Wide Area Networks Copyright 2005 John Wiley

Wide Area Networks Copyright 2005 John Wiley & Sons, Inc 1

Introduction • Metropolitan area networks (MANs) – Span from 3 to 30 miles and connect backbone networks (BNs) and LANs • Wide area networks (WANs) – Connect BNs and MANs across longer distances, often hundreds of miles or more • Typically built by using leased circuits from common carriers such as AT&T – Most organizations cannot afford to build their own MANs and WANs, Copyright 2005 John Wiley & Sons, Inc 2

Services Used by MANs/WANs • Circuit Switched Network Services • Dedicated Circuit Networks Services • Packet Switched Networks Services • Virtual Private Networks Services Copyright 2005 John Wiley & Sons, Inc 3

Circuit Switched Services • Oldest and simplest MAN/WAN approach • Uses the Public Switched Telephone Network (PSTN) – i. e. , telephone networks • Provided by common carriers like AT&T and Ameritech • Basic types in use today: – POTS (Plain Old Telephone Service) • Via use of modems to dial-up and connect to ISPs – ISDN (Integrated Services Digital Network ) Copyright 2005 John Wiley & Sons, Inc 4

Basic Architecture of Circuit Switched Services “Cloud” architecture Simpler design: What happens inside of network is hidden from the user Can be expensive (connection and traffic based payment) A computer using modem dials the number of a another computer and creates a temporary circuit When session is completed, circuit is disconnected. Copyright 2005 John Wiley & Sons, Inc 5

POTS based Circuit Switched Services • Use regular dial-up phone lines and a modem – Modem used to call another modem – Once a connection is made, data transfer begins • Commonly used to connect to the Internet by calling an ISP’s access point • Wide Area Telephone Services (WATS) – Wholesale long distance services used for both voice and data – Users buy so many hours of call time per month (e. g. , 100 hours per month) for one fixed rate Copyright 2005 John Wiley & Sons, Inc 6

ISDN based Circuit Switched Services • Combines voice, video, and data over the same digital circuit • Sometimes called narrowband ISDN • Provides digital dial-up lines (each requires): – An “ISDN modem” which sends digital transmissions is used • Also called: Terminal Adapter (TA) – An ISDN Network Terminator (NT-1 or NT-2) • Each NT needs a unique Service Profile Identifier (SPID) • Acceptance has been slow – Lack of standardization, different interpretations. and relatively high cost • ISDN: I Still Don’t Know Copyright 2005 John Wiley & Sons, Inc 7

Types of ISDN Services • Basic rate interface (BRI) – Basic access service or 2 B+D • Two 64 Kbps bearer ‘B’ channels (for voice or data) • One 16 Kbps control signaling ‘D’ channel – Can be installed over existing telephones lines (if less than 3. 5 miles) – Requires BRI specific end connections • Primary rate interface (PRI) – Primary access service or 23 B+D • Twenty three 64 Kbps ‘B’ channels • One 64 Kbps ‘D’ channel (basically T-1 service) – Requires T 1 like special circuit Copyright 2005 John Wiley & Sons, Inc 8

Broadband ISDN • A circuit-switched service but it uses ATM to move data • Backwardly compatible with ISDN. • B-ISDN services offered: – Full duplex channel at 155. 2 Mbps – Full duplex channel at 622. 08 Mbps – Asymmetrical service with two simplex channels (Upstream: 155. 2 Mbps, downstream: 622. 08 Mbps) Copyright 2005 John Wiley & Sons, Inc 9

Circuit Switched Services • Simple, flexible, and inexpensive – When not used intensively • Main problems – Varying quality • Each connection goes through the regular telephone network on a different circuit, – Low Data transmission rates • Up to 56 Kbps for POTS, and up to 1. 5 Mbps for ISDN • An alternative – Use a private dedicated circuit • Leased from a common carrier for the user’s exclusive use 24 hrs/day, 7 days/week Copyright 2005 John Wiley & Sons, Inc 10

Dedicated Circuits • Leased full duplex circuits from common carriers • Used to create point to point links between organizational locations – Routers and switches used to connect these locations together to form a network • Billed at a flat fee per month (with unlimited use of the circuit) • Require more care in network design • Basic dedicated circuit architectures – Ring, star, and mesh • Dedicated Circuit Services – T carrier services – Synchronous Optical Network (SONET) services Copyright 2005 John Wiley & Sons, Inc 11

