Data Communications

LAN Systems

ALOHAPacket RadioWhen station has frame, it sendsStation listens (for max round trip time)plus small

incrementIf ACK, fine. If not, retransmitIf no ACK after repeated transmissions, give upFrame check sequence (as in HDLC)If frame OK and address matches receiver, send ACKFrame may be damaged by noise or by another

station transmitting at the same time (collision)Any overlap of frames causes collisionMax utilization 18%

Slotted ALOHA

Time in uniform slots equal to frame transmission time

Need central clock (or other sync mechanism)

Transmission begins at slot boundaryFrames either miss or overlap totallyMax utilization 37%

CSMAPropagation time is much less than transmission timeAll stations know that a transmission has started

almost immediatelyFirst listen for clear medium (carrier sense)If medium idle, transmitIf two stations start at the same instant, collisionWait reasonable time (round trip plus ACK

contention)No ACK then retransmitMax utilization depends on propagation time (medium

length) and frame lengthLonger frame and shorter propagation gives better utilization

If Busy?

If medium is idle, transmitIf busy, listen for idle then transmit

immediatelyIf two stations are waiting, collision (1-

persistent algorithm)With non-persistent algorithm, if medium is

busy, go away and come back laterP-persistent algorithm – if medium busy, wait;

when idle, transmit with probability p (backoff with probability 1-p); p = 0.1 or 0.01

CSMA/CD

With CSMA, collision occupies medium for duration of transmission

Stations listen while transmittingIf medium idle, transmitIf busy, listen for idle, then transmitIf collision detected, jam then cease

transmissionAfter jam, wait random time then start again

Binary exponential back off

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 distanceFor twisted pair (star-wired bus) activity on

more than one port is collision

IEEE 802.3 Frame Format

Ethernet (CSMA/CD)

Carrier Sense Multiple Access with Collision Detection

Xerox created Ethernet back in the 70sIEEE 802.3 became the standard802.3 and Ethernet are not exactly the

same

10Mbps Specification (Ethernet)<data rate><Signaling method><Max segment

length>

10Base5 10Base2 10Base-T 10Base-FP

Medium: Coaxial Coaxial UTP 850nm fiberSignaling: Baseband Baseband Baseband Manchester Manchester Manchester Manchester

On/OffTopology: Bus Bus Star-wired bus

Star-wrd busNodes: 100 30 <100 33

100Mbps (Fast Ethernet) 100Base-TX 100Base-FX 100Base-T4

2 pair, Cat 5 UTP 2 optical fiber 4 pair, cat 3,4,54B5B, MLT-3 4B5B, NRZI 8B6T, NRZ

Also known as IEEE 802.3u.

100Base-TX most popular, uses same RJ-45 connectors and pin configuration as 10BaseT.

100Mbps (Fast Ethernet)

EIA/TIA recommends 90m between end-points (to allow for 10m in jumper cables) with 100Base-TX.

100Base-FX can have 412m segments using 62.5/125 micron fiber.

1000 Mbps (Gigabit) Ethernet

IEEE 802.3z standard for fiber, 802.3ab for twisted pair

Data transfers over cables at 1 billion bpsIn 1998, prices per port for 10Mbps - $212;

100Mbps - $432, 1000Mbps - $2,200.In 2003, prices per port for 10Mbps - $12;

100Mbps - $35; 1000Mbps - $100.

Gigabit Ethernet - Potential

Gbps throughput across network backbone between switches

Gbps throughput between backbone switches and file servers

Improved workstation and client/server application performance

Gbps throughput across the client end of a small business network

Gigabit Ethernet – Potential Drawbacks

Different patch cable formats may cause some interconnection difficulties

Network devices that process TCP/IP packets in software as opposed to in chips may create bottleneck situations

Gigabit Ethernet not supported everywhere yet

Need Cat 5e or betterNICs need to be PCI-X or 64-bit bus

architecture

Gigabit Ethernet Configuration

Gigabit Ethernet - Physical

1000Base-SXShort wavelength, multimode fiber

1000Base-LXLong wavelength, Multi or single mode fiber

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

1000Base-T4 pairs, cat 5/5e UTP

Signaling - 8B/10B

Gigabit Ethernet – Other Terms

Backpressure – a flow-control technique that avoids frame loss by impeding external traffic from sending frames to congested interfaces

GBIC (gigabit interface converter) – type of hot-swappable, standards-based transceiver used with switches. It converts electrical signals to optical signals and vice versa, and is usually used for connections on or to the backbone.

