktop image processing y. High capacity local storage z Backbone LANs y. Interconnect low speed local LANs y. Reliability y. Capacity y. Cost

LAN Architecture z Protocol architecture z Topologies z Media access control z Logical Link Control

Protocol Architecture z Lower layers of OSI model z IEEE 802 reference model z Physical z Logical link control (LLC) z Media access control (MAC)

IEEE 802 v OSI

802 Layers Physical z Encoding/decoding z Preamble generation/removal z Bit transmission/reception z Transmission medium and topology

802 Layers Logical Link Control z Interface to higher levels z Flow and error control

802 Layers Media Access Control z Assembly of data into frame with address and error detection fields z Disassembly of frame y. Address recognition y. Error detection z Govern access to transmission medium y. Not found in traditional layer 2 data link control z For the same LLC, several MAC options may be available

LAN Protocols in Context

Topologies z Tree z Bus y. Special case of tree x. One trunk, no branches z Ring z Star

LAN Topologies

Bus and Tree z Multipoint medium z Transmission propagates throughout medium z Heard by all stations y. Need to identify target station x. Each station has unique address z Full duplex connection between station and tap y. Allows for transmission and reception z Need to regulate transmission y. To avoid collisions y. To avoid hogging x. Data in small blocks - frames z Terminator absorbs frames at end of medium

Frame Transmission - Bus LAN

Ring Topology z Repeaters joined by point to point links in closed loop y. Receive data on one link and retransmit on another y. Links unidirectional y. Stations attach to repeaters z Data in frames y. Circulate past all stations y. Destination recognizes address and copies frame y. Frame circulates back to source where it is removed z Media access control determines when station can insert frame

Frame Transmission Ring LAN

Star Topology z Each station connected directly to central node y. Usually via two point to point links z Central node can broadcast y. Physical star, logical bus y. Only one station can transmit at a time z Central node can act as frame switch

Media Access Control z Where y. Central x. Greater control x. Simple access logic at station x. Avoids problems of co-ordination x. Single point of failure x. Potential bottleneck y. Distributed z How y. Synchronous x. Specific capacity dedicated to connection y. Asynchronous x. In response to demand

Asynchronous Systems z Round robin y. Good if many stations have data to transmit over extended period z Reservation y. Good for stream traffic z Contention y. Good for bursty traffic y. All stations contend for time y. Distributed y. Simple to implement y. Efficient under moderate load y. Tend to collapse under heavy load

MAC Frame Format z MAC layer receives data from LLC layer z MAC control z Destination MAC address z Source MAC address z LLS z CRC z MAC layer detects errors and discards frames z LLC optionally retransmits unsuccessful frames

Logical Link Control z Transmission of link level PDUs between two stations z Must support multiaccess, shared medium z Relieved of some link access details by MAC layer z Addressing involves specifying source and destination LLC users y. Referred to as service access points (SAP) y. Typically higher level protocol

LLC Services z Based on HDLC z Unacknowledged connectionless service z Connection mode service z Acknowledged connectionless service

LLC Protocol z Modeled after HDLC z Asynchronous balanced mode to support connection mode LLC service (type 2 operation) z Unnumbered information PDUs to support Acknowledged connectionless service (type 1) z Multiplexing using LSAPs

Typical Frame Format

Bus LANs z Signal balancing y. Signal must be strong enough to meet receiver’s minimum signal strength requirements y. Give adequate signal to noise ration y. Not so strong that it overloads transmitter y. Must satisfy these for all combinations of sending and receiving station on bus y. Usual to divide network into small segments y. Link segments with amplifies or repeaters

Transmission Media z Twisted pair y. Not practical in shared bus at higher data rates z Baseband coaxial cable y. Used by Ethernet z Broadband coaxial cable y. Included in 802. 3 specification but no longer made z Optical fiber y. Expensive y. Difficulty with availability y. Not used z Few new installations y. Replaced by star based twisted pair and optical fiber

Baseband Coaxial Cable z Uses digital signaling z Manchester or Differential Manchester encoding z Entire frequency spectrum of cable used z Single channel on cable z Bi-directional z Few kilometer range z Ethernet (basis for 802. 3) at 10 Mbps z 50 ohm cable