Ring Architecture • Reliability – Messages can be rerouted around the failed link (Data can flow in both directions (full-duplex circuits)) – With the expense of dramatically reduced performance • Performance – Messages need to travel through many nodes before reaching their destination Copyright 2005 John Wiley & Sons, Inc 12

Star Architecture • Easy to manage – Central computer routes all messages in the network • Reliability – Failure of central computer brings the network down – Failure of any circuit or computer affects one site only • Performance – Central computer becomes a bottleneck under high traffic central routing computer Copyright 2005 John Wiley & Sons, Inc 13

Mesh Architectures • Combine performance benefits of ring and star networks • Use decentralized routing, with each computer performing its own routing • Impact of losing a circuit is minimal (because of the alternate routes) • More expensive than setting up a star or ring network. • Setting up alternate routes between computers Full mesh Partial mesh • Expensive, seldom used • More practical Copyright 2005 John Wiley & Sons, Inc 14

T-Carrier Services • Most commonly used dedicated digital circuits in North America • Units of the T-hierarchy – DS-0 (64 Kbps); Basic unit – T-1 (a. k. a. DS-1) (1. 544 Mbps) • Allows 24 simultaneous 64 Kbps channels which transport data or voice messages using PCM – T-2 (6. 312 Mbps) multiplexes 4 T-1 circuits – T-3 (44. 376 Mbps); 28 T-1 capacity – T-4 (274. 176 Mbps); 178 T-1 capacity (672 DS-0 channels) – Fractional T-1, (FT-1) offers a portion of a T-1 Copyright 2005 John Wiley & Sons, Inc 15

T-Carrier Digital Hierarchy T-Carrier Designation DS Designation Data Rate DS-0 64 kbps T-1 DS-1 1. 544 Mbps T-2 DS-2 6. 312 Mbps T-3 DS-3 33. 375 Mbps T-4 DS-4 274. 176 Mbps Copyright 2005 John Wiley & Sons, Inc 16

Synchronous Optical Network (SONET) • ANSI standard for optical fiber transmission in Gbps range – Similar to ITU-T-based, synchronous digital hierarchy (SDH) – SDH and SONET can be easily interconnected • SONET hierarchy – Begins with OC-1 (optical carrier level 1) at 51. 84 Mbps – Each succeeding SONET hierarchy rate is defined as a multiple of OC-1 Copyright 2005 John Wiley & Sons, Inc 17

SONET Digital Hierarchy SONET Designation SDH Designation OC-1 Data Rate 51. 84 Mbps OC-3 STM-1 155. 52 Mbps OC-9 STM-3 466. 56 Mbps OC-12 STM-4 622. 08 Mbps OC-18 STM-6 933. 12 Mbps OC 24 STM-8 1. 244 Gbps OC-36 STM-12 1. 866 Gbps OC-48 STM-16 2. 488 Gbps OC-192 Copyright 2005 John Wiley & Sons, Inc 9. 952 Gbps 18

Packet Switched Services • In both circuit switched and dedicated services – A circuit established between two computers • Solely assigned for use only between these two computers • Data transmission provided only between these two computers • No other transmission possible until the circuit is closed – Packet switched services • Enable multiple connections to exist simultaneously between computers over the same physical circuits • User pays a fixed fee for the connection to the network plus charges for packets transmitted Copyright 2005 John Wiley & Sons, Inc 19

Basic Architecture of Packet Switched Services Packet assembly/ disassembly device (PAD). Owned by the customer or the common carrier Users buy a connection into the common carrier network, and connect via a PAD Point-of-Presence (POP) leased dedicated circuits Copyright 2005 John Wiley & Sons, Inc 20

Packet Switching • Interleave packets from separate messages for transmission – Most data communications consists of short burst of data – Packet switching takes advantage of this burstiness • Interleaving bursts from many users to maximize the use of the shared network Copyright 2005 John Wiley & Sons, Inc 21

