EEE449 Computer Networks

1. EEE449 Computer Networks Local Area Network (LAN)

2. LAN • Topology • The way in which the end points attached to the network are interconnected • Common topologies are bus, tree, ring and star

3. LAN Topologies

4. LAN Topology • Bus • Use of a multipoint medium • All stations are attached directly to a linear transmission medium through a tap • Full duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus • A transmission from any station propagates the length of the medium in both directions and can be received by all other stations • At each end of the bus is a terminator which absorbs any signal and remove it from the bus

5. LAN Topology • Tree • The transmission medium is a branching cable with no closed loops • The layout begins at a point known as the headend • One or more cables start at the headend, and each of these may have branches • The branches in turn may have additional branches • A transmission from any station propagates throughout the medium and can be received by all other stations

6. LAN Topology • Ring • The network consists of a set of repeaters joined by point-to-point links in a closed-loop • Links are unidirectional • Each stations attached to the network at a repeater and transmit data onto the network through the repeater • A transmission circulates past all the other stations until it returns to the source station, where it is removed • Need medium access control

7. LAN Topology • Star • Each station is directly connected to a common central node typically via two point-to-point links

8. Transmission

9. Transmission

10. LAN interconnections • Bridges and routers interconnect LANs and connect LAN to WAN • Bridge for interconnecting LANs • Routers – interconnecting variety of LANs and WANs • Bridge • Between LANs that use identical protocols for the physical and link layers • reasons for use: reliability, performance, security, geography

11. LAN interconnect

12. LAN interconnections • Bridge • no modification to frame content or format • no encapsulation • exact bitwise copy of frame • Large buffer space for minimal buffering to meet peak demand • contains routing and address intelligence • may connect more than two LANs • bridging is transparent to stations

13. LAN interconnections • Router • Connect two networks that may or may not be similar • Employs internet protocol • Network layer (layer 3) device • More later

14. LAN interconnections • Hubs • active central element of star layout • each station connected to hub by two lines • hub acts as a repeater • limited to about 100 m • optical fiber may be used out to 500m • Can have multiple levels involving a header hub and intermediate hubs

16. LAN interconnections • Layer 2 switches • Has replaced hub in popularity particularly for high-speed LANs • aka a switching hub • Multiplying capacity of LAN • store-and-forward switch • accepts frame on input line, buffers briefly, routes to destination port • see delay between sender and receiver • better integrity • cut-through switch • use destination address at beginning of frame • switch begins repeating frame onto output line as soon asdestination address recognized • highest possible throughput • risk of propagating bad frames

17. LAN interconnections

18. LAN interconnections • Layer 2 switches • no changeto attached devices to convert bus LAN or hub LAN to switched LAN • e.g. Ethernet LANs use Ethernet MAC protocol • have dedicated capacity equal to original LAN • scales easily • additional devices attached to switch by increasing capacity of layer 2

19. LAN interconnections • Layer 3 switch • Implements packet-forwarding function of the router in hardware • packet by packet • operates like a traditional router • flow-based switch • enhances performance by identifying flows of IP packets with same source and destination • by observing ongoing traffic or using a special flow label in packet header (IPv6) • a predefined route is used for identified flows to speed up flow

20. LAN interconnections

21. LAN Protocol

22. LAN Protocol • Includes physical, MAC and LLC layers • Physical layer – • Encompasses topology and transmission medium • MAC • Control access to the medium for an orderly and efficient use of the capacity • Centralised or decentralised control • Synchronous (dedicated capacity) or asynchronous (on demand) • Synchronous access is not suitable for LAN due to unpredictable station demand

23. LAN Protocol • MAC • Asynchronous access can be based on round robin, reservation or contention • Round robin • Efficient when many stations have data to transmit over an extended period of time • Considerable overhead in passing turns when only few stations transmit

24. LAN Protocol • Reservation • Time on the medium is divided into slots • Stations can reserved future slots • Suitable for streaming • Contention • All stations contend for the time slots • Suitable for bursty traffic

25. LAN Protocol • LLC • transmission of link level PDUs between stations • must support multiaccess, shared medium • but MAC layer handles link access details • addressing involves specifying source and destination LLC users • referred to as service access points (SAP) • typically higher level protocol

26. LAN protocol

27. Bridge protocol • IEEE 802.1D • Station address is designated at the MAC level • bridge does not need LLC layer • can pass frame over external comms system • capture frame • encapsulate it • forward it across link • remove encapsulation and forward over LAN link • e.g. WAN link

