Chapter 13 Local Area Network Technology LAN Applications

Chapter 13. Local Area Network Technology • LAN Applications • LAN Architecture ØProtocol Architecture ØTopologies ØMAC • Bus LANs • Ring LANs • Star LANs • Extended LANs: Bridge 9/25/2020 Spring, 2003 EE 4272

LAN Applications • Personal computer LANs Low cost Ø Limited data rate Ø • Backend networks and storage area networks Ø Interconnecting large systems (mainframes and large storage devices) Ø Ø Ø High data rate High speed interface Distributed access Limited distance Limited number of devices • High speed office networks Desktop image processing Ø High capacity local storage Ø • Backbone LANs Interconnect low speed local LANs Ø Reliability Ø Capacity Ø Cost Ø 9/25/2020 Spring, 2003 EE 4272

LAN Architecture • • Protocol architecture Topologies Media access control (MAC) Logical Link Control 9/25/2020 Spring, 2003 EE 4272

Protocol Architecture • Higher layer protocol are independent of network architecture • Lower layers of OSI model -> IEEE 802 reference model Physical Layer Ø Logical link control (LLC) Ø Media access control (MAC) Ø • IEEE 802 vs. OSI Model 9/25/2020 Spring, 2003 EE 4272

802 Layers • Physical Layers Ø Ø Encoding/decoding Preamble generation/removal Bit transmission/reception Transmission medium and topology • Logical Link Control Ø Ø Interface to higher levels Flow and error control • Media Access Control Ø Ø Assembly of data into frame with address and error detection fields Disassembly of frame Ø Ø Ø Govern access to transmission medium Ø Ø Address recognition Error detection Not found in traditional layer 2 data link control For the same LLC, several MAC options may be available 9/25/2020 Spring, 2003 EE 4272

LAN Topologies 9/25/2020 Spring, 2003 EE 4272

Bus and Tree • Multipoint medium • Transmission propagates throughout medium • A transmission heard by all stations Ø Need to identify target station: Each station has unique address • Full duplex connection between station and tap Ø Allows for transmission and reception • Need to regulate transmission To avoid collisions Ø To avoid hogging: Data in small blocks - frames Ø • Terminator absorbs frames at end of medium: no special needs to remove frames 9/25/2020 Spring, 2003 EE 4272

Frame Transmission - Bus LAN 9/25/2020 Spring, 2003 EE 4272

Ring Topology • Repeaters joined by point-to-point links in closed loop Receive data on one link and retransmit on another Ø Links unidirectional Ø Stations attach to repeaters Ø • Data in frames Circulate past all stations Ø Destination recognizes address and copies frame Ø Frame circulates back to source where it is removed Ø • Media access control determines when station can insert frame 9/25/2020 Spring, 2003 EE 4272

Star Topology • Each station connected directly to central node Ø Usually via two point-to-point links • Central node can broadcast Physical star, logical bus Ø Only one station can transmit at a time Ø • Central node can act as frame switch 9/25/2020 Spring, 2003 EE 4272

Media Access Control • Where Ø Central Ø Greater control (e. g. priority, guaranteed capacity) Ø Simple access logic at each station Ø Avoids problems of co-ordination Ø Single point of failure Ø Potential bottleneck Ø Distributed • How Synchronous: Specific capacity dedicated to connection (STDM/FDM) Ø Asynchronous: In response to demand (dynamic/statistic) Ø 9/25/2020 Spring, 2003 EE 4272

Asynchronous Systems • Round robin Ø Good if many stations have data to transmit over extended period • Reservation: Good for stream traffic (similar to Synchronous TDM) • Contention Good for bursty traffic Ø All stations contend for time Ø Distributed Ø Simple to implement Ø Efficient under moderate load Ø Tend to collapse under heavy load Ø 9/25/2020 Spring, 2003 EE 4272

