Fast LANs

1
Fast LANs

• Rationale
• to solve speed / topology limitations imposed by
  conventional shared media LANs
• Driving force
• More users
• New high speed applications

2
Fast LANs

• Solutions
• Bridging can solve problem to a certain extent
• Better solution is to adopt star/hub topology
• (in contrast to shared media topology)
• Solution adopted by new generation of LANs
• Competing technologies
• In the long term - ATM
• In the short / medium term
• 100VG-AnyLAN (802.12)
• Fast Ethernet (100BaseT)
• More recently Gigabit Ethernet

3
Fast LANs

• Switch based LANs
• Devices operate at device comparable speeds
• No absolute capacity ceiling
• May have concurrent transmissions to different
  destinations
• Expensive, complex and vulnerable

• Shared Media LANs
• Devices must operate at media speed
• Max capacity os media bandwidth
• Distributed control

4
100VG-AnyLAN (802.12)

• Main objectives of 802.12 subcommittee
• Should use Unshielded Twisted Pair (UTP)
• Defined for 10BaseT
• Very commonlu used
• Support new applications
• Compatible with existing LAN software
• Evolutionary technology

5
100VG-AnyLAN (802.12)

• Characteristics
• Allows 802.3/802.5 frame formats
• Can build large networks
• hierarchical topology
• implements two priority classes

6
Fast LANs
7
100VG-AnyLAN (802.12)

• Demand Priority MAC Protocol
• (Consider Single-Hub first)
• Each node is connected to Hub by 4UTP cables
• Transmission of data is spread across all 4 pairs
  _at_ 30Mbps
• Data encoded - 5 data bits across 6 transmission
  bits
• Gives 5/6 X 30Mbps X 4 100Mbps

8
100VG-AnyLAN (802.12)

• Signalling
• Only 2 of 4 pairs used for signalling
• One from station to Hub
• One from Hub to station
• Signalling used by node to get permission to
  transmit

9
100VG-AnyLAN (802.12)

• Gaining Access to Media
• When network is idle, all signalling lines are
  IDLE
• Node sends REQ when it wants to transmit
• Hub gives permission to transmit by turning off
  IDLE signal to requesting node
• Hub simultaneously other stations are alerted by
  INCOMING signal
• These stations respond by turning off IDLE signal
• When transmission begins, Hub reads the
  destination in frame header and relays the
  incoming frame accordingly
• When transmission finished
• Destination returns to IDLE
• Source may return to IDLE or issue another REQ
• Stations which have been unsuccessful will
  reassert REQ immediately
• Note however, destinations must wait for end of
  transmission before reasserting REQ.

10
Fast LANs
11
100VG-AnyLAN (802.12)

• Priority Access
• Needed to satisfy QoS requirements for delay
  sensitive traffic
• Two priority requests
• REQ-N
• REQ-H
• Hub strictly services high priority first, on
  round robin basis
• no pre-emption of normal priority frames
• Two pointers are maintained for queues
• High Next Port pointer
• Normal Next port pointer
• These give next port to service
• Normal requests waiting for gt 250milli secs are
  promoted to high priority

12
100VG-AnyLAN (802.12)

• Multi level configuration
• Connected in a hierarchical topology
• REQs passed onto higher layer
• Ultimately Root hub is responsible for granting
  access
• (via intermediate hubs)
• Essentially stations are searched like a tree

13
Fast LANs
14
100VG-AnyLAN (802.12)

• Training Cycle
• On joining network a training cycle is invoked
• Link quality is checked
• Hub can learn stations MAC address
• Determine which frame format will be used
• 802.3 / 802.5

15
100VG-AnyLAN (802.12)

• Satisfying QoS
• Use of priorities makes it fair
• B/W is shared equally among high priority REQs
• Remainder shared among normal priority REQs
• Delay is deterministic
• Max limit N X T max
• Can limit size of network to give a maximum delay
• B/w allocation strategies are also being
  considered
• Essentially similar to CAC used in ATM
• Negotiate requirements police
• More complex

16
100-Base-T

• Also known as Fast Ethernet
• Upgrade of 10-Base-T
• Star / hub topology
• Each station has its own 4 wire connection to hub
• Assuming full duplex transmission (option)
• only possibility of a collision on station / hub
  link
• 2 or more frames destined for same destination
• IEEE 802.3 frame format

17
100-Base-T

• Being championed by Fast Ethernet Alliance -
• headed by 3Com
• gt 100 companies
• Standardised by IEEE 802.3 committee
• 802.30

