13. LAN Systems

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13. LAN Systems
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Contents

• Ethernet(CSMA/CD)
• Token Ring
• IEEE 802.5
• FDDI(fiber distributed data interface)
• ATM LAN
• Fibre Channel
• Wireless LAN

3
Ethernet

• Ethernet is increasingly the LAN technology of
  choice.
• Simple, flexible, inexpensive, and high
  performance.
• Use a shared network, so that we need a protocol
  that tells the computers how to behave.

Protocol for Speaking in a Meeting 1. Listen
to see if anyone else is speaking. 2. When there
is a pause in the conversation, begin
phrasing the question. 3. At the same time,
continue listening just to make sure no one
else also used the opportunity to begin
speaking.
4
Ethernet (CSMA/CD)

• MAC (Medium Access Control)
• CSMA/CD (carrier sense multiple access with
  collision detection)
• IEEE 802.3 standard
• Contention (random access)
• ALOHA
• developed for packet radio at Univ. of Hawaii
• Any node with a newly generated packet
• immediately transmits the packet
• waits for a round-trip interval for ACK for
  packet
• If ACK is not received, waits for a random
  timeout interval and retries
• also called pure-ALOHA Talk when you please
• maximum utilization about 18

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Precusors
6
Precursors

• slotted ALOHA
• uniform time slots whose size is equal to frame
  transmission time
• transmission is only allowed at the beginning of
  slot
• maximum utilization about 37
• disadvantage of ALOHA and slotted ALOHA
• poor channel utilization

7
CSMA

• CSMA(carrier sense multiple access)
• transmitting station
• listen to the medium to determine if another is
  in progress(carrier sense)
• if the medium is in use, then waits, else
  transmits
• after transmitting it waits ACK
• if two stations send frames at the same time , a
  collision occurs.
• disadvantage of CSMA
• when two frames collide, the medium remains
  unusable for the duration of transmission of both
  damaged frames.

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CSMA
9
CSMA/CD

• CSMA CD(collision detection) --gt CSMA/CD
• listen before transmission till channel is free
• additionally continue to monitor channel during
  transmission
• if collision is detected, then abort transmission

10
CSMA/CD

• the amount of wasted capacity is reduced to the
  time it takes to detect a collision
• (cf. In case of CSMA, it should wait for ACK
  during a certain amount of time)
• frames should be long enough to allow collision
  detection prior to the end of transmission

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Collision Detection

• On baseband bus, collision produces much higher
  signal voltage than signal
• Collision detected if cable signal greater than
  single station signal
• Signal attenuated over distance
• Limit distance to 500m (10Base5) or 200m
  (10Base2)
• For twisted pair (star-topology) activity on more
  than one port is collision
• Special collision presence signal

12
CSMA/CD

• MAC frame(IEEE 802.3)
• preamble A 7-octet pattern of alternate 0s and
  1s used by the receiver to establish bit
  synchronization
• start frame delimiter(SFD) sequence 10101011,
  which indicates the actual start of the frame
• destination address(DA) and source address(SA)
• Unique 48 bit system address. IEEE assigns 3
  bytes organization ID addresses.
• Length length of the LLC data field
• LLC data data unit supplied by LLC
• Pad octets added to ensure that the frame is
  long enough for proper CD operation
• frame check sequence(FCS) error checking using
  32bit CRC

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CSMA/CD

• IEEE 802.3 10-Mbps Specifications(Ethernet)
• many alternative physical configurations
• 10BASE5, 10BASE2
• 10BASE-T (twisted pair)
• 10BASE-F (optical fiber)

14
CSMA/CD

• IEEE 802.3 100-Mbps Specifications (Fast
  Ethernet)
• low-cost, Ethernet-compatible LAN operating at
  100Mbps
• 100BASE-X
• physical medium specification
• requires the installation of new cable
• 100BASE -T4
• use low-quality Category 3 cable, thus taking
  advantage of large installation base of Category3
  cable in office building

15
Token Ring

• Token ring is the most commonly used MAC protocol
  for ring topology.
• Drawbacks of Ethernet
• Due to the collisions and random delay, it is
  impossible to predict exactly how long it will
  take a system to transmit a frame
• Non-deterministic protocol
• Token ring operates at speeds of 4Mbit/s or
  16Mbit/s.
• Two standard LAN using token ring MAC
• IEEE 802.5
• FDDI(fiber distributed data interface)

