DC 8 - 1

DATACOMM

John Abbott College JPC

Data Transport Networks

M. E. Kabay, PhD, CISSP

Director of Education, ICSA

President, JINBU Corp

Copyright © 1998 JINBU Corp.

All rights reserved

DC 8 - 2

Data Transport Networks

OSI lower-level functions– Physical layer (1)– Data link layer (2)– Network layer (3)

Key technologies– Local Area Networks (LANs)– Wide-Area Networks / Internetworking

(WANs)– Metropolitan Area Networks (MANs)– Packet-Switching Networks (PSN)

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Local Area Networks

Definition LAN Topologies Baseband vs Broadband Transmission LAN Access Methods Priority and Random Backoff LAN Standards Widely-Used LANs Higher-Speed LANs

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Local Area Networks

A local area network (LAN) is a user-owned communications mechanism linking information-processing and -storage equipment within one building or a cluster of buildings within a circumscribed geographical area.

No absolute distinction between a LAN and a WAN (wide-area network)

LANs evolved because of desire to – share expensive resources– share information

Networks linking dumb terminals to hosts are not considered LANs

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Local Area Networks

Features Continuous connection Interconnectivity Variety of hardware permitted Relatively inexpensive High speeds (2.5-100 Mbps)

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Local Area Networks

LAN Topologies

STAR RING NET/MESH BUS/TREE

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Local Area Networks

Star topology Failure of CPU / Hub downs entire network Performance is function of node at centre Costs largely due to central node

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Local Area Networks

Ring topology Failure of any one node downs entire network Performance declines as # nodes increases

n– P{network failure} = 1 - (1-p)

Relatively low cost

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Local Area Networks

Net/Mesh topology Network survives node failure Performance declines as # nodes increases Higher cost

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Local Area Networks

Bus/Tree topology Network survives node failure Performance declines as nodes increase Medium cost

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Local Area Networks

Baseband vs Broadband Transmission Baseband lower installation cost Broadband higher bandwidth

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Baseband Broadband

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Local Area Networks

LAN Access Methods Devices may accidentally transmit at same

time: collision Most access methods use CSMA (Carrier

Sense Multiple Access)– Will not begin transmitting while another

node is transmitting CSMA/CA (Collision Avoidance)

– If acknowledgement of message not received, node retransmits

– But both nodes wait fairly long

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Local Area Networks

LAN Access Methods CSMA/CD (Collision Detection)

– Nodes can detect collision quickly– Both nodes immediately stop transmitting

when collision occurs Wait a certain amount of time before starting

again

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Local Area Networks

Priority Backoff and Random Backoff In CSMA/CD, what determines when node

starts transmitting again? Priority backoff

– each node waits a fixed amount of time before retransmitting

– short-wait nodes have priority over long-wait nodes

Random backoff– each node waits a random time– equalizes access to network

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Local Area Networks

Token Passing (1)

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Local Area Networks

Token Passing (2)

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Local Area Networks

Token Passing (3)

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Local Area Networks

Token Passing (4)

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Local Area Networks

Token Passing (5)

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Local Area Networks

Token Passing (6)

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Local Area NetworksIEEE LAN Standards 802.1: Encapsulation standards for CSMA/CD 802.2: Logical link protocols 802.3: Broadband & baseband bus using

CSMA/CD 802.4: Broadband and baseband bus using

token passing 802.5: Token-passing rings 802.6: Metropolitan-area networks using

cable TV facilities 802.7: Other broadband systems 802.8: Fibre optics

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Local Area Networks

Widely-Used LANs Ethernet (IEEE 802.3)

– 10 Mbps commonplace (10Base-T)– twisted pair– 100 m max distance between nodes

IBM Token Ring (IEEE 802.5)– 4 or 16 Mbps

Banyan VINES (IEEE 802.5)

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Local Area Networks

Higher-Speed LANs ANSI Fiber Distributed Data Interface (FDDI)

– Fibre optics– 100 Mbps– Similar to IEEE 802.5– Double rings for increased robustness

100Base-T (IEEE 802.3) 100VG-Any-LAN

– IEEE 802.12– Demand priority scheme

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Wide-Area Networks (WANs): Internetworking Definition: an internet is a collection of linked

LANs Ordinary internets are built of

– LANs– Repeaters– Bridges– Routers– Gateways

A WAN is an extension of an internet: the connection of LANs not physically co-located

THE Internet is something else….