18
100-Base-T

• Can use UTP (evolutionary considerations)
• 100-Base-TX - Cat 5 UTP
• 100-Base-T4 - Cat 3,4,5 UTP cat 1 STP
• 100-Base-Fx - Fibre
• Typically
• Star topology
• 100m connections

19
100-Base-T

• Minimum frame length on 802.3 relates station
  being able to detect collisions
• propagation time for 2 X 2500 m safety margin
• gives 50 Mu secs
• 50 Mu secs _at_ 10Mbps 500 bits
• 512 bits set as min frame length
• keep minimum frame length and reduce maximum
  connection length
• increase the transmission rate
• still detect collisions
• Basic philosophy of 100-Base-T

20
100-Base-T

• Max length of connection from station to hub is
  100m
• Network diameter cannot exceed 250 m
• Need new network adapter cards, new hubs
  /switched
• operating at 100 Mbps
• Evolutionary upgrade
• allows mix of 10 100 Mbps connections
• at power up, adapter and hub exchange information
• media
• full / half duplex
• speed

21
Gigabit Ethernet

• Similarities with other Ethernet standards
• Max network diameter is 200m - same as for Fast
  Ethernet
• Doesnt (cant) deliver QoS
• Differences compared with other Ethernet
  standards
• operates at 1 billion bps
• Mac layer is modified
• Needs fibre as medium
• Cat 5 not suitable

22
Gigabit Ethernet

• Further 10 fold increase in bit rate implies
• corresponding decrease in network diameter
• assuming same logic as for Fast Ethernet
• A 20m network diameter is not practical
• Thus a different strategy applied

23
Gigabit Ethernet

• Carrier Extension Mechanism
• 200m network diameter maintained
• Instead minimum frame size increased to 512 bytes
• (from 512 bits)
• If a frame less that 512 bytes is transmitted it
  is padded out by Extended Carrier symbols
• Collisions are detected as with other Ethernet
  standards
• Thus Frame Extended Carrier symbols will last
  for a minimum of 512 bytes

24
Gigabit Ethernet

• Disadvantages of Carrier Extension Mechanism
• Network Utilisation can be low for short frames
  lengths
• Worst Case
• 64 / 512 bytes
• one eighth of 1Gbps
• 125Mbps

25
Gigabit Ethernet

• Frame Bursting
• Another technique designed to overcome the poor
  utilisation
• Shorter frames grouped together to ensure 512
  bytes contains user data
• Not always practical

26
Gigabit Ethernet

• Note 512 bytes is not 10 times 512 bits
• Other less obvious changes made to standard
• No of repeaters allowed has been reduced
• Generous error margins build into slower speed
  Ethernets have been reduced

27
Gigabit Ethernet

• Note, Carrier Extension and Frame Bursting only
  required in half-duplex mode
• Full duplex mode eliminates need for CSMA/CD
• Transmit and Receive on different wires
• Full duplex generally only applied on
  point-to-point arrangements

28
Now Politics

• 100VG-AnyLAN versus 100-Base-T
• HP and few associates versus Fast Ethernet
  Alliance
• Both standarised by IEEE

29
100VG-AnyLAN

• Remember defined for 100Mbps operation
• Can run at 400Mbps - UTP
• HP have trials which take it to 1Gbps
• believe they can operate at 4Gbps
• Advantages
• Has inbuilt priority
• good for satisfying QoS requirements in mixed
  traffic environment
• Disadvantages
• No point-to-point connections,
• essentially shared media

30
Fast Ethernet

• Disadvantages
• No inbuilt priority
• Advantages
• Point-to-point connections
• But Fast Ethernet must be feeling the heat
• Come up with way of implementing priorities
• Priority Access Control Enabled
• makes Ethernet fairer
• allows priorities
• taking this to IEEE for standardisation
• Watch this space next year

31
Other Technologies

• FDDI
• Been around since late 80s
• Dual ring (for reliability)
• ring max 100Km , 2Km max between stations
• 100 Mbps
• Really a MAN technology
• Fast Token Ring

32
Other Technologies

• Fibre Channel Standard (FCS)
• Star topology
• Fibre
• Station to switch connection - max of 10 Km
• 4 classes of service
• class 1 - circuit switched
• time critical
• non-bursty connections
• class 2 - connectionless
• guaranteed delivery
• etc
• Only competitor to ATM

33

• Short Term
• Evolutionary considerations will dictate that it
  will be
• 100VG-AnyLAN versus 100-Base-T
• Long Term
• ATM will rule the telecommunications world
• local and wide are networks