VLAN – Virtual LAN – logical grouping of nodes regardless of their physical location

Gigabit Ethernet – Other TermsGVRP (Generic VLAN Registration Protocol) –

A specification defining parameters that switches use to exchange information for registering VLANs on a Spanning-Tree network and to facilitate direct communication

With GVRP, the switch can exchange VLAN configuration information with other GVRP switches, prune unnecessary broadcast and unknown unicast traffic, and dynamically create and manage VLANs on switches connected through 802.1Q trunk ports.

Gigabit Ethernet – Other Terms

IEEE 802.1p – A standard that provides quality of service in Ethernet networks. It relies on packet tags and allows switches to transmit packets in order of priority. These tags are in the 802.1q header.

Possible priorities are 7 – network control; 6 – voice; 5 – video; 0 – best effort

Gigabit Ethernet – Other Terms

IEEE 802.1q – A standard that defines Ethernet frame tags that carry VLAN identifiers

The tag is inserted into the CSMA/CD frame between the SA and Ethertype/Length fields

Tag is 4 bytes – first 2 bytes is Tag Protocol Identifier. Next 3 bits is Priority field (802.1p). Next bit is Canonical Format Indicator (used in Token Ring and FDDI). Final 12 bits are VLAN ID.

Gigabit Ethernet – Other Terms

IEEE 802.3x – A standard that defines Ethernet frame start and stop requests and timers on full-duplex links; flow control dialog

Port mirroring – A transmission method in which frames transmitted and received on one port can be duplicated on another port for diagnostic purposes

Port trunking – A link aggregation technique that creates a single, high-speed logical link from several lower-speed physical links

Token Ring (802.5)MAC protocol

Small frame (token) circulates when idleStation waits for tokenChanges one bit in token to make it SOF for data frameAppend 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 arrivesUnder light loads, some inefficiencyUnder heavy loads, round robin

Token RingOperation

Token Ring MAC Frame

Priority Scheme

802.5 Physical LayerData Rate 4 16 100Medium UTP,STP,FiberSignaling Differential ManchesterMax Frame 4550 18200 18200Access Control TP or DTR TP or DTR DTR

Note: 1Gbit in development (yeah, right)

FDDI

100MbpsLAN and MAN applicationsToken Ring on steroids

FDDI MAC Frame Format

FDDI MAC Protocol

Same as 802.5 except:Station seizes token by aborting token

transmissionOnce token captured, one or more data frames

transmittedNew token released as soon as transmission

finished (early token release in 802.5)

FDDI Operation

FDDI Physical Layer

Medium Optical Fiber Twisted Pair

Data rate 100 100Signaling 4B/5B/NRZI MLT-3Max repeaters 100 100Between repeaters 2km 100m

Fiber Channel - BackgroundI/O channel

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

destination buffer

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

Fiber Channel

Best of both technologiesChannel oriented

Data type qualifiers for routing frame payloadLink level constructs associated with I/O opsProtocol interface specifications to support existing

I/O architecturese.g. SCSI

Network orientedFull multiplexing between multiple destinationsPeer to peer connectivityInternetworking to other connection technologies

Fiber Channel Elements

End systems - NodesSwitched elements - the network or fabricCommunication across point to point links

Fiber Channel Network

Fiber Channel Protocol Architecture (1)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

fiber channele.g. IEEE 802, ATM, IP, SCSI

Fiber Channel Protocol Architecture (2)

If 100 workstations are connected to a CSMA/CD LAN with a 0.01 persistence algorithm, how many stations might try to transmit at the same time?

In what type of scenario would we use CSMA over CSMA/CD?

What is the Pad field (in CSMA/CD) used for?Why does a token ring station take its own

data off the ring?Why is the ED field in token ring after the

CRC?

Review Questions