Multiple Access and Local Area Networks

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• Multiple Access and Local Area Networks

2G1316 2G1317Data Communications and Computer
Networks
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Illustrations in this material are collected from
Behrouz A Forouzan, Data Communications and
Networking, 3rd edition, McGraw-Hill.
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This Lecture

• Multiple access CSMA/CD, CSMA/CA, token passing,
  channelization
• LAN characteristics, basic principles
• Protocol architecture
• Topologies
• LAN systems Ethernet
• Extending LANs repeater, bridge, router
• Virtual LANs

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Communication on LANs

• Goal simple and cheap solution
• Characteristics
• small area, limited number of users, all nodes
  can communicate directly
• short and long sessions
• The use of shared medium and broadcast
  transmission
• simple network elements, simple network
  management
• Property of LANs
• propagation time ltlt frame transmission time
• (Tpr ltlt Ttr)
• if a station transmits, all other will soon know
  about it

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Multiple Access

• How to access a shared media in a controlled
  fashion

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MAMultiple Access

• Aloha
• Packet radio protocol
• Random-access method based on acknowledgements
  and backoffs

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Aloha Protocol
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Carrier Sense Multiple Access (CSMA)

• Carrier sense
• Listen (sense) before sending
• Do not send unless the medium is idle
• Reduces the possibility of collisions
• Does not eliminate collisions
• Propagation delay
• Takes time before all other stations can sense a
  transmission

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Persistence Strategy

• 1-persistent
• Send as soon as channel is idle

• Non-persistent
• Wait a random period of time before sensing again

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p-Persistent

• When channel is idle
• Send with probability p
• Wait and then sense again with probability (1-p)

Sense carrier
Yes
Busy?
Wait
No
r random(0, 1)
No
r lt p ?
Yes
Send frame
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CSMA with Collision Detection (CSMA/CD)

• Exponential back-off
• Wait 2N max_propagation_time after collision,
  where N is number of transmission attempts
• Send jam so other stations detect collision as
  well

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Collision Detection

• Requires that stations still transmit when the
  colliding packet arrives
• Collisions detected by all stations
• Minimum packet size and maximum bus length
• 72 bytes (64 bytes at data link layer) and 500
  meters for 10Base5 (Thick Ethernet)

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Wireless LAN (CSMA/CA)

• Problem with CSMA/CD in combination with radio
  signals
• Collision detection is not reliable

• E.g., asymmetry computer 3 might not receive the
  signals from computer 1 to computer 2
• Hidden station problem, signal fading
• Requires ability to send data and detect
  collisions at the same time
• More costly hardware, higher bandwidth

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CSMA/CA (contd)

• CSMA/CA
• Carrier Sense Multiple Access with Collision
  Avoidance
• Control signals before transmission
• Carrier sense
• do not transmit immediately when medium gets idle
  (p-persistence)
• Wait random time
• lower collision probability

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CSMA/CA Wait Procedure

• When medium is busy
• Wait a random amount of time
• But only decrement timer when medium is idle
• IEEE 802.11 Wireless LAN

t random(0, maxwait)
Sense carrier
Yes
Busy?
No
No
Decrement t
No
t 0 ?
Yes
Send frame
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CSMA/CA Procedure
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CSMA/CA With RTS/CTS

• Request-to-send/clear-to-send (RTS/CTS) handshake
• RTS/CTS frames contain Duration field
• Period of time the medium is reserved for
  transfer
• Other stations remain quite during this period
• Need not be used for all frames
• Overhead too high for small frames

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Controlled Access
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Token Passing

• Token (a control frame) circulates among the
  nodes
• The node that holds the token has the right to
  transmit
• Used in Token Ring LAN

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Token Passing Procedure
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Channelization

• FDMA
• A station is allocated a frequency band on an FDM
  link
• TDMA
• Entire bandwidth is one channel
• A station is allocated time slots on a TDM link
• CDMA (Code Division Multiple Access)
• Entire bandwidth is one channel
• Data from all inputs are transmitted at the same
  time
• Based on coding theory, and uses sequences of
  numbers called chips

