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CSCI1412 Lecture 18

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  1. phones off(please) CSCI1412Lecture 18 Networks 4 Topologies and Protocols Dr John Cowell

  2. Overview • Network topologies • ring, star, bus and mesh topologies • combined topologies • Communication protocols • message switching • LAN • Ethernet, Token Ring • MAN • FDDI, DQDB • WAN • ATM, ISDN

  3. Network Topologies

  4. Network Topologies • Topology means physical layout of a network • issues of: • hardware • cabling, desking • software • protocol, applications • Each workstation attached to network is known as a node • Types of Networks • ring, star, bus, mesh/distributed • combined

  5. Ring Networks • Ring must be maintained at all times • network crashes if break in cable ring • Higher speeds available at cost • Can be inflexible in wiring • Addition/deletion cause problems

  6. Star Networks • Server dependent - often a mainframe • Good central control • Robust - no problems if cable/node breaks • Large amounts of cable

  7. (Common) Bus Networks • Also known as multidrop • Each node has a feed to the bus • Easy to add new nodes • Can be inflexible - limits to cable length • Coax connectors cause problems – only used with 10 MHz Ethernet

  8. Mesh Networks • Fully connected network • Used in WANs, not LANs • No competition for common lines • fast connections • Wasteful of connectivity potential

  9. Combined Topologies

  10. Combined Topologies 2 • Combinations of topologies are common • ‘dynamically grown’ networks • Common bus - backbone • LAN’s cope with most users • occasional access to mainframe is sufficient • Bridges convert between protocols

  11. Communication Protocols

  12. Protocols • Set of rules governing communication issues • a standard • IEEE define the 802 protocol standards • Different protocols exist for different geographical area topologies • LANs • Ethernet (802.3), Token Bus (802.4), Token Ring (802.5), Wireless - WiFi (802.11[a|b|g|n]) • MAN • FDDI (802.5), DQDB (802.6) • WANs • ATM, ISDN, Frame Relay

  13. Connection Strategies • Circuit switched • ‘permanent’ circuit established for call duration • Message switched • a message is sent from node to node • message is stored temporarily at intermediate nodes • problem if message is too long for buffer • security issues • Packet switched - 2 types • datagram and virtual circuit

  14. Packet Switching • Packet switched - 2 types • datagram • each packet transmitted separately (e.g. TCP/IP) • connectionless service • virtual circuit • protocols establish a circuit, used for all packets (e.g. ATM) • segments may be shared with other virtual circuits • connection oriented • There are possible routes between A and F • each packet may have new route • packet 1: A  B  Fpacket 2: A  C  E  F (could arrive last of three)packet 3: A  C  F

  15. LAN Protocols Ethernet and Token Ring

  16. Ethernet • Node connects to bus cable via a transceiver Sending Network interface card receives a frame from PC • Transceiver says when cable clear • frame is put onto cable by transceiver • transceiver listens for collision (CSMA/CD) • if collision occurs, waits random time, then tries again ReceivingTransceiver monitors all traffic • all incoming frames address field are read • if for node, bits are sent to NIC • NIC does error check • PC executes network software to move frame from NIC to memory for further processing

  17. Ethernet Cable Specifications • The 802.3 standard specifies limits • maximum backbone length • maximum number of nodes attached • common data rates Max distanceCable Type Backbone between nodes Data Rate 10 BASE 5 50 ohm coax, 500 10 Mbps ‘Thick’ 10 mm diameter 10 BASE 2 50 ohm coax, 185 10 Mbps ‘Thin’ 5 mm diameter 10 BASE-T UTP 100 10 Mbps star topology 100 BASE-T UTP 100 100 Mbps ‘Fast’ 1000 BASE-T UTP 100 1000 Mbps Gigabit Ethernet

  18. Distance limitations • 10 BASE 5 was limited to 500 metres • signals degrade in copper cable • Used repeaters (amplifiers) to join 500 meter sections together • maximum of four repeaters (between two nodes) • the limit comes from the collision detection protocol • if the transceiver hears a collision, it must still have part of frame to transmit, in order to abandon it • minimum frame size of 64 bytes • maximum frame size to prevent network hogging • Faster Ethernet reduces cable length • e.g. 100 base T limited to 100 metre cables from central switch/hub

