Local Area Network LAN l Topology l Transmission

Local Area Network : LAN l. Topology l. Transmission medium l. Wiring layout l. Medium access control 7 Sep 2013

Local Area Networks (LANs) usually owned by the organization that is using the network to interconnect equipment Ø key elements: Ø l l topology transmission medium wiring layout medium access control

LAN Topologies

Bus and Tree Bus: Tree: • stations attach through tap to bus • full duplex allows transmission and reception • transmission propagates throughout medium • heard by all stations • terminator at each end • a generalization of bus • branching cable with no closed loops • tree layout begins at headend and branches out • heard by all stations

Frame Transmission on Bus LAN

Ring Topology a closed loop of repeaters joined by point-topoint links Ø receive data on one link & retransmit on another Ø l l Ø data transmitted in frames l l l Ø links unidirectional stations attach to repeaters circulate past all stations destination recognizes address and copies frame circulates back to source where it is removed medium access control determines when a station can insert frame

Frame Transmission Ring LAN

Star Topology Ø each station connects to common central node l usually via two point-to-point link • one for transmission and one for reception central node • • operate in broadcast fashion physical star, logical bus only one station can transmit at a time (hub) can act as frame switch

Choice of Topology l l l reliability medium wiring layout access control factors: performance expandability

Bus LAN Transmission Media twisted pair • early LANs used voice grade cable • scaling up for higher data rates not practical baseband coaxial cable • uses digital signaling • original Ethernet cont…

Bus LAN Transmission Media broadband coaxial cable • used in cable TV systems • analog signals at radio and TV frequencies • expensive, hard to install and maintain optical fiber • expensive taps • better alternatives available only baseband coaxial cable has achieved widespread use

Ring and Star Topologies Ring • very high speed links over long distances • potential of providing best throughput • single link or repeater failure disables network Star • uses natural layout of wiring in building • best for short distances • high data rates for small number of devices

Choice of Medium Ø constrained by LAN topology Ø capacity l to support the expected network traffic Ø reliability l to meet requirements for availability Ø types of data supported l tailored to the application Ø environmental scope l provide service over the range of environments

Media Available Voice grade unshielded twisted pair (UTP) Shielded twisted pair / baseband coaxial Cat 3 phone, cheap, low data rates Broadband cable even more expensive, higher data rate High performance UTP more expensive, higher data rates Cat 5+, very high data rates, suited for star topology Optical fibre security, high capacity, small size, high cost

LAN Protocol Architecture

IEEE 802 Layers Ø Physical Layer l l Encoding / decoding of signals preamble generation / removal bit transmission / reception transmission medium and topology

IEEE 802 Layers Ø Logical Link Control Layer (LLC) l l provide interface to higher levels perform flow and error control Ø Media Access Control l l on transmit assemble data into frame on reception disassemble frame, perform address recognition and error detection govern access to transmission medium for same LLC, may have several MAC options

LAN Protocols in Context

Logical Link Control Ø transmission of link level PDUs between stations Ø must support multi-access, shared medium Ø relieved of some details of link access by the MAC layer Ø addressing involves specifying source and destination LLC users l referred to as service access points (SAPs)

LLC Services unacknowledged connectionless service • data-gram style service • delivery of data is not guaranteed connection-mode service • logical connection is set up between two users • flow and error control are provided acknowledged connectionless service • datagrams are to be acknowledged, but no logical connection is set up

LLC Service Alternatives unacknowledged connectionless service • requires minimum logic • avoids duplication of mechanisms • preferred option in most cases connection-mode service • used in simple devices • provides flow control and reliability mechanisms acknowledged connectionless service • large communication channel needed • time critical or emergency control signals

LLC Protocol Ø modeled after HDLC Ø asynchronous balanced mode l connection mode (type 2) LLC service Ø unacknowledged connectionless service l using unnumbered information PDUs (type 1) Ø acknowledged connectionless service l using 2 new unnumbered PDUs (type 3) Ø permits multiplexing using LSAPs

MAC Frame Format

Medium Access Control (MAC) Protocol Ø controls access to the transmission medium Ø key parameters: l where • greater control, single point of failure • more complex, but more redundant l how • synchronous l capacity dedicated to connection, not optimal • asynchronous l l response to demand round robin, reservation, contention

Asynchronous Systems round robin reservation contention • each station given turn to transmit data • divide medium into slots • good for stream traffic • all stations contend for time • good for bursty traffic • simple to implement • tends to collapse under heavy load

MAC Frame Handling MAC layer receives data from LLC layer Ø PDU is referred to as a MAC frame Ø fields: Ø l l l MAC control destination MAC address source MAC address LLC CRC MAC layer detects errors and discards frames Ø LLC optionally retransmits unsuccessful frames Ø