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IEEE 802.5

• use of small frame called token
• wait for a token passing by
• seize a token and begin to transmit frames(no
  token on the ring)
• insert a new token in the ring when
• the station has completed transmission of its
  frame
• the leading edge of the transmitted frame has
  returned to the station
• the next station wishing to transmit can seize
  the newly inserted token
• advantage fair use of token(higher traffic
  load)
• disadvantage inefficiency (lower traffic load)

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IEEE 802.5
SD Starting Delimiter ACAccess Control FCFrame
Control DADestination Address SASource
Address FCSFrame Check Sequence EDEnding
Delimiter FSFrame Status

• MAC Frame

octet
1
1
1
2 or 6
2 or 6
gt0
4
1
1
SD
AC
FC
DA
SA
Data unit
FCS
ED
FS
P P P
T
M
R R R
J K I J K I
I
E
A
r r
C
A
C
r r
Access Control Field
Frame Status
Ending Delimiter Field
PPPpriority bits T Token bit M
Monitor bit RRRReservation bits
J, K Nondata bits I Intermediate-frame
bits E Error-detected bit
A Addressed recognized bit C Copied bit
General Frame Format
T0 token 1 frame
SD
AC
FC
Token frame format
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IEEE 802.5

• Example(single-priority case)
• transmitting station
• seize token T0 --gt T1
• token field acts as first two fields of the
  outgoing frame
• when the AC frame of last transmitted frame
  returns T1 -gt T0
• issuing token to the ring
• receiving station
• check passing frames for errors and set E1 if
  error detected
• if its own MAC address A--gt1
• may also copy the frame C--gt1
• A0, C0 destination station nonexistent, not
  active
• A1, C0 destination station exists, but frame
  not copied
• A1, C1 Frame received

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IEEE 802.5

• Token Ring Priority optional
• Eight levels of priority PPP bits
• A station having a higher priority frame to
  transmit than the current frame can reserve the
  next token for its priority level as the frame
  passes by.
• When the next token is issued, it will be at the
  reserved priority level.
• Stations of lower priority level cannot seize the
  token
• The station that upgraded the priority level
  should downgrade it to its former level when all
  higher-priority stations are finished

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IEEE 802.5

• Early Token Release
• lower ring utilization
• A transmitting station should wait until the
  leading edge of the frame returns before issuing
  a token.
• If the frame is shorter than the bit length of
  the ring, then low utilization.
• ETR allows a transmission station to release a
  token as soon as it completes frame transmission,
  whether or not the frame header has returned to
  the station
• Disadvantage
• Priority traffic

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IEEE 802.5

• physical layer medium alternatives

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IEEE 802.5 (Token Ring)

• Today, Token Rings are not built by chaining
  computers together in a ring.
• Instead, physically configured as a star, but
  logically ring.

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FDDI

• A token ring scheme similar to IEEE802.5
• Several differences to accommodate the higher
  data rate
• 100 Mbps
• MAC frame

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FDDI

• Difference from IEEE 802.5
• includes preamble to aid clocking, which is
  demanding at higher data rate
• both 16 , 48 bit addresses are allowed
• does not include priority and reservation bits
• MAC protocol
• similar to IEEE 802.5 , but two key differences
  exists because of higher data rate
• A station waiting for a token seizes the token by
  aborting(failing to repeat) the token
  transmission as soon as the token frame is
  recognized.
• A station that has been transmitting data frames
  release a new token as soon as it completes data
  frame transmission, like ETR in IEEE 802.5
• No use of priority bit

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FDDI

• A awaits token
• A seizes token, begins transmitting frame F1
  addressed to C
• A appends token to end transmission
• C copies frame as it goes by
• C continues to copy F1 B seizes token and
  transmits frame F2 to D
• B emits token D copies F2 A absorbs F1
• A let F2 and token pass B absorbs F2
• B let token pass

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ATM LANs

• Three generations for premises network
• First generation
• focused on CSMA/CD token ling LANs
• terminal-to-host connectivity client/server
  architecture
• Second generation
• focused on FDDI
• backbone LANs high-performance workstations
• Third generation
• focused on ATM LANs
• aggregate throughputs real-time transport
  guarantees

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ATM LANs

• Requirements for a third generation LAN
• Support multiple, guaranteed classes of service
• Provide scalable throughput
• Facilitate the interworking between LAN and WAN
  technology
• ATM LAN is suitable for this requirements
• Using virtual path virtual channel
• multiple classes of services are easily
  accommodated
• Easily scalable
• adding more ATM switching nodes
• using higher data rated devices