Packet Routing Methods • Describe which intermediate devices the data is routed through • Connectionless (Datagram) – Adds a destination and sequence number to each packet – Individual packets can follow different routes – Packets reassembled at destination (by using their sequence numbers) • Connection Oriented (Virtual Circuit (VC)) – Establishes an end-to-end circuit between the sender and receiver (before the packets sent) – All packets for that transmission take the same route over the virtual circuit established – Same physical circuit can carry many VCs Copyright 2005 John Wiley & Sons, Inc 22

Types of Virtual Circuits • Permanent Virtual Circuit (PVCs) – Established for long duration (days or weeks) – Changed only by the network manager – More commonly used – Packet switched networks using PVCs behave like a dedicated circuit networks • Switched Virtual Circuit (SVC) – Established dynamically on a per call basis – Disconnected when the call ends Copyright 2005 John Wiley & Sons, Inc 23

Data Rates of Virtual Circuits • Users specify the rates per PVC via negotiations – Committed information rate (CIR) • Guaranteed by the service provider • Packets sent at rates exceeding the CIR are marked discard eligible (DE), – Discarded if the network becomes overloaded – Maximum allowable rate (MAR) • Sends data only when the extra capacity is available Copyright 2005 John Wiley & Sons, Inc 24

Packet Switched Service Protocols • X. 25 • Asynchronous Transfer Mode (ATM) • Frame Relay • Switched Multimegabit Data Service (SMDS) • Ethernet/IP packet networks Copyright 2005 John Wiley & Sons, Inc 25

X. 25 • • Oldest packet switched service A standard developed by ITU-T Offers SVC and PVC services Uses LAPB and PLP protocols at the data link and network layers, respectively – Requires protocol translations at PADs (for those users who use different protocols at their LANs) • A reliable protocol (it performs error control and retransmits bad packets) • Widely used in Europe • Not in widespread use in North America – Low data rates (64 Kbps) (available now at 2. 048 Mbps) Copyright 2005 John Wiley & Sons, Inc 26

Asynchronous Transfer Mode (ATM) • Newer than X. 25; also standardized • ATM in MAN/WAN similar to ATM technology discussed for BNs • Similar to X. 25 – Provides packet switching service • Different than X. 25: Operating characteristics – Performs encapsulation (no translation) of packets – Provides no error control (an unreliable protocol) – Provides extensive Qo. S information – Scaleable (easy to multiplex ATM circuits onto much faster ones) Copyright 2005 John Wiley & Sons, Inc 27

Error Control in X. 25 vs. ATM Error control in ATM is handled typically the transport layer (providing end-to-end communications) ACKs sent immediately by each node ACKs sent by final destination Copyright 2005 John Wiley & Sons, Inc 28

ATM Features • Uses fixed length, 53 byte “cells” – 5 bytes of overhead and 48 bytes of user data – More suitable for real time transmissions. • Provides extensive Qo. S information – Enables setting of precise priorities among different types of transmissions (i. e. voice, video & e-mail) • Data Rates – Same rates as SONET: 51. 8, 466. 5, 622. 08 Mpbs – New versions: T 1 ATM (1. 5 Mbps), T 3 ATM (45 Mbps) Copyright 2005 John Wiley & Sons, Inc 29

Frame Relay • Another standardized technology • Faster than X. 25 but slower than ATM • Encapsulates packets – Packets delivered unchanged through the network • Unreliable, like ATM – Up to the end-points to control the errors • NO Qo. S support (under development) • Common CIR speeds: – 56, 128, 256, 384 Kbps, 1. 5, 2, and 45 Mbps Copyright 2005 John Wiley & Sons, Inc 30

SMDS • A non-standardized technology – Developed by Telcordia for local phone companies • Unreliable, like ATM • Encapsulates packets • Originally developed for MANs, but could be used for WANs as well • Transmission speeds offered: – 56 Kbps to 45 Mbps • Uncertain future – Not standardized; competition from FR, ATM, and others Copyright 2005 John Wiley & Sons, Inc 31

Ethernet/IP Packet Networks • Offer Ethernet/IP packet services for building MAN/WAN networks – Gigabit Ethernet fiber optic networks (bypassing common carrier network) • Currently offer CIR speeds from 1 Mbps to 1 Gbps at 1/4 the cost of more traditional services • No need to translate LAN protocol (Ethernet/IP) to the protocol used in MAN/WAN services – X. 25, ATM, Frame Relay and SMDS use different protocols requiring translation from/to LAN protocols • Emerging technology; expect changes Copyright 2005 John Wiley & Sons, Inc 32

