High Speed LANs

Data and Computer Communications

IntroductionIEEE 802.3 standard (Ethernet) cover data rates

of:10 Mbps(Ethernet), 100 Mbps(Fast Ethernet)

CSMA/CD MAC protocol 1Gbps and 10 Gbps (Gigabit Ethernet)

switched technique

Range of technologiesFast and Gigabit Ethernet

Extention of 10 Mbps CSMA/CD to higher speedsFibre Channel switched technique

Provides a low-cost, easy approach to achieve very high data rates

High Speed Wireless LANs

(CSMA/CD : carrier sense multiple access with collision detection)

Why High Speed LANs?Office LANs used to provide basic connectivity

Connecting PCs and terminals to mainframes and midrange systems that ran corporate applications

Providing workgroup connectivity at departmental level

Emphasis on file transfer and electronic mailSpeed and power of PCs has risen

Graphics-intensive applications and GUIsMIS (Management information Systems)

organizations recognize LANs as essential Began with client/server computing

Now dominant architecture in business environment IntranetworksFrequent transfer of large volumes of data 

'MIS' is a planned system of collecting, storing and disseminating data in the form of information needed to carry out the functions of management.

Applications Requiring High Speed LANsCentralized server farms

User needs to draw huge amounts of data from multiple centralized servers(servers farms)

E.g. Color publishing Servers contain tens of gigabytes of image data Downloaded to imaging workstations

Power workgroupsSmall number of cooperating usersDraw massive data files across networkE.g. Software development group testing new software

version or computer-aided design (CAD) running simulations

High-speed local backboneProcessing demand growsLANs grow at siteHigh-speed interconnection is necessary

IEEE802.3 Medium Access ControlCDMA/CD and its ancestor can be termed random access, or contention, techniques.

Random Access Stations access medium randomly

Contention (fight)Stations content for time on medium

Scheme from which CSMA/CD evolved ALOHA / pure ALOHASlotted ALOHACSMA algorithm used: nonpersistent CSMA,

1-persistent protocol, p-persistent

ALOHA / pure ALOHADeveloped for packet radio network

a station can transmit a frame at any timeWhen station has frame, it sends

Station listens (for max round trip time)plus small increment

If ACK, fine. If not, retransmitIf no ACK after repeated transmissions, give upReceiving station:Frame check sequence (as in HDLC)If frame OK and address matches receiver, send ACK

Frame may be damaged by noise or by another station transmitting at the same time (collision)

Any overlap of frames causes collisionIf frame received is determined to be invalid, receiving

station ignores itMax utilization 18%

(collision rises rapidly with increased load)

Slotted ALOHATo prove efficiency, time on the channel is

organized into uniform slots equal to frame transmission time

Need central clock (or other sync mechanism)

Transmission permitted only to begin at slot boundary

Frames either miss or overlap totallyMax utilization 37%

Pure ALOHA vs Slotted ALOHAIn pure Aloha, a user can transmit at any

time but risks collisions with other users' messages.

"Slotted Aloha" reduces the chance of collisions by dividing the channel into time slots and requiring that the user send only at the beginning of a time slot.

CSMA (carrier sense multiple access)A station wishing to transmit first listens to the

medium to determine if another transmission is in progress (carrier sense)

All stations know that a transmission has started almost immediatelyFirst listen for clear medium (carrier sense)If medium idle, transmit

If two stations start at the same instant, collisionWait reasonable time (round trip plus ACK

contention)No ACK then retransmit

No ACK = collision

CSMA (carrier sense multiple access)Max utilization depends on propagation time

(medium length) and frame lengthLonger frame and shorter propagation gives

better utilizationBetter than ALOHA and slotted ALOHA

With CSMA, an algorithm is needed to specify what a station should do if the medium is found busy.nonpersistent CSMA,1-persistent protocol p-persistent

Nonpersistent CSMA- attempts to reduce collision1. If medium is idle, transmit; otherwise, go to 22. If medium is busy, wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

 Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time While another transmission is in progress

If both stations delay same time before retrying, both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