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ATM LANs

• Possible types of ATM LANs
• Gateway to ATM WAN
• Act as a router and traffic concentrator
• Backbone ATM switch
• Single ATM switch or local network of ATM
  switches
• Workgroup ATM
• End systems connect directly to an ATM switch
• Practical usage of ATM LAN
• mixture of two or all three of above types of
  network

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ATM LANs

• Example of backbone ATM LAN

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ATM LANs

• Advantages of ATM LAN configuration
• Relatively painless method for inserting a
  high-speed backbone
• Simple to increase the capacity of the backbone
• Shortcomings of previous ATM LAN configuration
• Cant accommodate all local needs of LAN
  structures
• The end systems remain attached to shared-media
  LANs
• go through limitations on data rate imposed by
  shared medium

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ATM LANs

• Advanced approach (ATM hub)

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ATM LANs

• ATM hub approach (cont.)
• Structure
• Each ATM hub includes a number of ports
• Each port operate at different data rate and
  protocols
• consists of a number of rack-mounted modules
• Difference between the previous structure ATM
  hub structure
• Each end system has a dedicated p-to-p link to
  the hub
• (Not by the shared medium)
• Ex. 10-Mbps Ethernet port module
• Use same CSMA/CD protocol but does not share the
  medium
• We can expect more data rate close to the
  maximum 10-Mbps

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ATM LANs

• Advantages of ATM LAN structure
• Existing LAN installations and LAN hardware can
  be used
• Disadvantages
• Mixed-protocol structure
• Requires protocol conversion capability

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FIBRE CHANNEL

• Issuing background
• Growth of data size of application
• Growth of complexity of application
• Growth of data speed which processor handles
• Two method of data communication schemes are
  affected
• I/O channel
• Data path which used device-to-device data
  transfer
• CD-ROM, video I/O devices etc.
• Network communications
• Collection of interconnected access points
• Requires software protocol (flow control, error
  detection, error recov.)

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FIBRE CHANNEL

• Fibre Channel
• Designed to combine best features of both
  technology
• Speed of channel communication
• Flexibility interconnectivity of
  protocol-based network
• Allows system designers to combine every sort
  of current scheme

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FIBRE CHANNEL

• Document on requirements of Fibre Channel FCA94
• Full duplex links with two fibers per link
• Performance from 100 Mbps to 800 Mbps on a single
  link
• Support for distance up to 10km
• Small connectors
• High-capacity utilization with distance
  insensitivity
• Greater connectivity than existing multidrop
  channels
• Broad availability
• Support for multiple cost/performance
  levels(small system super computer)

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FIBRE CHANNEL

• Fibre Channel elements
• Node
• End system
• N_ports (interconnection)
• Fabric
• Collection of switching elements
• F_ports
• Routing between N_ports
• Buffering capacity

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FIBRE CHANNEL

• Difference between other LANs
• Much like traditional circuit-switched network
• Do not share the medium
• Need not Medium Access Control
• Easy to scale (for N_port, data rates, distance)
• Its switching network based
• Easy to accommodate new transmission media
• by adding new N F_port on existing fabric

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FIBRE CHANNEL

• Fibre Channel Protocol Architecture
• Organized into five levels

Defined currently only for FC-0 FC-2 Theres
no final standard for FC-3 FC-4
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WIRELESS LANs

• Wireless LAN Model (developed by IEEE 802.11)

• BSS(basic service set)
• Smallest building block
• Sharing same medium
• Use same MAC protocol
• Consists of some stations
• ESS(extended service set)
• two or more BSS set inter-connected by
  distribution system(e.g. wired backbone LAN)

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WIRELESS LANs

• Physical Medium Specification
• Infrared 1Mbps and 2Mbps (850 950 nm)
• Direct sequence spread spectrum 1Mbps and 2Mbps
  (2.4GHz)
• Frequency-hopping spread spectrum (2.4GHz)

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WIRELESS LANs

• Medium Access Control
• Distributed coordination function (DCF)
• like CSMA/CD (contention based)
• Adapted for ad hoc network
• Point coordination function(PCF)
• Adapted for network which has centralized station
  attached back-bone wired LAN (contention free)
• 802.11 specification
• DCF based
• PCF optional

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WIRELESS LANs

• Distributed Coordination Function (DCF)
• Using simple CSMA algorithm
• Do not render CD function(impractical for
  wireless medium)
• Using IFS (interframe space) as waiting time
  (exponential backoff)