IEEE 802' LAN Standards

1
IEEE 802. LAN Standards
10 Mbits/s CSMA/CD (802.3) 100 Mbits/s CSMA/CD
(802.3u) 1000 Mbits/s CSMA/CD (802.3z) Token
Bus (800.4) Token Ring (800.5) 100VG-AnyLan
(802.12)
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IEEE LAN Standards and the OSI Reference Model
Network
802.1 Architecture and Overviews
802.2 Logical Link Control (LLC)
Data Link
802.3
802.4
802.5
MAC
MAC
MAC
Physical
802.3
802.4
802.5
Physical
Physical
Physical
(CSMA/CD)
(Token-bus)
(Token-Ring)
OSI Reference Model
IEEE LAN Standards
3
Network 802.3 Specifications
10Base5
10Base-T
10Base2
Thick 50 Ohm Coaxial cable
Thin Coaxial cable
Twisted Pair
Medium
Topology
Bus
Bus
Star
Segment distance
500 m
200 m
100 m
Data Rate
10 Mbits/s
10 Mbits/s
10 Mbits/s
Maximum Nodes/Seg
100
30
1024
4
Thick and Thin Ethernet Wiring
Thick Ethernet cable
Thick Ethernet Wiring
Transceiver
Terminator
AUI cable
Terminator
Thin Ethernet Wiring
BNC connector
Thin Ethernet cable
5
Twisted Pair Ethernet
twisted pair wiring
Hub
RJ-45 connector

• Does not have a shared physical medium
• Twisted pair with RJ-45 connectors
• Components in a hub simulate a physical cable
  making entire system operate like a conventional
  Ethernet
• Each computer uses CSMA/CD to access network and
  Ethernet frame format

6
Shared Ethernet
tap

Hub (repeater)

Ethernet segment
7
Ethernet Frame Format
Minimum length of a valid frame 64 bytes
662464 802.3
8
6
6
2
46 - 1500 bytes
4
Preamble
DA
SA
Type
Data
CRC
DA Destination address SA Source Address Type
identifies upper layer protocol (demultiplex
key) Data 0 1500 bytes (min 46, pad if
necessary)
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• Ethernet addresses are 48-bit unicast assigned
  to each adapter
• Example 62b5e001d
• Broadcast (to all stations) all 1s
• Multicast (to a specific group of stations)
  first bit is 1
• Stations can be in promiscuous mode accept all
  frames
• (even if not addressed to them)

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Ethernet Experience

• Very popular and successful working example of
  CSMA/CD
• Cost-effective, cable cheap, no switching
  equipment
• Easy to administer, maintain, reliable
• Typically 10-200 hosts, within less 1500 m
• Works well for low loads, under heavy traffic
  much of networks
• bandwidth is wasted due to collisions
• Emerging technologies Switched Ethernet, Fast
  Ethernet,
• Gigabit Ethernet

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• A collision is detected when the transmitters
  read back is different
• from what was transmitted
• - minimum frame length must be 512 bit times
  (51.2 ms x 10 Mbits/s)
• Uses Manchester encoding
• Baseband signaling signals transmitted without
  modulation
• - entire spectrum is used by signals
• - if digital, 0 and 1 are represented as
  voltages (e.g. using
• Manchester encoding
• Broadband signal data transmitted as analog
  signals over multiple
• frequency channels (e.g. using a coaxial cable)
• - digital signals require modulation

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Token Bus

• IEEE 802.4 standard (1985)
• Token special control bit-sequence
• Physically, a token-bus is a linear or
  tree-shaped (broadband coaxial)
• cable with stations attached to it
• Logically it operates as a ring, cable is a
  broadcast medium each
• station receives a frame
• In 802.3 a station may wait for an undetermined
  amount of time
• before it can transmit, does not allow priorities
• With Token Bus, a station does not wait more
  than a fixed amount
• of time to send a frame

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Token Passing

• With Token Bus, there are no collisions, one
  station transmits at a time

Broadband coax cable
token

• Token is passed from node to node in a
  predefined order
• Once a station acquires a token (as it moves
  around), it can start
• transmitting
• After it finishes transmission, it passes the
  token to the next station
• (its logical neighbor)

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Token Bus MAC Frame Format
1
1
6
6
0 - 8174 bytes
4
1
1
End delimiter
Frame control
Start delimiter
Preamble
Data
FCS
DA
SA
Preamble used for synchronization Frame
control distinguishes control from data frames
(in data frames, it indicates priority), also
used to indicate the destinations ACK Control
frames include token and other maintenance frames
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Token Bus Performance

• Efficiency still poor due to round-trip
  propagation delay
• Maintenance and administration
• - Initialization/add new stations
• - Lost token problem (e.g. station holding the
  token goes down)
• MAC layer protocol complex to implement
• It can implement 4 priority levels
• Token bus is suitable for automated assembly
  lines
• Problems with token bus ?

