WANs connect remote sites. Connection requirements vary depending on user requirements, cost, and availability.

Transcription

1 WAN Review A WAN makes data connections across a broad geographic area so that information can be exchanged between distant sites. This topic defines the characteristics of a WAN. WAN Overview WANs connect remote sites. Connection requirements vary depending on user requirements, cost, and availability. 2003, Cisco Systems, Inc. All rights reserved. ICND v A WAN is different from a LAN. Unlike a LAN, which connects workstations, peripherals, terminals, and other devices in a single building or other small geographic area, a WAN makes data connections across a broad geographic area. Companies use the WAN to connect various company sites so that information can be exchanged between distant offices. Because the cost of building a global network to connect remote sites can be astronomical, WAN services are generally leased from service providers. You must subscribe to an outside WAN provider to use network resources that your organization does not own. The service provider will use the portion of its network that you leased to transport the information. Note A metropolitan-area network (MAN) leverages the high-speed communication infrastructure built around large cities. A MAN supports higher bandwidth than is typically afforded by a WAN, but is limited in scope to the high-speed infrastructure contained in the metropolitan area. Copyright 2003, Cisco Systems, Inc. Establishing Serial Point-to-Point Connections 5-5

2 WAN Connection Types Some of the WAN connection types available are leased line, circuit-switched, and packetswitched. This topic describes the different WAN connection types and the components that provide the connection. WAN Connection Types: Layer , Cisco Systems, Inc. All rights reserved. ICND v Some of the WAN connection types that you can select are as follows: Leased line: A leased line, also known as a point-to-point or dedicated connection, provides a single, preestablished WAN communications path from the customer premises, through a service provider network, to a remote network. The service provider reserves this connection for private use by the client. Leased lines eliminate the issues that arise with a shared connection, but they are costly. Leased lines are typically employed over synchronous serial connections up to T3 speeds, operating at 45 Mbps. Circuit-switched: Circuit switching is a switching system in which a dedicated circuit path must exist between sender and receiver for the duration of the call. Service provider networks use circuit switching to provide basic telephone service or ISDN. Circuitswitched connections are commonly used in environments that require only sporadic WAN usage. Basic telephone service is typically employed over an asynchronous serial connection. Packet-switched: Packet switching is a WAN switching method in which network devices share a common backbone to transport packets from a source to a destination across a carrier network. Packet-switched networks use virtual circuits (VCs) that provide end-toend connectivity. Programmed switching devices provide the physical connections. Packet headers generally identify the destination. Packet switching offers services similar to those of leased lines; however, the line is shared and the cost of the service is lower. Like leased lines, packet-switched networks are often employed over serial connections with speeds ranging from 56 kbps to T3 speeds (45 Mbps). Cell switching is similar to packet switching, but instead of packets, data is divided into fixed-length cells and then transported across VCs. Cell-switched connections can range in speed from T1 (1.544 Mbps) to DS-3 (45 Mbps) using copper cabling, and up to OC-192 (10 Gbps) using fiber cabling. 5-6 Interconnecting Cisco Network Devices (ICND) v2.1 Copyright 2003, Cisco Systems, Inc.

3 Interfacing Between WAN Service Providers Provider assigns connection parameters to subscriber. 2003, Cisco Systems, Inc. All rights reserved. ICND v When your organization subscribes to an outside WAN service for network resources, the provider assigns your organization the parameters for making the WAN link. Commonly used terms for the main physical parts are as follows: Customer premises equipment (CPE): Devices physically located on subscriber premises. The equipment includes devices that are owned by the subscriber and devices that are leased to the subscriber by the service provider. Demarcation (or demarc): The juncture at which the CPE ends and the local loop portion of the service begins. It often occurs at a telecommunication closet. Local loop (or last-mile ): Cabling (usually copper wiring) that extends from the demarcation point into the WAN service provider central office (CO). CO switch: A switching facility that provides the nearest point of presence (POP) for the provider WAN service. Inside the long-distance toll network are several types of COs. Toll network: The collective switches and facilities (called trunks) of the WAN provider. The call may cross a trunk to a primary center and then go to a sectional center and then to a regional or international carrier center as the call travels the long distance to its destination. Switches operate in provider offices, with toll charges based on tariffs or authorized rates. Copyright 2003, Cisco Systems, Inc. Establishing Serial Point-to-Point Connections 5-7

