1. Local Area Networks

2. 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

3. 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

4. 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

5. LAN Topologies LANs may be categorised according to topology Star Ring
Bus

6. 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

7. 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

8. Ethernet Wiring

9. Original Thick Ethernet Wiring
10Base5
Coaxial cable and AUI (Attachment Unit Interface) connector
Network hardware involves two components
network interface card
transceiver (transmitter-receiver)

10. Connection Multiplexing
Allows multiple computers to share a transceiver and makes wiring simpler

11. Thin Ethernet Wiring 10Base2
Thinner wire, no transceiver and simple BNC connector

12. Thin Ethernet Wiring

13. Twisted Pair Ethernet 10BaseT
Twisted pair cable connects all computers to a central hub

14. Ethernet Wiring

15. Wiring Schemes

17. 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!!

18. 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

19. NIC
Connectors
NICs often support all three kinds of connections

20. 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

21. 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

22. 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

23. Explicit Frame Types

24. 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

25. 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.

26. 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

27. (a)The hidden terminal problem. (b) The exposed station problem.
CSMA Problems in Wireless Networks
(a)The hidden terminal problem. (b) The exposed station problem.
From the book Computer Networks by Tanenbaum

28. 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

29. 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

30. Example Ring Network - FDDI
Fiber Distributed Data Interconnect
Overcomes token ring weakness to failure, through two counter-rotating cables

31. Example Star Network - ATM
Asynchronous Transfer Mode
Uses pairs of optical fibres to connect computers to a central Switch

32. LAN Extensions Fiber modems Repeaters
extend connection between computer and transceiver
Repeaters
join LAN segments
regenerate signals without knowledge of frames

33. 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

34. 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)

35. 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

36. Bridge Example of Learning

37. 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

38. 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.

39. 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