Topic 8: WAN- Chapter 11 & 12: Wide Area Networks Business Data Communications, 4e
LANs, WANs, and MANs • Ownership • WANs can be either public or private • LANs are usually privately owned • Capacity • LANs are usually higher capacity, to carry greater internal communications load • Coverage • LANs are typically limited to a single location • WANs interconnect locations • MANs occupy a middle ground
Network Services Available for MAN and WAN • Dialed Circuit Services • Direct Dialing (DD) & Wide Area Telephone Services (WATS) • Dedicated Circuit Services • Voice-grade circuits • Wideband Analog Services • T-Carrier Circuits • Synchronous Optical Network (SONET) • Circuit-Switched Services • Integrated Services Digital Network (Narrowband & Broadband) • Packet-Switched Services • X.25, Frame Relay, ATM, SMDS, and Ethernet/IP
Switching Methods • Circuit Switching: Requires a dedicated communication path for duration of transmission; wastes bandwidth, but minimizes delays • Message Switching: Entire path is not dedicated, but long delays result from intermediate storage and repetition of message • Packet Switching: Specialized message switching, with very little delay
Circuit-Switching • Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes • Dominant in both voice and data communications today • e.g. PSTN is a circuit-switched network • Relatively inefficient (100% dedication even without 100% utilization)
Circuit-Switching Stages • Circuit establishment • Transfer of information • point-to-point from endpoints to node • internal switching/multiplexing among nodes • Circuit disconnect
Circuit Establishment • Station requests connection from node • Node determines best route, sends message to next link • Each subsequent node continues the establishment of a path • Once nodes have established connection, test message is sent to determine if receiver is ready/able to accept message
Information Transfer • Point-to-point transfer from source to node • Internal switching and multiplexed transfer from node to node • Point-to-point transfer from node to receiver • Usually a full-duplex connection throughout
Circuit Disconnect • When transfer is complete, one station initiates termination • Signals must be propagated to all nodes used in transit in order to free up resources
Subscribers Local loop Connects subscriber to local telco exchange Exchanges Telco switching centers Also known as end office >19,000 in US Trunks Connections between exchanges Carry multiple voice circuits using FDM or synchronous TDM Managed by IXCs (inter-exchange carriers) Public Switched Telephone Network (PSTN) Services: 1. Dial-up line 2. Dedicated line
Integrated Service Digital Network (ISDN) • 1st generation: narrowband ISDN • Basic Rate Interface (BRI) • two 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps • circuit-switched • 2nd generation: broadband ISDN (B-ISDN) • Primary Rate Interface (PRI) • twenty-three 64Kbps bearer channels + 64 data channel (23B+D) = 1.536 Mbps • packet-switched network • development effort led to ATM/cell relay
Past Criticism of ISDN • “Innovations Subscribers Don’t Need” , “It Still Doesn’t Network” , “It Still Does Nothing” • Why so much criticism? • overhyping of services before delivery • high price of equipment • delay in implementing infrastructure • incompatibility between providers' equipment. • Didn’t live up to early promises
ISDN Principles • Support of voice and nonvoice using limited set of standard facilities • Support for switched and nonswitched applications • Reliance on 64kbps connections • Intelligence in the networks • Layered protocol architecture (can be mapped onto OSI model) • Variety of configurations
ISDN Network Architecture • Physical path from user to office • subscriber loop, a.k.a. local loop • full-duplex • primarily twisted pair, but fiber use growing • Central office connecting subscriber loops • B channels: 64kbps • D channels: 16 or 64kbps • H channels: 384, 1536, or 1920 kbps
ISDN B Channel • Basic user channel (aka “bearer channel”) • Can carry digital voice, data, or mixture • Mixed data must have same destination • Four kinds of connections possible • Circuit-switched • Packet-switched • Frame mode • Semipermanent
ISDN D Channel • Carries signaling information using common-channel signaling • call management • billing data • Allows B channels to be used more efficiently • Can be used for packet switching
ISDN H Channel • Only available over primary interface • High speed rates • Used in ATM
ISDN Basic Access • Basic Rate Interface (BRI) • Two full-duplex 64kbps B channels • One full-duplex 16kbps D channel • Framing, synchronization, and overhead bring total data rate to 192kbps • Can be supported by existing twisted pair local loops • 2B+D most common, but 1B+D available
ISDN Primary Access • Primary Rate Interface (PRI) • Used when greater capacity required • No international agreement on rates • US, Canada, Japan: 1.544mbps (= to T1) • Europe: 2.048mbps • Typically 23 64kbps B + 1 64kbps D • Fractional use of nB+D possible • Can be used to support H channels
Wide Area Networking Issues • Trend towards distributed processing architectures to support applications and organizational needs. • Expansion of wide area networking technologies and services available to meet those needs. • Dedicated vs. Switched WAN Services
X.25 The oldest packet switched service is X.25, a standard developed by ITU-T. X.25 offers datagram, switched virtual circuit, and permanent virtual circuit services (Data link layer protocol: LAPB (Link Access Procedure-Balanced), network layer protocol PLP). Although widely used in Europe, X.25 is not widespread in North America. The primary reason is transmission speed, now 2.048 Mbps (up from 64 Kbps).
Frame Relay Characteristics • Frame relay is a packet switching technology that transmits data faster than X.25. It differs from X.25 and traditional networks in three important ways: 1. Frame relay only operates at the data link layer. 2. Frame relay networks do not perform error control. 3. Frame relay defines two connection data rate that are negotiated per connection and for each virtual circuit as it is established: Committed information rate (CIR) and Maximum allowable rate (MAR). • Transmission speeds: 56 Kbps to 45 Mbps. • Frame relay lacks of standards.
