Fast LANs

• Rationale– to solve speed / topology limitations imposed

by conventional shared media LANs

• Driving force– More users

– New high speed applications

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

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

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

100VG-AnyLAN (802.12)

• Characteristics– Allows 802.3/802.5 frame formats

– Can build large networks

• hierarchical topology

– implements two priority classes

Fast LANs

Root hub

100VG AnyLAN Four Port Hub - Root

Storage 100VG intermediate hub

3 port device

ServerServer w/sw/sw/s

w/sw/s

4 port 100VG hub

Figure 1. 100VG-AnyLAN example topology.

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 @ 30Mbps

– Data encoded - 5 data bits across 6 transmission bits

– Gives 5/6 X 30Mbps X 4 = 100Mbps

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

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.

Fast LANs

Hub

Source Dest

REQ

(a) Remaining signalling lines IDLE

Hub

Source Dest

REQ

Hub

Source Dest

Silence; Incoming(b) (c)

Frame

Start of frame transmission

Hub

Source Dest

Hub

Source Dest

Hub

Source Dest

Frame Frame Frame

(d) Address known- relay frame. (e) Silence; (f) Frame relayed - all lines IDLE

Figure 2. The Demand Priority MAC Protocol - single layer hierarchy

Indicates Silence;

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 > 250milli secs are promoted to high priority

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

Fast LANs

DTE

Route Hub

Intermediate Hub Intermediate Hub

Intermediate Hub

DTE DTE DTE

DTEDTE

DTEDTE DTE

DTE

1 2 3 4 5 6

2.1 2.2 2.3 2.4

2.3.1 2.3.2 2.3.3 2.3.4

5.1 5.2 5.3 5.4

DTE DTE DTE

n/c

n/c

Normal Servicing Order :1 ,2.1, 2.3, 2.3.1, 2.3.3, 2.3.4, 2.4, 3, 4, 5.1, 5.2, 5.3, 5.4, 6.

Figure 3. Service order in a multi-level hierarchical 802.12 LAN.

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

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

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

100-Base-T

• Being championed by Fast Ethernet Alliance - – headed by 3Com

• > 100 companies

• Standardised by IEEE 802.3 committee– 802.30

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

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 @ 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

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

Gigabit Ethernet

– Similarities with other Ethernet standards

• Max network diameter is 200m - same as for Fast Ethernet

• Doesn’t (can’t) 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

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

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

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

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

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

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

Now Politics

• 100VG-AnyLAN versus 100-Base-T

• HP and few associates versus Fast Ethernet Alliance

• Both standarised by IEEE

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

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

Other Technologies

• FDDI– Been around since late 80’s

– Dual ring (for reliability)

– ring max 100Km , 2Km max between stations

– 100 Mbps

– Really a MAN technology

• Fast Token Ring

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

• 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