Bridges connects similar LANs with identical physical and link layer protocols Ø minimal processing Ø can map between MAC formats Ø reasons for use: Ø l l reliability performance security geography

Bridge Function

Bridge Design Aspects Ø no modification to frame content or format Ø no encapsulation Ø exact bitwise copy of frame Ø buffering to meet peak demand Ø contains routing and address intelligence Ø may connect more than two LANs Ø bridging is transparent to stations

Bridge Protocol Architecture IEEE 802. 1 D defines architecture Ø MAC level designates endpoint Ø bridge does not need LLC layer Ø captures frame removes encapsulation encapsulates it forwards it across link transmits to destination

Connection of Two LANs

Bridges and LANs with Alternative Routes

Fixed Routing simplest and most common Ø suitable for Internets that are stable Ø a fixed route is selected for each pair of LANs Ø • usually least hop route only changed when topology changes Ø widely used but limited flexibility Ø

Spanning Tree Ø bridge automatically develops routing table Ø automatically updates routing table in response to changing topology algorithm consists of three mechanisms: frame forwarding address learning loop resolution

Frame Forwarding maintain forwarding database for each port Ø for a frame arriving on port X: Ø search forwarding database to see if MAC address is listed for any port except X if address not found, forward to all ports except X if address listed for port Y, check port Y for blocking or forwarding state if not blocked, transmit frame through port Y

Address Learning can preload forwarding database Ø when frame arrives at port X, it has come from the LAN attached to port X Ø use source address to update forwarding database for port X to include that address Ø have a timer on each entry in database Ø if timer expires, entry is removed Ø each time frame arrives, source address checked against forwarding database Ø l l if present timer is reset and direction recorded if not present entry is created and timer set

Spanning Tree Algorithm Ø address learning works for tree layout if there are no alternate routes in the network l alternate route means there is a closed loop for any connected graph there is a spanning tree maintaining connectivity with no closed loops Ø algorithm must be dynamic Ø IEEE 802. 1 Spanning Tree Algorithm: • • each bridge assigned unique identifier cost assigned to each bridge port exchange information between bridges to find spanning tree automatically updated whenever topology changes

Loop of Bridges

Interconnecting LANs - Hubs active central element of star layout Ø each station connected to hub by two UTP lines Ø hub acts as a repeater Ø limited to about 100 m by UTP properties Ø optical fiber may be used out to 500 m Ø physically star, logically bus Ø transmission from a station seen by all others Ø if two stations transmit at the same time have a collision Ø

Two Level Hub Topology

Buses, Hubs and Switches bus configuration all stations share capacity of bus (e. g. 10 Mbps) only one station transmitting at a time hub uses star wiring to attach stations transmission from any station received by hub and retransmitted on all outgoing lines Ø only one station can transmit at a time total capacity of LAN is 10 Mbps can improve performance using a layer 2 switch l l can switch multiple frames between separate ports multiplying capacity of LAN

Shared Medium Bus and Hub

Layer 2 Switch Benefits Ø no changeto attached devices to convert bus LAN or hub LAN to switched LAN l Ø have dedicated capacity equal to original LAN l Ø e. g. Ethernet LANs use Ethernet MAC protocol assuming switch has sufficient capacity to keep up with all devices scales easily l additional devices attached to switch by increasing capacity of layer 2

Types of Layer 2 Switches Ø store-and-forward switch l l l accepts frame on input line, buffers briefly, routes to destination port see delay between sender and receiver boosts overall integrity Ø cut-through switch l l use destination address at beginning of frame switch begins repeating frame onto output line as soon asdestination address is recognized highest possible throughput risk of propagating bad frames

Layer 2 Switch vs. Bridge Ø differences between switches & bridges: Bridge frame handling done in software Switch performs frame forwarding in hardware analyzes and forwards one frame at a time can handle multiple frames at a time uses store-andforward operation can have cutthrough operation layer 2 switch can be viewed as full-duplex hub Ø incorporates logic to function as multiport bridge Ø new installations typically include layer 2 switches with bridge functionality rather than bridges Ø

A Partitioned LAN Configuration

Virtual LANs (VLANs) Ø subgroup within a LAN Ø created by software Ø combines user stations and network devices into a single broadcast domain Ø functions at the MAC layer Ø router required to link VLANs Ø physically dispersed but maintains group identity

A VLAN Configuration

Defining VLANs Ø broadcast domain consisting of a group of end stations not limited by physical location and communicate as if they were on a common LAN Ø membership by: l l l port group MAC address protocol information

Communicating VLAN Membership Switches need to know VLAN membership Ø configure information manually Ø network management signaling protocol Ø frame tagging (IEEE 802. 1 Q)

Summary Ø LAN topologies and transmission media l bus, tree, ring, star Ø LAN protocol architecture l IEEE 802, LLC, MAC Ø bridges, hubs, layer 2 switches Ø virtual LANs