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Multiple Access and Local Area Networks

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Title: Multiple Access and Local Area Networks


1
  • Multiple Access and Local Area Networks

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

4
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

5
Multiple Access
  • How to access a shared media in a controlled
    fashion

6
MAMultiple Access
  • Aloha
  • Packet radio protocol
  • Random-access method based on acknowledgements
    and backoffs

7
Aloha Protocol
8
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

9
Persistence Strategy
  • 1-persistent
  • Send as soon as channel is idle
  • Non-persistent
  • Wait a random period of time before sensing again

10
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
11
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

12
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)

13
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

14
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

15
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
16
CSMA/CA Procedure
17
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

18
Controlled Access
19
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

20
Token Passing Procedure
21
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

22
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)

23
IEEE Project 802
24
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

25
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)

26
10Base5 (Thick Ethernet)
27
Switched Ethernet
  • One port per station
  • Full-duplex mode
  • No need for CSMA/CD
  • MAC control sublayer added
  • Flow and error control

28
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

29
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

30
Gigabit Ethernet
  • 1 Gb/s
  • Full duplex without CSMA/CD
  • Mostly used
  • Half-duplex with CSMA/CD
  • Two-wire and four-wire

31
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)

32
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)

33
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

34
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,

35
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

36
Bridged Ethernet
  • Share of bandwidth increases
  • Probability of collisions decreases
  • Smaller collision domains

37
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

38
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

39
Learning Bridges Example
40
Learning BridgesLoop Problem
41
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)

42
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

43
Before Spanning Tree
44
Applying Spanning Tree
45
Forwarding Ports and Blocking Ports
46
Routers
  • Connects LAN segments on Internet (IP) level
  • MAC protocols on the connected segments can be
    different

47
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

48
VLAN Divides LAN Into Logical Groups
49
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

50
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

51
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

52
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
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