5 Bus and Tree Transmission propagates throughout medium Heard by all stationsNeed to identify target stationEach station has unique addressFull duplex connection between station and tapAllows for simultaneous transmission and receptionNeed to regulate transmissionTo avoid collisionsTo avoid hoggingData in small frames (fragmentation!)Terminator absorbs frames at end of mediumPrevent from being reflected into the channel
7 Ring Topology Repeaters joined by point to point links in closed loop Receive data on one link and retransmit on anotherLinks unidirectionalStations attach to repeatersData in framesCirculate past all stationsDestination recognizes address and copies frameFrame circulates back to source where it is removedMAC protocol determines when station can insert frame
9 Star Topology Each station connected directly to central node Usually via two point to point linksCentral node can broadcastOnly one station can transmit at a timeOr central node can act as frame switchMore stations can transmit at a time
11 802 Layers - Physical Encoding/decoding Preamble generation/removal 7 bytes with pattern followed by one byte with patternused to synchronise receiver, sender clock ratesBit transmission/receptionTransmission medium and topology
12 802 Layers - Logical Link Control Based on HDLCProvides interface to higher levelsTransmission of LLC PDU between two stationsFlow and error controlMust support multiaccess, shared LAN mediaLink access handled by MAC layer
13 LLC Services Unacknowledged connectionless service No handshake and no ack (unreliable)Connection mode serviceUse handshake and ackAcknowledged connectionless serviceNo handshake but uses ack
14 Media Access ControlAssembly of data into frame with address and error detection fieldsDisassembly of frameAddress recognitionError detectionGovern access to transmission medium
15 MAC Frame Format MAC layer receives data from LLC layer and adds: MAC controlDestination MAC address (6-octet or 48-bit)Source MAC addressCRCMAC layer detects errors and discards framesMAC broadcast address: FF FF FF FF FF FF16LLC optionally retransmits unsuccessful frames
16 IEEE 802.3 MAC Frame Format Addresses: 6 octets LengthAddresses: 6 octetsif adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocolotherwise, adapter discards frameLength: length of data field in octets, max frame size is 1518 octets (excluding preamble & SFD)CRC: checked at receiver, if error is detected, the frame is simply dropped (32-bit CRC)
17 MAC protocols Assume single shared broadcast channel Two or more simultaneous transmissions by nodes will cause interferenceonly one node can send successfully at a timeMAC protocol:distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
18 MAC Protocols: A taxonomy Three broad classes:Channel Partitioning or Reservationdivide channel into smaller “pieces” (time slots, frequency, code)allocate a piece to node for exclusive useRandom Access or Contentionchannel not divided, thus can’t avoid collisionsNeed to “recover” from collisions“Taking turns” or Round Robintightly coordinate shared access to avoid collisions
19 Address Resolution Protocol (ARP) Even if you have the IP address of your destination, you need its MAC to get your data across a physical networkSo, we need a way to do this mappingARP performs dynamic mapping between IP and MACAny resolved mapping is stored in a host’s ARP cache
23 Encapsulation of ARP Packet LengthMcGraw-HillThe McGraw-Hill Companies, Inc., 2004
24 Interconnecting LAN segments HubsBridgesSwitches
25 Hubs Hub acts as a repeater (physical layer device) When single station transmits, hub repeats signal on outgoing line to each stationLimited to about 100 mOptical fibre may be usedMax about 500 mPhysically star, logically busTransmission from any station received by all other stationsForms a single collision domainTwo stations transmit at the same time collision!!
26 Interconnecting with hubs Backbone hub interconnects LAN segmentsExtends max distance between stationsBut individual segments’ collision domain become one large collision domainwhen a node in CS and a node in EE transmit at same time collision!!Can’t interconnect 10BaseT & 100BaseT
27 Bridges Link layer device (layer-2 device) stores and forwards Ethernet framesexamines frame header and selectively forwards frame based on MAC dest addresstransparentstations are unaware of presence of bridgesplug-and-play, self-learningbridges do not need to be configured
28 Bridges: traffic isolation Bridge installation breaks LAN into LAN segmentsbridges filter packets:same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domainsbridgecollisiondomain= hub= stationLAN segmentLAN segmentLAN
