Data Transport Networks

Transcription

1 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 (LANs) Wide-Area Networks / Internetworking () Metropolitan Area Networks (MANs) (PSN) DC 8-3 Definition LAN Topologies Baseband vs Broadband Transmission LAN Access Methods Priority and Random Backoff LAN Standards Widely-Used LANs Higher-Speed LANs DC 8-4 A local area network (LAN) is a user-owned communications mechanism linking informationprocessing 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 Features Continuous connection Interconnectivity Variety of hardware permitted Relatively inexpensive High speeds ( Mbps) LAN Topologies STAR RING NET/MESH BUS/TREE DC 8-5 DC 8-6

2 Star topology Failure of CPU / Hub downs entire network Performance is function of node at centre Costs largely due to central node 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 DC 8-7 DC 8-8 Net/Mesh topology Network survives node failure Performance declines as # nodes increases Higher cost Bus/Tree topology Network survives node failure Performance declines as nodes increase Medium cost DC 8-9 DC 8-10 DC 8-11 Baseband vs Broadband Transmission Baseband lower installation cost Broadband higher bandwidth Baseband Broadband DC 8-12 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

3 LAN Access Methods CSMA/CD (Collision Detection) s can detect collision quickly Both nodes immediately stop transmitting when collision occurs Wait a certain amount of time before starting again 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 longwait nodes Random backoff each node waits a random time equalizes access to network DC 8-13 DC 8-14 Token Passing (1) Token Passing (2) DC 8-15 DC 8-16 Token Passing (3) Token Passing (4) DC 8-17 DC 8-18

4 Token Passing (5) Token Passing (6) DC 8-19 DC 8-20 DC 8-21 IEEE 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 DC 8-22 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) Higher-Speed LANs ANSI Fiber Distributed Data Interface (FDDI) Fibre optics 100 Mbps Similar to IEEE Double rings for increased robustness 100Base-T (IEEE 802.3) 100VG-Any-LAN IEEE Demand priority scheme Wide-Area Networks (): 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. DC 8-23 DC 8-24

5 Repeaters 7-Applications Bridges 7-Applications LANs on each floor 10Base-T Repeaters on each floor Local bridge 100Base-T Remote bridge Fibre optic link under roadway Digital Leased Line ThickLAN backbone risers Remote bridge DC 8-25 DC Applications 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 Routers & Brouters Intelligence: can be addressed Requires protocol agreement Can select alternate routes Bridges becoming smarter now called brouters Montreal 7-Applications Québec DC 8-27 DC 8-28 Halifax Gateways 7-Applications 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 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 DC 8-29 DC 8-30

6 Internetworking Transmission Options: T-carriers (leased lines) T1: Mbps 24 voice T1C: Mbps 48 voice T2: Mbps 96 voice 4 T1 T3: 44 Mbps 672 voice 28 T1 T4: 274 Mbps 4032 voice 168 T1 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 DC 8-31 DC 8-32 Wireless Data Transport Wireless LANs radio infrared Broadcast beepers stock quotes Two-way cellular modems Cellular Digital Packet Data (CDPD) Public X.25 PSN Services Routing Data in PSNs Frame Relay Networks DC 8-33 DC 8-34 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 DATA I/O PAD DATA DATACOMM OVERHEAD Destination Sequence ID CRC Route ID PACKET DC 8-35 DC 8-36

7 Montreal Buffers Processor Montreal Buffers Processor Circuits Packets Circuits Packets Vancouver Halifax Vancouver Halifax DC 8-37 DC 8-38 Public PSNs Widely-available public nodes Charge by kilopacket Datapac (Stentor / Bell Canada) Telenet (SPRINT) Tymnet (MCI) ARPANET (US govt) CCITT 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 DC 8-39 DC 8-40 PSN 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 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 DC 8-41 DC 8-42

8 Frame 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 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. DC 8-43 DC 8-44