10 Base 5 z Ethernet and 802. 3 originally used 0. 4 inch diameter cable at 10 Mbps z Max cable length 500 m z Distance between taps a multiple of 2. 5 m y. Ensures that reflections from taps do not add in phase z Max 100 taps z 10 Base 5

10 Base 2 z Cheapernet z 0. 25 inch cable y. More flexible y. Easier to bring to workstation y. Cheaper electronics y. Greater attenuation y. Lower noise resistance y. Fewer taps (30) y. Shorter distance (185 m)

Repeaters z Transmits in both directions z Joins two segments of cable z No buffering z No logical isolation of segments z If two stations on different segments send at the same time, packets will collide z Only one path of segments and repeaters between any two stations

Baseband Configuration

Ring LANs z Each repeater connects to two others via unidirectional transmission links z Single closed path z Data transferred bit by bit from one repeater to the next z Repeater regenerates and retransmits each bit z Repeater performs data insertion, data reception, data removal z Repeater acts as attachment point z Packet removed by transmitter after one trip round ring

Ring Repeater States

Listen State Functions z Scan passing bit stream for patterns y. Address of attached station y. Token permission to transmit z Copy incoming bit and send to attached station y. Whilst forwarding each bit z Modify bit as it passes ye. g. to indicate a packet has been copied (ACK)

Transmit State Functions z Station has data z Repeater has permission z May receive incoming bits y. If ring bit length shorter than packet x. Pass back to station for checking (ACK) y. May be more than one packet on ring x. Buffer for retransmission later

Bypass State z Signals propagate past repeater with no delay (other than propagation delay) z Partial solution to reliability problem (see later) z Improved performance

Ring Media z Twisted pair z Baseband coaxial z Fiber optic z Not broadband coaxial y. Would have to receive and transmit on multiple channels, asynchronously

Timing Jitter z Clocking included with signal ye. g. differential Manchester encoding y. Clock recovered by repeaters x. To know when to sample signal and recover bits x. Use clocking for retransmission y. Clock recovery deviates from midbit transmission randomly x. Noise x. Imperfections in circuitry z Retransmission without distortion but with timing error z Cumulative effect is that bit length varies z Limits number of repeaters on ring

Solving Timing Jitter Limitations z Repeater uses phase locked loop y. Minimize deviation from one bit to the next z Use buffer at one or more repeaters y. Hold a certain number of bits y. Expand contract to keep bit length of ring constant z Significant increase in maximum ring size

Potential Ring Problems z Break in any link disables network z Repeater failure disables network z Installation of new repeater to attach new station requires identification of two topologically adjacent repeaters z Timing jitter z Method of removing circulating packets required y. With backup in case of errors z Mostly solved with star-ring architecture

Star Ring Architecture z Feed all inter-repeater links to single site y. Concentrator y. Provides central access to signal on every link y. Easier to find faults y. Can launch message into ring and see how far it gets y. Faulty segment can be disconnected and repaired later y. New repeater can be added easily y. Bypass relay can be moved to concentrator y. Can lead to long cable runs z Can connect multiple rings using bridges

Star LANs z Use unshielded twisted pair wire (telephone) y. Minimal installation cost x. May already be an installed base x. All locations in building covered by existing installation z Attach to a central active hub z Two links y. Transmit and receive z Hub repeats incoming signal on all outgoing lines z Link lengths limited to about 100 m y. Fiber optic - up to 500 m z Logical bus - with collisions

Two Level Star Topology

Hubs and Switches z Shared medium hub y. Central hub y. Hub retransmits incoming signal to all outgoing lines y. Only one station can transmit at a time y. With a 10 Mbps LAN, total capacity is 10 Mbps z Switched LAN hub y. Hub acts as switch y. Incoming frame switches to appropriate outgoing line y. Unused lines can also be used to switch other traffic y. With two pairs of lines in use, overall capacity is now 20 Mbps