29 Forwarding How to determine to which LAN segment to forward frame? Looks like a routing problem...
30 Self learning A bridge has a bridge table entry in bridge table: (Station MAC Address, Bridge Interface, Timestamp)stale entries in table dropped (TTL can be ~ 60 min)bridges learn which hosts can be reached through which interfaceswhen frame received, bridge “learns” location of sender: incoming LAN segmentrecords sender/location pair in bridge table
31 Bridge exampleSuppose C sends frame to D and D replies back with frame to C.Bridge receives frame from from Cupdates bridge table, C is on interface/port 1because D is not in table, bridge sends frame into interfaces 2 and 3frame received by D
32 Bridge Learning: example C 1D generates frame for C, and sends itbridge receives framenotes in bridge table that D is on interface 2bridge knows C is on interface 1, so selectively forwards frame to interface 1
33 Interconnection without backbone Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment
34 Backbone configuration Recommended !Note: A bridge does not change the physical (MAC) addresses in a frame.
36 Spanning Tree Algorithm Address learning works for tree layouti.e. no closed loops (or cycles)But not for cyclic connected graph!Spanning Tree Algo. builds a network including all the nodes with selected links (i.e. edges) without closed loopsKnown as a spanning tree!
37 Spanning Treefor increased reliability, desirable to have redundant, alternative paths from source to dest but need to avoid cyclessolution: organize bridges in a spanning tree by disabling subset of interfacesDisabled
38 Some bridge featuresIsolates collision domains resulting in higher total max throughput (i.e. amount of data transmitted within an interval)Transparent (“plug-and-play”): no configuration necessary
39 Routers vs. Bridges (1) both store-and-forward devices routers: network layer devices (examine network layer headers)bridges are link layer devicesrouters maintain routing tables, implement routing algorithmsbridges maintain bridge tables, implement filtering, learning and spanning tree algorithms
40 Routers vs. Bridges (2) Bridges pros (+) and cons (-) + Bridge operation is simpler requiring less data unit processing+ Bridge tables are self learning- All traffic confined to spanning tree, even when alternative bandwidth is available- Bridges do not offer protection from broadcast storms (i.e. forwarding of broadcast traffic)
41 Routers vs. Bridges (3) Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)+ provide protection against broadcast storms- require IP address configuration (not plug and play)- require higher packet processingbridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)
42 Ethernet Switches Essentially a multi-interface bridge layer 2 (frame) forwarding, filtering using LAN addressesIncoming frame from particular station switched to appropriate output lineUnused lines can switch other trafficMore than one station can transmit at a timeMultiplying capacity of LAN
44 Types of Ethernet Switches Store-and-forward switchAccepts frame on input lineBuffers it briefly, then forwards it to appropriate output lineError checking, boosts integrity of networkCut-through switchTakes advantage of dest address appearing at beginning of frameSwitch begins repeating frame onto output line as soon as it recognizes dest addressHighest possible throughputRisk of propagating bad framesSwitch unable to check CRC prior to retransmissionNetgear GS108UK GB SwitchLatency ~ 10 µs for 64-byte framesThroughput 32 MfpsMAC database (8000 entries)
45 Ethernet Switch Benefits No change to attached stations to convert bus LAN or hub LAN to switched LANFor Ethernet LAN, each station uses Ethernet MAC protocolEach station has dedicated capacity equal to original LANAssuming switch has sufficient capacity to keep up with all devicesSwitch scales easilyCon: still has broadcast storm problem!
46 Subnetwork with layer-3 device! Solution: break up network into subnetworks connected by routers or layer-3 switch (faster!)Packet forwarding done in the hardwareMAC broadcast frame limited to stations and switches contained within a single subnetwork
47 Typical Large LAN Organization Thousands to tens of thousands of stationsDesktop systems links 10 Mbps to 100 MbpsInto layer 2 switchWireless LAN connectivity available for mobile usersLayer 3 switches at local network's coreForm local backboneInterconnected at 1 GbpsConnect to layer 2 switches at 100 Mbps to 1 GbpsServers connect directly to layer 2 or layer 3 switches at 1 Gbps