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Token Ring

• IEEE 802.5 standard, IBM chose it as its LAN
• Stations are physically configured as a
  (unidirectional) ring
• It is not a broadcast medium, collection of
  point-to-point links
• that happen to form a circle

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1-bit delay
Ring Interface
To station
From station
From station
To station
Listen mode
Transmit mode

• It uses a token passing scheme
• When a station seizes the token, it inverts a
  single bit in the 3-byte
• token which changes it into a data frame
• Frames on the ring make a full rotation and then
  they are absorbed
• by the source
• 1-bit buffer every bit is copied to a 1-bit
  buffer and then copied
• out to the ring

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Token Ring MAC Frame Format
gt 0
1
1
1
2/6
2/6
4
1
DA
SA
Data
FCS
ED
FS
FC
AC
SD
End delimiter
Frame control
Frame Status
Access control
Start delimiter
Access control has the format PPPTMRRR - PPP
priority field - T token bit (frame is token or
data) - M monitor bit (if set frame is removed,
i.e. frame has being going around many
times) - RRR reservation field (a station can
reserve next token) Frame control distinguishes
control and data frames
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• Frame Status contains the address recognized (A)
  and
• frame-copied (C) bits (duplicated, not included
  in CRC)
• A 0 C 0 destination not alive
• A 1 C 0 destination alive, but frame not
  accepted
• (e.g. error, no buffer)
• A 1 C 1 destination alive and frame
  copied
• Frame Check Sequence 32-bit CRC that covers the
  address and data
• A monitor station can be responsible for ring
  maintenance
• - token maintenance
• - removed garbled or orphan frames
• - each station has the capability of detecting a
  break in the ring
• Token Ring uses Differential Manchester Encoding

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Ethernet VS Token Ring

• Token ring is efficient at heavy traffic while
  Ethernet is not
• Token ring has guaranteed delay
• Active interfaces is a disadvantage for the
  token ring
• Token ring can support optical fiber
• Token ring allow for priorities while Ethernet
  does not
• Token ring has less complex analog components
• Major drawback in token ring is its centralized
  monitoring function
• Ethernet protocol is popular, simple, stations
  can be added on the fly

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High-Speed LANs

• FDDI
• 100VG-AnyLAN (IEEE 802.12)
• Fast Ethernet (IEEE 802.3u)
• Gigabit Ethernet (IEEE 802.3z)
• ATM LAN

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FDDI

• Fiber Distributed Data Interface (ANSI)
• A high-speed (100 Mbits/s) LAN (token ring)
  technology
• Suitable also for MAN
• Uses fiber as its transmission
• Can attach 1000 stations, can be up to 200 km
• Based on two counter-rotating (independent)
  fiber rings
• If the ring breaks, the two rings are joined
  forming a single ring
• Maximum frame size is 4500 bytes

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Secondary ring
SAS
DAS
Primary ring
Concentrator
DAS Dual Attachment Stations Concentrator
allows multiple stations on the ring (can attach
many devices at the concentrator) SAS Single
Attachment Stations
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How does it work

• A station wants to transmit it sees the token,
  removes it and inserts
• its own frame
• Each station downstream forwards the frame,
  destination copies it
• Each station introduces a 50 ns delay (10-bit
  buffer, waits for
• half-full buffer before it starts transmission)
• Once the frame returns to the source, it is
  removed
• - Token can be re-inserted once frame is
  transmitted
• (immediate or early release)
• - Upon frames return to the source (delayed
  release)

