Local Area Networks

Mesh Networks• Early local networks used

dedicated links between each pair of computers

• Some useful properties – hardware and frame details can be

tailored for each link

– easy to enforce security and privacy

• Poor scalability

• Many links would follow the same physical path

Shared Communication Channels

• Shared LANs invented in the 1960s• Rely on computers sharing a single medium• Computers coordinate their access

– Media Access Control (MAC)

• Low cost• But not suitable for wide area – communication

delays prevent coordination

Locality of Reference

• LANs now connect more computers than any other form of network

• The reason LANs are so popular is due to the principle of locality of reference – physical locality of reference – computers more likely to

communicate with those nearby

– temporal locality of reference – computer is more likely to communicate with the same computers repeatedly

LAN Topologies

• LANs may be categorised according to topology

Ring

Star

Bus

Advantages & Disadvantages

• Star is more robust but hub may be a bottleneck, and use up the most wiring

• Ring enables easy coordination but is sensitive to a cable cut

• Bus requires less wiring but is also sensitive to a cable cut

Example Bus Network - Ethernet• Single coaxial cable - the ether - to which computers connect

• Transmitter has exclusive use of the medium

• IEEE standard (802.3) specifies details – data rates

– maximum length and minimum separation

– electrical and physical details

– frame formats

Assignment: IEEE 802 standards

Ethernet Wiring

Original Thick Ethernet Wiring• 10Base5• Coaxial cable and AUI (Attachment Unit Interface)

connector• Network hardware involves two components

– network interface card– transceiver (transmitter-receiver)

Connection Multiplexing

• Allows multiple computers to share a transceiver and makes wiring simpler

Thin Ethernet Wiring

• 10Base2• Thinner wire, no transceiver and simple BNC

connector

Thin Ethernet Wiring

Twisted Pair Ethernet

• 10BaseT• Twisted pair cable connects all computers to a central

hub

Ethernet Wiring

Wiring Schemes

Comparing Wiring Schemes• Thicknet

– can change computers without disruption– Accessing remote transceivers can be difficult

• Thinnet – easier access– greater possibility of disconnection

• Twisted pair – cheapest– cable/connection failure only affects one machine

• Is 10BaseT Ethernet a bus or a star– physically it is a star– logically it is a bus

• Physical topology can differ from logical topology!!

Network Interface Hardware

• Network Adapter Card or Network Interface Card (NIC)– handles details of transmission independently of the CPU

– specific to a particular technology

– sometimes the NIC may need to connect to other hardware in order to connect to the network

NIC

Connectors• NICs often support

all three kinds of connections

Hardware Addressing• A LAN provides connectivity for all computers but most communications

are one-to-one– to avoid unnecessary interruptions, each NIC is assigned a unique numerical

address (48 bits) called its hardware address, physical address or MAC address

• Each frame transmitted across the network includes a destination and a

source MAC address field – the destination MAC address is decode to determine whether to accept it

– the source address field is used when a reply is needed

Other Addressing Modes

• Broadcasting– some applications require that a sender transmits a frame

to all stations on the network

– use a special reserved broadcast address as destination

– all NICs are configured to accept frames with their own address and the broadcast address

• Promiscuous Mode– NICs can often be configured to accept all packets

– useful for network analysers (sniffers).

– has security implications

Ethernet Transmission• Manchester encoding

– uses rising and falling edges to encode data

• Ethernet uses a standard frame format with 48 bits address and a 16 bit frame type field– Frame type field used by receivers to

decide follow-on processing required

• Standard frame types are defined by Ethernet standards

• Preambles consist of alternating 1’s and 0’s that are used for clock synchronization

Explicit Frame Types

Ethernet Coordination

• The computers can detect when a signal is on the Ether – carrier sense

• Can transmit only when the Ether is free – carrier sense with multiple access (CSMA), thus preventing a computer interrupting an ongoing transmission

• Collision can still occur if computers sense the media as free and proceed to transmit at the same time

• Each computer also senses the transmission to detect for a collision (if signal is garbled)

• Whole mechanism is called - carrier sense multiple access with collision detect - CSMA/CD

Collision Recovery

• Computers must wait after collision before retransmission

• Choose random delay up to specified maximum• Double the delay sets for each subsequent collision

– binary exponential backoff• More offered traffic results in more collisions,

more backing off, results in congestion and reduced throughput.

Example Bus Network - LocalTalk

• LAN technology for Apple computers• MAC protocol is CSMA/CA (collision avoidance)• Each computer first sends a small message to

reserve the bus• Another example is wireless LAN

– hidden station problem

– exposed station problem

(a)The hidden terminal problem. (b) The exposed station problem.

From the book Computer Networks by Tanenbaum

CSMA Problems in Wireless Networks

Example Ring Network – IBM Token Ring

• MAC protocol based on token passing• Computer must wait for permission before

transmitting• Computer controls the ring until finished• Data flows right round the ring

– receiver makes a copy

– transmitter checks for errors and then removes data and generates a new token

IBM Token Ring (Continued)

• Special message called token grants permission • Computer grabs token, removes it, sends one

frame, checks for errors when the frame returns then regenerates a new token into the network

Example Ring Network - FDDI

• Overcomes token ring weakness to failure, through two counter-rotating cables

Fiber Distributed Data InterconnectFiber Distributed Data Interconnect

Example Star Network - ATM

• Asynchronous Transfer Mode• Uses pairs of optical fibres to connect computers

to a central Switch

LAN Extensions

• Fiber modems – extend connection

between computer and transceiver

• Repeaters– join LAN segments

– regenerate signals without knowledge of frames

Multiple Repeaters

• Ethernet standard says no more than four repeaters between two computers

• Fiber modems can be used between repeaters for long distance extensions

• Biggest problem with repeaters is that all signals including collisions and noise will be transmitted

Bridges

• Connect two segments, but work at the frame level• Use promiscuous mode and forward all frames• Don’t forward incorrect frames (e.g. collisions and

noise)

Self-Learning Bridge

• Only forward a frame if necessary (frame filtering) – destination is on the other segment

– broadcast address is used

• Bridge learns which segment a computer is on when that computer sends a frame

• When a frame is received by the bridge – extracts source address and updates knowledge

– inspects destination address and consults the current knowledge for forwarding decisions

BridgeExample of Learning

Bridged Network Planning

• Parallelism for capacity and performance – bridges allow

communication on separate segments to occur at the same time

– computers that communicate frequently with one another should be attached to the same segment

Cycle of Bridges

• To prevent the problem of infinite loops, an algorithm call distributed spanning tree (DST) is used when a bridge first initializes

• DST enables a bridge to determine whether any other bridge is already performing forwarding operation in each LAN segment that it attaches

• If there is, some of its ports will be blocked, creating a logical tree topology on top of the physical one, thus preventing loops.

Switching• Switch – a single electronic

device that transfers frames between computers

• Whereas a hub simulates a shared medium, a switch simulated a bridged LAN with one computer per segment

• Advantage is greater data rate due to parallelism

• Hubs and switches are combined in LAN design to reduce cost