1-persistent CSMA-attempts to reduce idle time To avoid idle channel time, 1-persistent

protocol used Station wishing to transmit listens and

obeys following: 1. If medium idle, transmit; otherwise, go to

step 22. If medium busy, listen until idle; then

transmit immediately 1-persistent stations selfish If two or more

stations waiting, collision guaranteed Gets sorted out after collision

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules:1. If medium idle, transmit with probability p, and

delay one time unit with probability (1 – p) Time unit typically maximum propagation delay

2. If medium busy, listen until idle and repeat step 1

3. If transmission is delayed one time unit, repeat step 1

CSMA/CD With CSMA, collision occupies medium for

duration of transmission Medium unusable waste

CSMA/CD :To reduce waste, stations listen whilst transmitting

1. If medium idle, transmit, otherwise, step 22. If busy, listen for idle, then transmit3. If collision detected, transmit a jamming signal then

cease transmission (backoff)4. After jam, wait random time then start from step

1IEEE 802.3 uses 1-persistent IEEE 802.3 uses 1-persistent algorithm

CSMA/CDOperation

IEEE 802.3 Frame Format

10Mbps Specification (Ethernet)In Ethernet, IEEE defines types of cable, connections, and signals used( physical layer standards)

<data rate><Signaling method><Max segment length>

10Base5 10Base2 10Base-T 10Base-F

Medium Coaxial Coaxial UTP 850nm fiberSignaling Baseband Baseband Baseband Manchester

Manchester Manchester ManchesterOn/Off

Max length500m 185m 100m 500mTopology Bus Bus Star StarNodes 100 30 - 33

100Mbps Fast EthernetUse IEEE 802.3 MAC protocol and frame

format100BASE-X use physical medium

specifications from FDDITwo physical links between nodes

Transmission and reception

Star-wire topologySimilar to 10BASE-T

100Mbps Specification (Ethernet)

In Ethernet, IEEE defines types of cable, connections, and signals used( physical layer standards)

<data rate><Signaling method><Max segment length>

100Base-TX 100Base-TX 100Base-FX 100Base-T4

Medium 2 pair, STP 2 pair, Cat 5 UTP 2 optical fiber 4 pair, cat 3,4,5

Signaling MLT-3 MLT-3 NRZI NRZ

Max length 100m 100m 100m 100m

Data rate 100Mbps 100Mbps 100Mbps 100Mbps

100BASE-T Options

Full Duplex OperationTraditional Ethernet half duplex

Either transmit or receive but not both simultaneouslyWith full-duplex, station can transmit and receive

simultaneously100-Mbps Ethernet in full-duplex mode, theoretical

transfer rate 200 MbpsAttached stations must have full-duplex adapter cards

(traditional Ethernet: half-duplex adapter cards)Must use switching hub (traditional Ethernet: multiport repeater /hub)

Each station constitutes separate collision domainIn fact, no collisionsCSMA/CD algorithm no longer needed802.3 MAC frame format usedAttached stations can continue CSMA/CD

Mixed ConfigurationsFast Ethernet supports mixture of existing 10-

Mbps LANs and newer 100-Mbps LANsE.g. 100-Mbps backbone LAN to support 10-

Mbps hubsStations attach to 10-Mbps hubs using 10BASE-T Hubs connected to switching hubs using 100BASE-T

Support 10-Mbps and 100-MbpsHigh-capacity workstations and servers attach directly

to 10/100 switchesSwitches connected to 100-Mbps hubs using 100-Mbps

links100-Mbps hubs provide building backbone

Connected to router providing connection to WAN

Gigabit Ethernet Configuration

Gigabit Ethernet – PhysicalIn Ethernet, IEEE defines types of cable,

connections, and signals used( physical standards)

1000Base-SXShort wavelength, multimode fiber

1000Base-LXLong wavelength, Multi or single mode fiber

1000Base-CXCopper jumpers <25m, shielded twisted pair

1000Base-T4 pairs, cat 5 UTP

Signaling (encoding scheme) - 8B/10B

Gbit Ethernet Medium Options(log scale)