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• Early-release is more efficient (especially at
  high loads)
• Nodes are serviced in a round-robin
• Limitations cost (fiber wiring, adapters),
  station, token management
• FDDI can also be implemented over copper
  (cheaper, 100 km) CDDI
• THT (Token Holding Time) upper bound on how
  long a station can
• hold the token
• TRT (Token Rotation Time) interval between two
  successive arrivals
• of the token
• - TRT lt ActiveNodes X THT RingLatency

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Timed Token Protocol
1) Each station measures its TRT 2) If measured
TRT gt TTRT (Target TRT) token is too late,
station cannot retransmit 3) If TRT lt TTRT,
token is early, station can send data (THT lt
TTRT - TRT)
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• FDDI defines two classes of traffic
• - synchronous (e.g. voice) guaranteed bandwidth
  (always tx)
• - asynchronous (e.g. file transfer) (tx only
  when token is
• early and there is time left)
• Uses 4B/5B encoding (map 4-bit into 5-bit
  symbols)
• No retransmissions

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FDDI MAC Frame Format
lt 4500 bytes
64
8
48
48
32
4
12
bits
DA
SA
Data
FCS
ED
FS
FC
SD
SOF
SOF Start of Frame SD Start Delimiter FC
Frame control (indicates use of 2/6-byte
addresses, asynchronous or synchronous frame,
control frame) DA Destination address (2 or 6
bytes) SA Source address (2 or 6 bytes) FCS
32-bit CRC based on FC, DA, SA, and the data ED
Ending Delimiter FS Frame Status
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Fast Ethernet

• IEEE 802.3u, 1995
• - 100Base-T4 (CAT3, CAT5 UTP)
• - 100Base-TX (CAT5 UTP, STP)
• - 100Base-FX (fiber, 400m)
• Can be used in shared and switched environment
• 100 Mbits/s hubs, 10/100 Mbits/s network
  interface cards
• Same frame format and size, time slot (512
  bits), and MAC protocol
• Basic idea reduce bit time from 100 nsec to 10
  nsec
• Reduce maximum cable length by a factor of 10

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100VG-AnyLAN

• 100 Mbits/s shared-medium (HP ATT, IEEE
  802.12)
• VG voice-grade
• Configured as a hierarchical structure up to
  three levels
• Gives priority to time critical (e.g. voice)
  applications
• Can use either Ethernet or Token Ring frame
  formats
• Does not use CSMA/CD
• Uses a new MAC scheme called demand priority to
  determine
• the order in which stations share the
  bandwidth
• Limitations software has to be re-written

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Switched Ethernet

• Dedicated connections to multiport hub which
  acts as a switch
• no collisions, does not use coax
• Advantages
• - existing infrastructure preserved (use UTP,
  STP,
• fiber, adapters)
• - easily expandable, scalable (e.g. add more
  switches)
• - allow for multiple connections between
  stations (on
• different Ethernet segments)
• - high performance, better efficiency (break
  large shared
• LANs into smaller farms

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Shared Ethernet
Hub
Switched Ethernet
switch ports
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• Suitable as a backbone solution and for
  client-server applications
• Can support
• - full duplex Ethernet (up to 20 Mbits/s) using
  switching
• stations can transmit and listen at the same
  time (coaxial
• does not allow full-duplex operation)
• - fast Ethernet (100 Mbits/s)
• No changes in software/hardware

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Gigabit Ethernet

• IEEE 802.3z, 1996 - 1 Gbit/s
• Currently running over fiber (1000 Base-X)
• In the future UTP-5 (1000 Base-T)
• Switched, full-duplex operation
• Shared, half-duplex operation
• Same Ethernet frame format

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• Retains CSMA/CD protocol
• Slot time of 512 bytes (4.096 ms) exceeds
  transmission time (512 ns)
• of minimum length (64 bytes) frame by eightfold
• Still able to send 64-byte frames
• When frame less than 512 bytes is sent, MAC
  layer sends a
• special signal
• Frame bursting is used to pipeline several
  frames
• - by combining frames into larger superframe
  512-byte
• slot can be filled up

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• Not a 10-fold decrease in network diameter
• Suitable for backbone, and high-end server
  connections
• Limitations speed of light

10BASE-T
100BASE-X
Gigabit Ethernet
10
100
1000
Data rate (Mbits/s)
100
100
25-100 m
CAT5 UTP
500
100
25-100
STP/Coax
2 km
400m (half-duplex)
500 m
Multimode fiber
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