Virtual Private Networks • Provides equivalent of a private packet switched network over public Internet – Use PVCs (tunnels) that run over the Internet • Appear to the user as private networks – Encapsulate the packets sent over these tunnels • Using special protocols that also encrypt the IP packets they enclose • Provides low cost and flexibility – Uses Internet; Can be setup quickly • Disadvantages of VPNs: – Unpredictability of Internet traffic – Lack of standards for Internet-based VPNs, so that not all vendor equipment and services are compatible Copyright 2005 John Wiley & Sons, Inc 33

VPN Architecture ISP Access Server VPN Device leased circuits Office VPN Device Telephone Line Employee’s Home Internet Backbone VPN Tunnel • VPN is transparent to the users, ISP, and the Internet as a whole; • It appears to be simply a stream of packets moving across the Internet Copyright 2005 John Wiley & Sons, Inc VPN Device Office Backbone 34

Packet from the client computer PPP IP TCP Packet in transmission through the Internet SMTP ATM IP ISP Telephone Line Access Server L 2 TP PPP IP TCP SMTP L 2 TP: Layer 2 Tunneling Protocol (An emerging VPN Layer-2 access protocol) VPN Device Employee’s Home Packet from the VPN Tunnel PPP Outgoing packets from the VPN are sent through specially designed routers or switches. IP TCP SMTP Internet VPN Encapsulation of Packets Copyright 2005 John Wiley & Sons, Inc VPN Device Access Server Backbone 35

VPN Types • Intranet VPN – Provides virtual circuits between organization offices over the Internet • Extranet VPN – Same as an intranet VPN except that the VPN connects several different organizations, e. g. , customers and suppliers, over the Internet • Access VPN – Enables employees to access an organization's networks from remote locations Copyright 2005 John Wiley & Sons, Inc 36

MAN/WAN Design Practices • Difficult to recommend best practices – Services, not products, being bought – Fast changing environment with introduction of new technologies and services from non-traditional companies • Factors used – Effective data rates and cost – Reliability – Network integration • Design Practices – Start with flexible packet switched service – Move to dedicated circuit services, once stabilized – May use both: packet switched services as backup Copyright 2005 John Wiley & Sons, Inc 37

Improving MAN/WAN Performance • Handled in the same way as improving LAN performance – By checking the devices in the network, – By upgrading the circuits between computers – By changing the demand placed on the network Copyright 2005 John Wiley & Sons, Inc 38

Improving Device Performance • Upgrade the devices (routers) and computers that connect backbones to the WAN – Select devices with lower “latency” • Time it takes in converting input packets to output packets • Examine the routing protocol (static or dynamic) – Dynamic routing • Increases performance in networks with many possible routes from one computer to another • Better suited for “bursty” traffic • Imposes an overhead cost (additional traffic) – Reduces overall network capacity – Should not exceed 20% Copyright 2005 John Wiley & Sons, Inc 39

Improving Circuit Capacity • Analyze the traffic to find the circuits approaching capacity – Upgrade overused circuits – Downgrade underused circuits to save cost • Examine why circuits are overused – Caused by traffic between certain locations • Add additional circuits between these locations – Capacity okay generally, but not meeting peak demand • Add a circuit switched or packet switched service that is only used when demand exceeds capacity – Caused by a faulty circuit somewhere in the network • Replace and/or repair the circuit • Make sure that circuits are operating properly Copyright 2005 John Wiley & Sons, Inc 40

Reducing Network Demand • Determine impact on network – Require a network impact statement for all new application software • Use data compression of all data in the network • Shift network usage – From peak or high cost times to lower demand or lower cost times – e. g. , transmit reports from retail stores to headquarters after the stores close • Redesign the network – Move data closer to applications and people who use them – Use distributed databases to spread traffic across Copyright 2005 John Wiley & Sons, Inc 41

Implications for Management • Changing role of networking and telecom managers – Increased and mostly digitized data transmission causing the merger of these positions • Changing technology – Increasing dominance of VPNs, Frame Relay and Ethernet/IP – Decreasing cots of setting up MANs/WANs • Changing vendor profiles – From telecom vendors to vendors with Ethernet and Internet experiences Copyright 2005 John Wiley & Sons, Inc 42