Local Area Network
Lesson 7NETS2150/2850
Lesson OutlineCommon LAN topologiesLogical Link Control sublayerMedium Access Control sublayerARP protocol for IP MAC mapLAN interconnection devices
TopologiesLAN topology refers to the ways end systems are interconnectedCommon topologies:
TreeBus
Special case of tree
RingStar
LAN Topologies
Bus and TreeTransmission propagates throughout medium Heard by all stations
Need to identify target stationEach station has unique address
Full duplex connection between station and tapAllows for simultaneous transmission and reception
Need to regulate transmissionTo avoid collisionsTo avoid hogging
Data in small frames (fragmentation!)
Terminator absorbs frames at end of mediumPrevent from being reflected into the channel
Frame Transmissionon Bus LAN
Ring TopologyRepeaters joined by point to point links in closed loop
Receive data on one link and retransmit on anotherLinks unidirectionalStations attach to repeaters
Data in framesCirculate past all stationsDestination recognizes address and copies frameFrame circulates back to source where it is removed
MAC protocol determines when station can insert frame
Frame TransmissionRing LAN
Star TopologyEach station connected directly to central node
Usually via two point to point links
Central node can broadcastOnly one station can transmit at a time
Or central node can act as frame switch
More stations can transmit at a time
IEEE 802 v OSI RM
802 Layers - PhysicalEncoding/decodingPreamble generation/removal
7 bytes with pattern 10101010 followed by one byte with pattern 10101011used to synchronise receiver, sender clock rates
Bit transmission/receptionTransmission medium and topology
802 Layers -Logical Link Control
Based on HDLCProvides interface to higher levelsTransmission of LLC PDU between two stations
Flow and error control
Must support multiaccess, shared LAN media
Link access handled by MAC layer
LLC ServicesUnacknowledged connectionless service
No handshake and no ack (unreliable)
Connection mode serviceUse handshake and ack
Acknowledged connectionless service
No handshake but uses ack
Media Access ControlAssembly of data into frame with address and error detection fieldsDisassembly of frame
Address recognitionError detection
Govern access to transmission medium
MAC Frame FormatMAC layer receives data from LLC layer and adds:
MAC controlDestination MAC address (6-octet or 48-bit)Source MAC addressCRC
MAC layer detects errors and discards framesMAC broadcast address: FF FF FF FF FF FF16
LLC optionally retransmits unsuccessful frames
IEEE 802.3 MAC Frame Format
Addresses: 6 octetsif adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocolotherwise, adapter discards frame
Length: 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)
Octets: 8 6 6 2 46-1500 4
Length
MAC protocolsAssume single shared broadcast channel Two or more simultaneous transmissions by nodes will cause interference
only one node can send successfully at a time
MAC protocol:distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
MAC Protocols: A taxonomyThree broad classes:
Channel Partitioning or Reservationdivide channel into smaller “pieces” (time slots, frequency, code)allocate a piece to node for exclusive use
Random 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
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
ARP operation
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An ARP request is broadcast; an ARP reply is unicast.
NoteNote::
An ARP reply is only generated by the destined node.
ARP Packet Format
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Encapsulation of ARP Packet
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Length
Interconnecting LAN segments
HubsBridgesSwitches
HubsHub 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 used
Max about 500 m
Physically star, logically busTransmission from any station received by all other stationsForms a single collision domain
Two stations transmit at the same time collision!!
Interconnecting with hubsBackbone hub interconnects LAN segmentsExtends max distance between stationsBut individual segments’ collision domain become one large collision domain
when a node in CS and a node in EE transmit at same time collision!!
Can’t interconnect 10BaseT & 100BaseT
BridgesLink layer device (layer-2 device)
stores and forwards Ethernet framesexamines frame header and selectively forwards frame based on MAC dest address
transparentstations are unaware of presence of bridges
plug-and-play, self-learningbridges do not need to be configured
Bridges: traffic isolationBridge installation breaks LAN into LAN segments
bridges filter packets: same-LAN-segment frames not usually forwarded onto other LAN segmentssegments become separate collision domains
bridge collision domain
collision domain
= hub
= station
LAN
LAN segment LAN segment
Forwarding
How to determine to which LAN segment to forward frame?• Looks like a routing problem...
Self learningA bridge has a bridge tableentry 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 interfaces
when frame received, bridge “learns” location of sender: incoming LAN segmentrecords sender/location pair in bridge table
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 3
frame received by D
Bridge Learning: example
D generates frame for C, and sends it
bridge receives frame notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so selectively forwards frame to interface 1
C 1
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
Backbone configuration
Recommended !
Note: A bridge does not change the physical (MAC) addresses in a frame.
Loop of Bridges
Spanning Tree AlgorithmAddress learning works for tree layout
i.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 loops
Known as a spanning tree!
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 interfaces
Disabled
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
Routers vs. Bridges (1)both store-and-forward devices
routers: network layer devices (examine network layer headers)bridges are link layer devices
routers maintain routing tables, implement routing algorithmsbridges maintain bridge tables, implement filtering, learning and spanning tree algorithms
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)
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 processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)
Ethernet SwitchesEssentially a multi-interface bridge
layer 2 (frame) forwarding, filtering using LAN addresses
Incoming frame from particular station switched to appropriate output lineUnused lines can switch other trafficMore than one station can transmit at a time
Multiplying capacity of LAN
Shared Hub and Switch
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 network
Cut-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 throughput Risk of propagating bad frames
Switch unable to check CRC prior to retransmission
Netgear GS108UK GB SwitchLatency ~ 10 µs for 64-byte framesThroughput 32 MfpsMAC database (8000 entries)
Ethernet Switch BenefitsNo change to attached stations to convert bus LAN or hub LAN to switched LANFor Ethernet LAN, each station uses Ethernet MAC protocol Each station has dedicated capacity equal to original LAN
Assuming switch has sufficient capacity to keep up with all devices
Switch scales easilyCon: still has broadcast storm problem!
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
Typical Large LAN Organization
Thousands to tens of thousands of stationsDesktop systems links 10 Mbps to 100 Mbps
Into layer 2 switch
Wireless LAN connectivity available for mobile usersLayer 3 switches at local network's core
Form local backboneInterconnected at 1 GbpsConnect to layer 2 switches at 100 Mbps to 1 Gbps
Servers connect directly to layer 2 or layer 3 switches at 1 Gbps
Typical Large LAN OrganizationDiagram
Summary comparisonhubs bridges routers switches
traffi cisolation
no yes yes yes
plug & play yes yes no yes
optimalrouting
no no yes no
cutthrough
yes no no yes
SummaryLAN topologiesIEEE 802 reference modelTypes of MAC protocolsInterconnection Devices
Hubs, bridges, switches, routers
Read Stallings chapter 15Next: Specific MAC protocols