Technical Details of Bus LANs • Signal balancing Ø Signal must be strong enough to meet receiver’s minimum signal strength requirements Ø Give adequate signal-to-noise ration Ø Not too strong that it overloads transmitter Must satisfy signal balancing for all combinations of sending and receiving station on bus Ø Usual to divide network into small segments Ø Link segments with amplifies or repeaters Ø 9/25/2020 Spring, 2003 EE 4272

BUS LAN: Transmission Media • Twisted pair Ø Not practical in shared bus at higher data rates • Baseband coaxial cable: Used by Ethernet Ø Ø Ø Uses digital signaling Manchester or Differential Manchester encoding Entire frequency spectrum of cable used Bi-directional Few kilometer range, 50 ohm cable, Ethernet (basis for 802. 3) at 10 Mbps • Broadband coaxial cable (e. g. , cable TV) Ø Included in 802. 3 specification but no longer made • Optical fiber Ø Expensive, Difficulty with availability 9/25/2020 Spring, 2003 EE 4272

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 round ring 9/25/2020 Spring, 2003 EE 4272

Ring Media • • Twisted pair Baseband coaxial Fiber optic Not broadband coaxial Ø Would have to receive and transmit on multiple channels, asynchronously 9/25/2020 Spring, 2003 EE 4272

Timing Jitter • Clocking included with signal e. g. differential Manchester encoding Ø Clock recovered by repeaters Ø To know when to sample signal and recover bits Ø Use clocking for retransmission Ø Clock recovery deviates from mid-bit transmission randomly Ø Noise, imperfections in circuitry Ø • Retransmission without distortion but with timing error • Solutions: Repeater uses phase locked loop Ø Minimize deviation from one bit to the next Ø Use buffer at one or more repeaters Ø Hold a certain number of bits Ø Expand contract to keep bit length of ring constant Ø 9/25/2020 Spring, 2003 EE 4272

Potential Ring Problems • Break in any link disables network (link failure) • Repeater failure disables network (node failure) • Installation of new repeater to attach new station requires identification of two topologically adjacent repeaters • Timing jitter • Method of removing circulating packets required Ø With backup in case of errors • Mostly solved with star-ring architecture (e. g. FDDI) 9/25/2020 Spring, 2003 EE 4272

Star LANs • Use unshielded twisted pair wire (telephone) Ø Minimal installation cost May already be an installed base Ø All locations in building covered by existing installation Ø • Attach to a central active hub • Two links Ø Transmit and receive • Hub repeats incoming signal on all outgoing lines • Link lengths limited to about 100 m Ø Fiber optic - up to 500 m • Logical bus - with collisions 9/25/2020 Spring, 2003 EE 4272

Shared Medium Hubs and Switching Hubs • Shared medium hub Central hub Ø Hub retransmits incoming signal to all outgoing lines Ø Only one station can transmit at a time Ø With a 10 Mbps LAN, total capacity is 10 Mbps Ø • Switched LAN hub Hub acts as switch Ø Incoming frame switches to appropriate outgoing line Ø Unused lines can also be used to switch other traffic Ø With two pairs of lines in use, overall capacity is now 20 Mbps Ø 9/25/2020 Spring, 2003 EE 4272

Switched Hubs • • No change to software or hardware of devices Each device has dedicated capacity Scales well 2 Types Ø Store-and-forward switch Ø Accept Ø input, buffer it briefly, then output Cut-through switch Ø Take advantage of the destination address being at the start of the frame Ø Begin repeating incoming frame onto output line as soon as address recognized Ø May propagate some bad frames 9/25/2020 Spring, 2003 EE 4272

Bridges • • Ability to expand beyond single LAN Provide interconnection to other LANs/WANs Use Bridge or router Bridge is simpler Connects similar LANs Ø Identical protocols for physical and link layers Ø Minimal processing Ø • Router more general purpose Ø Interconnect various LANs and WANs • Ways of Interconnecting Networks: Repeater: By which multiple networks are interconnected at physical layer Bridge: By which multiple networks are interconnected at data link layer Router: By which multiple networks are interconnected at network layer Gateway: By which multiple networks are interconnected at higher layer 9/25/2020 Spring, 2003 EE 4272