  19. Token Ring • Used widely in industry • originally developed by IBM • time critical applications • Data rates of 1 Mbps, 4 Mbps, 16 Mbps • Nodes are connected in a ring topology • A token (special frame) is constantly circling • If network breaks, token disappears • need to restart (reboot) network • avoid this with wire centre hub • ‘star topology’, logical ring

  20. Sending Data on Token Ring • Node wishing to send frame waits for token • if token is ‘free’, it is replaced with data frame • Frame travels around ring • each node reads destination address • destination node • copies frame • resets status bits • puts frame back on ring • transmitting node • removes frame from ring • creates a new token, and puts it on ring • waits a period before next grabbing token • prevents hogging

  21. Token Ring Features • Unlike Ethernet, collisions cannot occur • a node must wait until the token is free • it is possible to calculate the maximum time that will pass before any node will be able to transmit • no wasted bandwidth from collisions • Token ring uses a sophisticated priority scheme • nodes can be assigned different priorities • only nodes with high priority can seize the token • If a sending device fails, its token may continue to circulate forever and lock the network • special monitor nodes can detect such a situation

  22. MAN Protocols FDDI and DQDB

  23. Fibre Distributed Data Interface • FDDI is essentially Token Ring over fibre • covered by the same IEEE 802.5 standard • no electromagnetic interference • better security • faster data rates • 100 Mbps around 200 Km ring with 200 nodes • Token is put back onto the ring immediately after a frame is transmitted • Capacity much greater than Token Ring • Recently a CDDI (Copper Distributed Data Interface) standard has also emerged

  24. Distributed Queue Dual Bus • Attributed to the PhD thesis of Robert Newman (Uni. Western Australia) • IEEE 802.6 is an example • High speed MAN (2 - 300 Mbps) • Range up to 34 miles. • Two parallel cables, if one breaks the signal can switch to the other bus. • Highly resistance to cable damage

  25. WAN Protocols ISDN, ATM & 10GbE

  26. Integrated Services Digital Network • Now largely replaced by broadband technologies • ISDN is digital network available over telephone • integrates voice & non-voice transmission • Basic installation uses three separate channels • 2 B channels = 64 Kbps - data channels • D channel = 16 Kbps - control channel • can also used for telemetry • remote (water/electricity) meter reading, alarm systems • the three channels share time on a cable • time-division multiplexing • Industry installation uses thirty B channels + D

  27. Asynchronous Transfer Mode • ATM is a very fast packet-switched protocol • 100 Mbps or greater, up to gigabits per second! • Uses small fixed-size packets • 53 octets (5 header, 48 payload) • very similar to DQDB • Connection oriented • every packet with same destination travels same route • virtual circuit number in header • requires dedicated routers • CRC in header allows error checking at network nodes • Protocol optimised for multimedia • Will probably be replaced by Gigabit Ethernet technologies for new implementations

  28. 10 GbE • 10 Gigabit Ethernet • 10 times as fast as Gb Ethernet • retains Ethernet frame format • Over 1.2 million ports shipped in 2008 • Different standards available • E.g. 10 GBASE-SR short range, 26m-85m. Low cost. • E.g. 10 GBASE-LR range up to 10km, but 25lm often achievable. • Fibre • LAN PHY, etc • Copper • 10GBaseT, etc • Greatest distance between hosts currently 80 km (10GBASE-ZR), but this is a de-facto standard only (not part of IEEE802) • See • http://en.wikipedia.org/wiki/10_gigabit_Ethernet • http://www.ieee802.org/3/10GBT/public/nov03/10GBASE-T_tutorial.pdf

  29. Summary • Network topologies • ring, star, bus and mesh topologies • combined topologies • Communication protocols • message switching • LAN • Ethernet, Token Ring • MAN • FDDI, DQDB • WAN • ATM, ISDN