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Local Area Networks (LANs)

• Ethernet by far the most popular
• Originally from Xeroxs Palo Alto Research Center
  (PARC) in 1976
• LAN standardization in IEEE Project 802
• Data link layer subdivision
• Logical Link Control (LLC)
• Medium Access Control (MAC)

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IEEE Project 802
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IEEE 802.3 MAC Frame Format
Data link layer frame

• 48-bit addresses
• Written as 009027253c4e or 00-90-27-25-3c-4e
• Multicast (8th bit is 1), unicast, or broadcast
  (all 1s)
• Length/PDU
• Length if less than 1518
• IEEE 802.3 format
• Otherwise PDU type
• DIX (DEC, Intel, Xerox) Ethernet format
• CRC-32

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Traditional Ethernet

• 10 Mb/s
• CSMA/CD access
• Manchester coding
• Several different physical layers
• Bus
• 10Base5 (thick coax)
• 10Base2 (thin coax)
• Star
• 10Base-T (twister pair)
• 10Base-FL (fiber link)

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10Base5 (Thick Ethernet)
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Switched Ethernet

• One port per station
• Full-duplex mode
• No need for CSMA/CD
• MAC control sublayer added
• Flow and error control

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Fast Ethernet (IEEE 802.3u)

• 100 Mb/s
• CSMA/CD
• Compatibility
• Autonegotiation
• 10/100 Mb/s, full/half duplex, etc
• Two-wire and four-wire

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Fast Ethernet Physical Layer

• Two-wire
• 100Base-TX
• Twister pair (cat 5 UTP or STP)
• 4B/5B block coding MLT-3 line coding
• 125 MHz bandwidth
• 100Base-FX
• Fiber-optic cables
• 4B/5B block coding NRZ-I line coding
• Four-wire
• 100Base-T4
• Twisted pair (cat 3 UTP or higher)
• 4 times 25 Mbps with 8B/6T NRZ coding

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Gigabit Ethernet

• 1 Gb/s
• Full duplex without CSMA/CD
• Mostly used
• Half-duplex with CSMA/CD
• Two-wire and four-wire

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Gigabit Ethernet Physical Layer

• Two wire
• 1000Base-X (IEEE 802.3z)
• 1000-BaseSX (shortwave fiber), 1000-BaseLX
  (longwave fiber), 1000-BaseCX (short copper
  jumpers)
• 8B/10B block coding and NRZ line coding
• Four wire
• 1000Base-T (IEEE 802.3ab)
• Cat 5 UTP
• 8B/10B block coding and 4D-PAM5 line coding
  (4-dimensional, 5-level pulse amplitude
  modulation)

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10G Ethernet (IEEE802.3ae)

• Serial transmission
• 10GBase-SR, 10GBase-SW, 10GBase-LR, 10GBase-LW,
  10GBase-ER, 10GBase-EW
• S Short wavelength (850 nm multimode, 300 m)
• L Long wavelength (1310 nm singlemode, 10 km)
• E Extra long wavelength (1550 nm singlemode, 40
  km)
• WAN/LAN varieties
• W WAN interoperability (SONET/SDH scrambling,
  STS-192c framing)
• R LAN
• 64B/66B coding
• Parallel transmission
• 10GBase-LX4
• 8B/10B coding
• 4 3.125 Gb/s W-WDM (Wide Wave Division
  Multiplexing)

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Extending LANs

• Why not one LAN?
• Signal quality and network performance
• Declines with number of connected devices and
  network diameter
• Reliability
• Several self-contained units
• Security
• Separation of traffic
• Geography
• Connect LANs at different locations

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Extending LANs

• Repeaters and hubs
• Connects segments of the same LAN
• Signal regeneration
• Bridges (two-layer switches)
• Routing at the data link layer
• Connects LANs that use same type of data link
  addresses
• Traffic filtering
• Router (three-layer switches)
• Routing at the network layer
• Connects LANs (or links in general) of different
  technologies
• Beware terminology is getting blurred!
• Smart switches, dual-speed hubs,