Bridges • Advantages Reliability: partition network into self-contained units Ø Performance Ø Security Ø Geography Ø • Functions of a Bridge Read all frames transmitted on one LAN and only accept those address to any station on the other LAN Ø Using MAC protocol for second LAN, retransmit each frame Ø Do the same the other way round Ø 9/25/2020 Spring, 2003 EE 4272

Bridge Design Aspects • • • No modification to content/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 Ø May be more than one bridge to cross Ø • May connect more than two LANs • Bridging is transparent to stations Ø Appears to all stations on multiple LANs as if they are on one single LAN 9/25/2020 Spring, 2003 EE 4272

Bridge Protocol Architecture • IEEE 802. 1 D • MAC level Ø Station address is at this level • Bridge does not need LLC layer Ø It is relaying MAC frames • Can pass frame over external comms system Ø Ø Ø e. g. WAN link Capture frame Encapsulate it Forward it across link Remove encapsulation and forward over LAN link 9/25/2020 Spring, 2003 EE 4272

Fixed Routing • Complex large LANs need alternative routes Load balancing Ø Fault tolerance Ø • 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 Ø Usually least hop route Ø Only changed when topology changes Ø 9/25/2020 Spring, 2003 EE 4272

Spanning Tree • Bridge automatically develops routing table • Automatically update in response to changes • 3 Mechanisms within the algorithm Address learning Ø Frame forwarding Ø Loop resolution Ø 9/25/2020 Spring, 2003 EE 4272

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 • Each time frame arrives, source address checked against forwarding database 9/25/2020 Spring, 2003 EE 4272

S 1 S 2 S 3 S 5 S 4 S 5 LAN 1 LAN 2 B 1 port 1 Address S 1 9/25/2020 Spring, 2003 port 2 LAN 3 B 2 port 1 Port Address 1 S 1 EE 4272 port 2 Port 1

S 1 S 2 S 3 LAN 1 S 2 LAN 2 B 1 port 1 9/25/2020 Spring, 2003 S 5 S 4 port 2 LAN 3 B 2 port 1 port 2 Address Port S 1 S 3 1 2 S 1 S 3 1 1 EE 4272

S 1 S 2 S 3 S 5 S 4 LAN 1 LAN 2 B 1 port 1 Address S 1 S 3 S 4 9/25/2020 Spring, 2003 port 2 LAN 3 B 2 port 1 Port 1 2 2 EE 4272 port 2 Address Port S 1 S 3 S 4 1 1 2 S 3

S 1 S 2 S 3 S 2 S 1 LAN 2 port 1 9/25/2020 Spring, 2003 S 5 S 4 Bridge 1 port 2 LAN 3 port 1 Bridge 2 port 2 Address Port S 1 S 3 S 4 S 2 1 2 2 1 S 3 S 4 1 1 2 EE 4272

Frame forwarding • Maintain forwarding database for each port Ø 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 Ø If address not found, forward to all ports except X Ø If address listed for port Y, check port Y for blocking or forwarding state Ø Ø Blocking Ø prevents port from receiving or transmitting If not blocked, transmit frame through port Y 9/25/2020 Spring, 2003 EE 4272

Spanning Tree Algorithm • Address learning works for tree layout Ø 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 spanning tree 9/25/2020 Spring, 2003 EE 4272

Loop in Spanning Tree Algorithm of LAN Bridge A • Loop Problem Causes: B B 3 the network is managed by more than one administrators Ø to provide redundancy in case of failure Ø C B 5 D B 2 B 7 E K F B 1 G H B 6 B 4 I J • Bridges run a distributed spanning tree algorithm to remove loops select which bridges actively forward frames Ø now IEEE 802. 1 specification Ø Automatically disable certain bridges (not physically remove them) Ø 9/25/2020 Spring, 2003 EE 4272