local area networks. mesh networks early local networks used dedicated links between each pair of...
TRANSCRIPT
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
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
Original Thick Ethernet Wiring• 10Base5• Coaxial cable and AUI (Attachment Unit Interface)
connector• Network hardware involves two components
– network interface card– transceiver (transmitter-receiver)
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
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
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
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