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WANs

Repeaters

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

Repeaters on each floor

ThickLAN backbone risers

Fibre optic link under roadway

LANs on each floor

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WANs

Bridges»

10Base-T

Local bridge 100Base-T

Remote bridge

Digital Leased Line

Remote bridge

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

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WANs

Bridges Protocol insensitive Learning

– modify routing table automatically as devices are added

Filtering– discard packets staying on local bus

Forwarding– send packets to right network

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7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

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WANsRouters &

Brouters

Intelligence: can be addressed Requires protocol agreement Can select alternate routes Bridges becoming smarter

– now called brouters

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Montreal

Québec

Halifax

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

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WANs

Gateways

Sometimes called protocol converters Can link LANs with different protocols Especially important in multi-vendor

internetworks; e.g., linking OSI system with SNA network

Multiprotocol switches are hardware Software protocol conversion also common

» 7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

7-Applications6-Presentation5-Session4-Transport3-Network2-Link1-Physical

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WANs

Internetworking Transmission Options Commercial services make internetworking

possible at low cost Switched Multi-Megabit Data Service (SMDS)

– offered by many carriers in Canada / US– connectionless: simply routes packets or

frames Connectionless Broadband Data Service

(CBDS)– popular in Europe

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WANs

Internetworking Transmission Options:

T-carriers (leased lines) T1: 1.544 Mbps 24 voice T1C: 3.152 Mbps 48 voice T2: 6.312 Mbps 96 voice 4 T1 T3: 44 Mbps 672 voice 28 T1 T4: 274 Mbps 4032 voice 168 T1

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WANs

The Internet TCP/IP based internetworking Store-and-forward technology Began as DARPA project in late 1960s Steady expansion during 1970s-80s Explosive growth late 1980s and in 90s Now thought to have several million hosts NOT the “Information Superhighway” More details in Hot Topics course

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WANs

Wireless Data Transport Wireless LANs

– radio– infrared

Broadcast– beepers– stock quotes

Two-way– cellular modems– Cellular Digital Packet Data (CDPD)

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Packet-Switching Networks Public Packet-Switching Networks X.25 PSN Services Routing Data in PSNs Frame Relay Networks

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Packet-Switching Networks Cost of leased lines can be prohibitive for

sporadic use Virtual circuits established for sessions at

low cost Packet Assembler-Disassembler (PAD)

– Links devices to PSN cloud– Data disassembled into packets– Packets routed through PSN cloud– Packets reassembled into data stream

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Packet-Switching Networks

DATA

DA

TA

I/O

PAD

PACKET

DATACOMMOVERHEAD

Destination

Sequence ID

CRC

Route ID

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Packet-Switching Networks

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MontrealNode

HalifaxNode

VancouverNode

Buffers

Processor

Circuits

Packets»

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Packet-Switching Networks

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MontrealNode

HalifaxNode

VancouverNode

Buffers

Processor

Circuits

Packets»

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Packet-Switching NetworksPublic PSNs Widely-available public nodes Charge by kilopacket Datapac (Stentor / Bell Canada) Telenet (SPRINT) Tymnet (MCI) ARPANET (US govt)

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Packet-Switching NetworksCCITT X.25 (“X-and-a-quarter”) Most common standard for PSN Functions divided into 3 levels that

correspond to OSI stack’s lower layers– Physical level: CCITT V.24/V.28 like RS-

232-C– Frame level: LAP-B data link like SDLC– Packet level: network addressing and

routing PAD used to convert asynch to X.25 flow

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Packet-Switching NetworksPSN Services Closed user group Incoming calls only Outgoing calls only Flow-control negotiation

– define packet size, other parms Throughput class negotiation

– define allowable use of bandwidth Reverse charging = collect calls

– like 800 number for datacomm

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Packet-Switching Networks Routing Data in PSNs Virtual circuit unlike telephone call circuit

– Applies to one packet at a time– No user control over how individual

packets reach destination Packets often arrive at destination nodes out

of sequence Destination nodes therefore buffer and

resequence the packets to reconstitute original data stream

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Packet-Switching NetworksFrame Relay Networks X.25 and other PSN have heavy overhead

– designed for analog phone circuits– extensive error correction

Digital circuits much higher reliability, lower noise

Frame Relay drops node-based error checking Functions at OSI layers 1 & 2 (application &

presentation) User systems do their own error-checking and

recovery

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Homework Read Chapter 8 of your textbook in detail,

adding to your workbook notes as appropriate. Review and be prepared to define or expand all

the terms listed at the end of Chapter 8 of your textbook (no hand-in required)

Answer all the exercises on page 187 of the textbook using a computer word-processing program or absolutely legible handwriting (hand in after quiz Monday morning)

Scan Chapters 9 and 10 of your textbook before coming to class on Day 4.