EEE 449 Computer Networks Local Area Network LAN

EEE 449 Computer Networks Local Area Network (LAN)

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

LAN Topologies

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

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

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

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

Transmission

Transmission

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

LAN interconnect

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

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

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 500 m – Can have multiple levels involving a header hub and intermediate hubs

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 as destination address recognized • highest possible throughput • risk of propagating bad frames

LAN interconnections

LAN interconnections • Layer 2 switches – no change to 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

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 (IPv 6) • a predefined route is used for identified flows to speed up flow

LAN interconnections

LAN Protocol

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

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

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

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

LAN protocol

Bridge protocol • • IEEE 802. 1 D 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

Bridge protocol

High-Speed LANs – Ethernet (IEEE 802. 3 10 -Mbps) – Fast Ethernet (IEEE 802. 3 100 -Mbps) – Gigabit Ethernet – 10 -Gbps Ethernet

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

Ethernet

Ethernet Preamble: A 7 -octet pattern of alternating 0 s and 1 s 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.

Ethernet IEEE 802. 3 10 Mbps <data rate in Mbps> <signaling method><max segment length in hundreds of meters>

Ethernet IEEE 802. 3 10 Mbps • Alternatives for 10 -Mbps are: • 10 BASE 5: 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. • 10 BASE 2: lower-cost alternative to 10 BASE 5 using a thinner cable, with fewer taps over a shorter distance than the 10 BASE 5 cable. • 10 BASE-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. • 10 BASE-F: Contains three specifications using optical fibre

Fast Ethernet (IEEE 802. 3 100 Mbps) • a low-cost, Ethernet-compatible LAN operating at 100 Mbps • All of the 100 BASE-T options use the IEEE 802. 3 MAC protocol and frame format.

Fast Ethernet (IEEE 802. 3 100 Mbps) • 100 BASE-X refers to a set of options that use two physical links between nodes; one for transmission and one for reception. • 100 BASE-TX makes use of shielded twisted pair (STP) or high-quality (Category 5) unshielded twisted pair (UTP). • 100 BASE-FX uses optical fiber. • For all of the 100 BASE-T options, the topology is similar to that of 10 BASE-T, namely a star-wire topology.

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 100 BASE-T and 10 BASE-T, preserving a smooth migration path. • As more organizations move to 100 BASE-T, putting huge traffic loads on backbone networks, demand for Gigabit Ethernet has intensified.

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.

Gigabit Ethernet

Gigabit Ethernet • 1000 BASE-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. • • 1000 BASE-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 singlemode fiber. Wavelengths are in the range of 1270 to 1355 nm. • • 1000 BASE-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. • • 1000 BASE-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.

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