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Repeaters

• Connects LAN segments on physical level
• To overcome distance limitations due to signal
  degradation
• Signal regenerator
• MAC protocol must be identical in all segments
• Collision propagates to all segments
• Hub is a multiport repeater

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Bridged Ethernet

• Share of bandwidth increases
• Probability of collisions decreases
• Smaller collision domains

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Bridges

• Forwards complete, correct frames
• Forwards frames only to segment where the
  destination address belongs
• Table with mapping from MAC addresses to ports
• Needs to learn the location of connected stations
• Filtering
• Buffer frames while ports are busy
• Can connect LANs with different data link layers
  protocols
• Ethernet LAN to Wireless LAN

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Learning Bridges

• How do bridges learn the location of the
  stations?
• A forwarding table that maps addresses to ports
• For each arriving frame
• Extract source address and add the port number
  and source address to the forwarding table
• Examine destination address and check if it is in
  the forwarding table
• if it is, transmit the frame on the respective
  port
• Otherwise, broadcast the frame on all ports

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Learning Bridges Example
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Learning BridgesLoop Problem
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Spanning Tree

• Purpose
• Bridges dynamically discover a subset of the
  topology that is loop-free (a tree)
• Just enough connectivity so that
• there is a path between every pair of segments
  where physically possible
• the tree is spanning
• Each bridge has a unique ID
• A cost can be calculated for each path between
  two bridges
• All bridges exchange configuration messages,
  called bridge protocol data units (BPDUs)

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Spanning Tree Process

• The node with the smallest ID is selected the
  root bridge
• Mark the port on each bridge with the least cost
  path (shortest path, typically) to the bridge as
  a root port
• On the root bridge, all ports are marked
• On each LAN segment, select a designated bridge
• Bridge with least cost path to root bridge
• If two bridges have the same least cost, the
  bridge with smallest ID is designated bridge
• Mark the corresponding port as the designated
  port
• Forward frames only on marked ports
• Designated ports and root ports
• Block on the others

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Before Spanning Tree
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Applying Spanning Tree
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Forwarding Ports and Blocking Ports
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Routers

• Connects LAN segments on Internet (IP) level
• MAC protocols on the connected segments can be
  different

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Virtuell LANs (VLAN)

• Need a way to divide the LAN into different parts
• Without physical reconfiguration
• Moving stations without reconfigurations
• Create virtual workgroups
• Keep broadcasts isolated
• Keep different protocols from each other

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VLAN Divides LAN Into Logical Groups
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VLAN Grouping

• How is VLAN membership determined?
• Port number
• Ports 1, 2, 7 VLAN 1
• Ports 3, 4, 5, 6 VLAN 2
• MAC address
• Frame tagging

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Frame Tagging
6 bytes
6 bytes
2 bytes
46-1500 bytes
6 bytes
2 bytes
Destinationaddress
Sourceaddress
TagHeader
Length/Type
DATA
CRC
TPID (81-00)
UserPriority
CFI
VLANIdentifier
16 bits
3 bits
1 bit
12 bits

• Tag header added to Ethernet header
• IEEE 802.1Q
• 12-bit VLAN ID allows for 4096 VLANs

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Summary

• Shared mediummultiple access
• CSMA/CD, CSMA/CA
• Channelization
• Ethernet
• Frame format
• Three generations
• LAN protocol stack and MAC layer
• Extending LANs repeaters, bridges, routers
• Spanning tree
• Virtual LANs

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Reading Instructions

• Behrouz A. Forouzan, Data Communications and
  Networking, third edition
• 14 Local Area Networks Ethernet
• 14.1 Traditional Ethernet
• 14.2 Fast Ethernet
• 14.3 Gigabit Ethernet
• 16 Connecting LANs, Backbone Networks, and
  Virtual LANs
• 16.1 Connecting Devices
• 16.3 Virtual LANs