10Gbps Ethernet - Uses High-speed, local backbone interconnection between

large-capacity switches Server farm Campus wide connectivity Enables Internet service providers (ISPs) and network

service providers (NSPs) to create very high-speed links at very low cost

Allows construction of (MANs) and WANsConnect geographically dispersed LANs between campuses

or points of presence (PoPs) Ethernet competes with ATM and other WAN

technologies 10-Gbps Ethernet provides considerable value over ATM

10Gbps Ethernet - Physical Maximum link distances cover 300 m to 40 km Full-duplex mode only 10GBASE-S (short):

850 nm on multimode fiberUp to 300 m

10GBASE-L (long)1310 nm on single-mode fiberUp to 10 km

10GBASE-E (extended)1550 nm on single-mode fiberUp to 40 km

10GBASE-LX4:1310 nm on single-mode or multimode fiberUp to 10 kmWavelength-division multiplexing (WDM) bit stream across four

light waves

10Gbps Ethernet Distance Options (log scale)

Token Ring (802.5)Developed from IBM's commercial token

ringBecause of IBM's presence, token ring has

gained broad acceptanceNever achieved popularity of EthernetCurrently, large installed base of token ring

products

Ring OperationEach repeater connects to two others via

unidirectional transmission linksSingle closed pathData transferred bit by bit from one repeater to

the nextRepeater regenerates and retransmits each bitRepeater performs data insertion, data

reception, data removalRepeater acts as attachment pointPacket removed by transmitter after one trip

round ring

802.5 MAC ProtocolSmall frame (token) circulates when idleStation waits for tokenChanges one bit in token to make it SOF (start of

frame) for data frameAttach rest of data frameFrame makes round trip and is absorbed by

transmitting stationStation then inserts new token when transmission

has finished and leading edge of returning frame arrives

Under light loads, some inefficiencyUnder heavy loads, round robin

Token RingOperation

Fibre Channel - BackgroundData comms with processor:I/O channel

Direct point to point or multipoint comms linkHardware basedHigh SpeedVery short distanceUser data moved from source buffer to destination

bufferNetwork connection

Interconnected access pointsSoftware based protocolFlow control, error detection &recoveryEnd systems connections

Fibre ChannelBest of both technologiesChannel oriented

Simplicity and speedNetwork oriented

Flexibility and interconnectivity

Fibre Channel RequirementsFull duplex links with two fibers per link100 Mbps to 800 Mbps on single line

Full duplex 200 Mbps to 1600 Mbps per linkUp to 10 kmSupports simple encoding and framing schemeIn turn supports a variety of channel and

network protocols

Fibre Channel ElementsEnd systems - NodesSwitched elements - the network or fabricCommunication across point to point links

Fibre Channel Network

Fibre Channel Protocol Architecture (1)Fibre Channel standard is organized into five levels:FC-0 Physical Media

Optical fiber for long distancecoaxial cable for high speed short distanceSTP for lower speed short distance

FC-1 Transmission Protocol8B/10B signal encoding

FC-2 Framing ProtocolTopologiesFraming formatsFlow and error controlSequences and exchanges (logical grouping of frames)

FC-3 Common ServicesIncluding multicasting

FC-4 MappingMapping of channel and network services onto fibre channel

e.g. IEEE 802, ATM, IP, SCSI

Fibre Channel Physical MediaProvides range of options for physical

medium, the data rate on medium, and topology of network

Shielded twisted pair, video coaxial cable, and optical fiber

Data rates 100 Mbps to 3.2 GbpsPoint-to-point from 33 m to 10 km

Fabric AdvantagesScalability of capacity

As additional ports added, aggregate capacity of network increases

Minimizes congestion and contentionIncreases throughput

Protocol independentDistance insensitiveSwitch and transmission link technologies may

change without affecting overall configurationBurden on nodes minimized

Fibre Channel node responsible for managing point-to-point connection between itself and fabric

Fabric responsible for routing and error detection

Alternative TopologiesPoint-to-point topology

Only two portsDirectly connected, with no intervening

switchesNo routing

Arbitrated loop topologySimple, low-cost topologyUp to 126 nodes in loopOperates roughly equivalent to token ring

Topologies, transmission media, and data rates may be combined

Five Applications of Fibre Channel

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