28. Bridge protocol

29. High-Speed LANs • Ethernet (IEEE 802.3 10-Mbps) • Fast Ethernet (IEEE 802.3 100-Mbps) • Gigabit Ethernet • 10-Gbps Ethernet

30. Ethernet • most widely used LAN standard • developed by IEEE 802.3 • IEEE 802.3 MAC – use CSMA/CD • Station continues to listen to the medium while transmitting • If the medium is idle, transmit • Otherwise if the medium is busy, continue to listen until the channel is idle and then transmit immediately • If a collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease transmission • After transmitting the jamming signal, wait a random amount of time (Backoff) then attempt to transmit again

31. Ethernet

32. Ethernet Preamble: A 7-octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronization. Start Frame Delimiter (SFD): The sequence 10101011, which indicates the actual start of the frame and enables the receiver to locate the first bit of the rest of the frame. Destination Address (DA): Specifies the station(s) for which the frame is intended. It may be a unique physical address, a group address, or a global address. Source Address (SA): Specifies the station that sent the frame. Length/Type: Length of LLC data field in octets, or Ethernet Type field, LLC Data: Data unit supplied by LLC. Pad: Octets added to ensure that the frame is long enough for proper CD operation. Frame Check Sequence (FCS): A 32-bit cyclic redundancy check, based on all fields except preamble, SFD, and FCS.

33. Ethernet IEEE 802.3 10Mbps <data rate in Mbps> <signaling method><max segment length in hundreds of meters>

34. Ethernet IEEE 802.3 10Mbps • Alternatives for 10-Mbps are: • 10BASE5: Specifies the use of 50-ohm coaxial cable and Manchester digital signaling. The maximum length of a cable segment is set at 500 meters. Can extend using up to 4 repeaters. • 10BASE2: lower-cost alternative to 10BASE5 using a thinner cable, with fewer taps over a shorter distance than the 10BASE5 cable. • 10BASE-T: Uses unshielded twisted pair in a star-shaped topology, with length of a link is limited to 100 meters. As an alternative, an optical fiber link may be used out to 500 m. • 10BASE-F: Contains three specifications using optical fibre

35. Fast Ethernet (IEEE 802.3 100Mbps) • a low-cost, Ethernet-compatible LAN operating at 100 Mbps • All of the 100BASE-T options use the IEEE 802.3 MAC protocol and frame format.

36. Fast Ethernet (IEEE 802.3 100Mbps) • 100BASE-X refers to a set of options that use two physical links between nodes; • one for transmission and one for reception. • 100BASE-TX makes use of shielded twisted pair (STP) or • high-quality (Category 5) unshielded twisted pair (UTP). • 100BASE-FX uses optical fiber. • For all of the 100BASE-T options, the topology is similar to that of 10BASE-T, • namely a star-wire topology.

37. Gigabit Ethernet • defines a new medium and transmission specification • retains the CSMA/CD protocol and Ethernet format of its 10-Mbps and 100-Mbps predecessors. • compatible with 100BASE-T and 10BASE-T, preserving a smooth migration path. • As more organizations move to 100BASE-T, putting huge traffic loads on backbone networks, demand for Gigabit Ethernet has intensified.

38. Gigabit Ethernet A 1-Gbps switching hub provides backbone connectivity for central servers and high-speed workgroup hubs. Each workgroup LAN switch supports both 1-Gbps links, to connect to the backbone LAN switch and to support high-performance workgroup servers, and 100-Mbps links, to support high-performance workstations, servers, and 100-Mbps LAN switches.

39. Gigabit Ethernet

40. Gigabit Ethernet • 1000BASE-SX: This short-wavelength option supports duplex links of up to 275 m using 62.5-µm multimode or up to 550 m using 50-µm multimode fiber. Wavelengths are in the range of 770 to 860 nm. • • 1000BASE-LX: This long-wavelength option supports duplex links of up to 550 m of 62.5-µm or 50-µm multimode fiber or 5 km of 10-µm single-mode fiber. Wavelengths are in the range of 1270 to 1355 nm. • • 1000BASE-CX: This option supports 1-Gbps links among devices located within a single room or equipment rack, using copper jumpers (specialized shielded twisted-pair cable that spans no more than 25 m). Each link is composed of a separate shielded twisted pair running in each direction. • • 1000BASE-T: This option makes use of four pairs of Category 5 unshielded twisted pair to support devices over a range of up to 100 m.

41. 10-Gbps Ethernet Higher-capacity backbone pipes will help relieve congestion for workgroup switches, where Gigabit Ethernet uplinks can easily become overloaded, and for server farms, where 1-Gbps network interface cards are already in widespread use. The goal for maximum link distances cover a range of applications: from 300 m to 40 km