Switched Hubs z No change to software or hardware of devices z Each device has dedicated capacity z Scales well z Store and forward switch y. Accept input, buffer it briefly, then output z Cut through switch y. Take advantage of the destination address being at the start of the frame y. Begin repeating incoming frame onto output line as soon as address recognized y. May propagate some bad frames

Hubs and Switches (diag)

Wireless LANs z Mobility z Flexibility z Hard to wire areas z Reduced cost of wireless systems z Improved performance of wireless systems

Wireless LAN Applications z LAN Extension z Cross building interconnection z Nomadic access z Ad hoc networks

LAN Extension z Buildings with large open areas y. Manufacturing plants y. Warehouses z Historical buildings z Small offices z May be mixed with fixed wiring system

Single Cell Wireless LAN

Multi Cell Wireless LAN

Cross Building Interconnection z Point to point wireless link between buildings z Typically connecting bridges or routers z Used where cable connection not possible ye. g. across a street

Nomadic Access z Mobile data terminal ye. g. laptop z Transfer of data from laptop to server z Campus or cluster of buildings

Ad Hoc Networking z Peer to peer z Temporary z e. g. conference

Wireless LAN Configurations

Wireless LAN Requirements z Throughput z Number of nodes z Connection to backbone z Service area z Battery power consumption z Transmission robustness and security z Collocated network operation z License free operation z Handoff/roaming z Dynamic configuration

Wireless LAN Technology z Infrared (IR) LANs z Spread spectrum LANs z Narrow band microwave

Bridges z Ability to expand beyond single LAN z Provide interconnection to other LANs/WANs z Use Bridge or router z Bridge is simpler y. Connects similar LANs y. Identical protocols for physical and link layers y. Minimal processing z Router more general purpose y. Interconnect various LANs and WANs ysee later

Why Bridge? z Reliability z Performance z Security z Geography

Functions of a Bridge z Read all frames transmitted on one LAN and accept those address to any station on the other LAN z Using MAC protocol for second LAN, retransmit each frame z Do the same the other way round

Bridge Operation

Bridge Design Aspects z No modification to content or format of frame z No encapsulation z Exact bitwise copy of frame z Minimal buffering to meet peak demand z Contains routing and address intelligence y. Must be able to tell which frames to pass y. May be more than one bridge to cross z May connect more than two LANs z Bridging is transparent to stations y. Appears to all stations on multiple LANs as if they are on one single LAN

Bridge Protocol Architecture z IEEE 802. 1 D z MAC level y. Station address is at this level z Bridge does not need LLC layer y. It is relaying MAC frames z Can pass frame over external comms system ye. g. WAN link y. Capture frame y. Encapsulate it y. Forward it across link y. Remove encapsulation and forward over LAN link

Connection of Two LANs

Fixed Routing z Complex large LANs need alternative routes y. Load balancing y. Fault tolerance z Bridge must decide whether to forward frame z Bridge must decide which LAN to forward frame on z Routing selected for each source-destination pair of LANs y. Done in configuration y. Usually least hop route y. Only changed when topology changes

Multiple LANs

Spanning Tree z Bridge automatically develops routing table z Automatically update in response to changes z Frame forwarding z Address learning z Loop resolution

Frame forwarding z Maintain forwarding database for each port y. List station addresses reached through each port z For a frame arriving on port X: y. Search forwarding database to see if MAC address is listed for any port except X y. If address not found, forward to all ports except X y. If address listed for port Y, check port Y for blocking or forwarding state x. Blocking prevents port from receiving or transmitting y. If not blocked, transmit frame through port Y

Address Learning z Can preload forwarding database z Can be learned z When frame arrives at port X, it has come form the LAN attached to port X z Use the source address to update forwarding database for port X to include that address z Timer on each entry in database z Each time frame arrives, source address checked against forwarding database

Spanning Tree Algorithm z Address learning works for tree layout yi. e. no closed loops z For any connected graph there is a spanning tree that maintains connectivity but contains no closed loops z Each bridge assigned unique identifier z Exchange between bridges to establish spanning tree

Loop of Bridges

Required Reading z Stallings chapter 13 z Loads of info on the Web