Asynchronous Transfer Mode (ATM) ATM has four important differences from frame relay: • ATM uses fixed packet lengths of 53 bytes (5 bytes of overhead and 48 bytes of user data), which is more suitable for voice transmissions. • ATM provides extensive quality of service information that enables the setting of very precise priorities among different types of transmissions (i.e. voice, video & e-mail; services include CBR, VBR, ABR & UBR). • ATM is scaleable. It is easy to multiplex basic ATM circuits into much faster ATM circuits. • ATM provides connection-oriented services only.
Virtual Channels & Virtual Paths • Logical connections in ATM are virtual channels • analogous to a virtual circuit in X.25 or a frame relay logical connection • used for connections between two end users, user-network exchange (control signaling), and network-network exchange (network management and routing) • A virtual path is a bundle of virtual channels that have the same endpoints.
Advantages of Virtual Paths • Simplified network architecture • Increased network performance and reliability • Reduced processing and short connection setup time • Enhanced network services
T Carrier Circuits T Carrier circuits are dedicated digital circuits and are the most commonly used form of dedicated circuit services in North America today. Instead of a modem, a channel service unit (CSU) or data service unit (DSU) are used to connect the circuit into the network.
T Carrier Circuits T-1 circuit (a.k.a. a DS-1 circuit) provides a data rate of 1.544 Mbps. T-1’s allow 24 simultaneous 64 Kbps channels (with TDM) which transport data, or voice messages using pulse code modulation. (64Kbps x 24 = 1.536Mbps) T-2 circuit (6.312 Mbps) is basically a multiplexed bundle of four T-1 circuits. T-3 circuit (44.376 Mbps) is equal to the capacity of 28 T-1 circuits (672 64Kbps channels). T-4 circuit (274.176 Mbps) is equal to the capacity of 178 T-1s. Fractional T-1, (FT-1) offers portions of a 1.544 Mbps T-1 for a fraction of its full costs.
T Carrier System T-Carrier Designation DS Designation Speed T-1 T-2 T-3 T-4 DS-0 DS-1 (24 DS-0) DS-2 (96 DS-0) DS-3 (672 DS-0) DS-4 (178 T-1) 64 Kbps 1.544 Mbps 6.312 Mbps 44.375 Mbps 274.176 Mbps
*Digital signal X (DS-x) A term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel. Both the North American T-carrier system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.
*E Carrier Circuits (European Standard) • E1 - 2.048 Mbps (32 DS-0). E1 carries at a higher data rate than T-1 because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use. • E2 - 8.448 Mbps. • E3 - 16 E1 signals, 34.368 Mbps. • E4 - four E3 channels, 139.264 Mbps. • E5 - four E4 channels, 565.148 Mbps.
Synchronous Optical Network (SONET) • An Optical Network for Dedicated Connection Services. • SONET has been accepted by the U.S. Standards Agency (ANSI) as a standard for optical (fiber) transmission at gigabits per second speed. • The International Telecommunications Standards Agency (ITU-T) also standardized a version of SONET under the name of synchronous digital hierarchy (SDH). The two are very similar and can be easily interconnected.
SONET SONET Designation SDH Designation Speed OC-1 OC-3 OC-9 OC-12 OC-18 OC24 OC-36 OC-48 OC-192 51.84 Mbps 155.52 Mbps 466.56 Mbps 622.08 Mbps 933.12 Mbps 1.244 Gbps 1.866 Gbps 2.488 Gbps 9.952 Gbps STM-1 STM-3 STM-4 STM-6 STM-8 STM-12 STM-16
*Switched Multimegabit Data Service (SMDS) Characteristics of SMDS: • Uses ATM-like 53-byte cells, but a different address format. • Provides datagram-based transmission services. So, it is a connectionless service. • Data unit is large enough to encapsulate frames of Ethernet, token ring and FDDI. • An unreliable packet service like ATM and frame relay. Like ATM and frame relay, SMDS does not perform error checking; the user is responsible for error checking. • Speed ranging 56kbps - 44.375Mbps. • Not yet a widely accepted standard. • Its future is uncertain.
*SMDS Network Components SNI: Subscriber network interface CPE: Customer premises equipment
*SMDS Interface Protocol (SIP) SIP is used for communications between CPE and SMDS carrier equipment
Ethernet/IP Packet Network • A MAN/WAN service started in 2000 • X.25, ATM, frame relay and SMDS use traditional PSTN and thus provided by the common carrier such as AT&T and BellSouth. ISP with Ethernet/IP packet service laid their own gigabit Ethernet fiber-optic networks in large cities. • All traffic entering the network must be Ethernet using IP.
Multiprotocol Label Switching (MPLS) • MPLS is a standards-approved technology for speeding up network traffic flow and making it easier to manage. • MPLS sets up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to. • MPLS is called multiprotocol because it works with the IP, ATM, and frame relay network protocols. • MPLS allows most packets to be forwarded at the layer 2 (switching) level rather than at the layer 3 (routing) level. • In addition to moving traffic faster overall, MPLS makes it easy to manage a network for quality of service (QoS).
MPLS Services in the Market In January 1999, AT&T announced the first VPN services to be based on MPLS --- its IP-Enabled Frame Relay service. Cable & Wireless and Cisco Systems conducted a trial of IP-VPN service based on MPLS with Hongkong Telecom in March, 1999. MCI/Worldcom Started to offer MPLS-based IP-VPN service in March, 1999.
*Internet Backbone Networks-- Major companies AT&T Network Services (http://www.ipservices.att.com/backbone/) BBN Planet (GTE) Cable & Wireless USA Sprintlink UUNET, a part of MCI WorldCom