4 WAN Cabling Cisco routers support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 standards for serial connections. This topic describes the cabling available for WAN connections. Serial Point-to-Point Connections 2003, Cisco Systems, Inc. All rights reserved. ICND v To make a synchronous serial leased-line or packet-switched WAN connection, such as a Frame Relay connection, Cisco devices support the following serial standards: EIA/TIA-232 EIA/TIA-449 V.35 X.21 EIA/TIA-530 When you order the cable, you will receive a shielded serial transition cable that has the appropriate connector for the standard that you specify. The router end of the shielded serial transition cable has a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card (WIC). Because five different cable types are supported with this port, it is sometimes called a 5-in-1 serial port. The other end of the serial transition cable is available with the connector that is appropriate for the standard you specify. The documentation for the device to which you want to connect should indicate the standard used for that device. Your CPE, in this case a router, is the data terminal equipment (DTE). The data circuitterminating equipment (DCE), commonly a modem or channel service unit (CSU) and/or data service unit (DSU), is the device that is used to convert the user data from the DTE into a form acceptable to the WAN service provider. The synchronous serial port on the router is configured as DTE or DCE (except EIA/TIA-530, which is DTE only), depending on the attached cable, which is ordered as either DTE or DCE to match the router configuration. If the 5-8 Interconnecting Cisco Network Devices (ICND) v2.1 Copyright 2003, Cisco Systems, Inc.

5 port is configured as DTE (the default setting), it will require external clocking from the CSU/DSU or other DCE device. Note To support higher densities in a smaller form factor, Cisco has introduced a smart serial cable. The serial end of the smart serial cable is a 26-pin connector that is much smaller than the DB-60 connector used to connect to a five-in-one serial port. These transition cables support the same five serial standards, are available in either DTE or DCE configuration, and are used with the two-port serial and two-port asynchronous and/or synchronous WICs. Copyright 2003, Cisco Systems, Inc. Establishing Serial Point-to-Point Connections 5-9

6 Layer 2 Encapsulation Protocols To encapsulate data for crossing a WAN link, you can choose from a variety of Layer 2 protocols, including High-Level Data Link Control (HDLC), PPP, Serial Line Internet Protocol (SLIP), X.25 and/or Link Access Procedure, Balanced (LAPB), Frame Relay, and ATM. This topic describes the different encapsulation protocols. Typical WAN Encapsulation Protocols: Layer , Cisco Systems, Inc. All rights reserved. ICND v On each WAN connection, data is encapsulated into frames before crossing the WAN link. To ensure that the correct protocol is used, you will need to configure the appropriate Layer 2 encapsulation type. The choice of protocol depends on the WAN technology and the communicating equipment. Typical WAN protocols include the following: HDLC: The Cisco default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections. It is used typically when communicating between two Cisco devices. It is a bit-oriented synchronous data-link layer protocol. (HDLC is covered in more detail later in this lesson.) PPP: Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP was designed to work with several network-layer protocols, such as IP. PPP also has built-in security mechanisms such as Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). SLIP: A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely replaced by PPP. X.25 and/or LAPB: International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data-link layer protocol that manages the communication between DTE and DCE, including packet framing, ordering, and error checking. X.25 is a predecessor to Frame Relay Interconnecting Cisco Network Devices (ICND) v2.1 Copyright 2003, Cisco Systems, Inc.

7 Frame Relay: Industry standard, switched data-link layer protocol that handles multiple VCs. It is a successor to X.25 that is streamlined to eliminate some of the time-consuming processes (such as error correction and flow control) that were employed in X.25 to compensate for older, less-reliable communication links. ATM: The international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. ATM, a cell-switched technology, uses fixed-length cells, which allow processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as T3, E3, and SONET. Note SLIP, X.25, and ATM are not covered further here. Copyright 2003, Cisco Systems, Inc. Establishing Serial Point-to-Point Connections 5-11

8 Summary This topic summarizes the key points discussed in this lesson. Summary A WAN makes data connections across a broad geographic area so that information can be exchanged between distant sites. Some of the WAN connection types available are leased line, circuit-switched, and packet-switched. Cisco routers support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 standards for serial connections. To encapsulate data for crossing a WAN link, you can choose from a variety of Layer 2 protocols, including HDLC, PPP, SLIP, X.25/LAPB, Frame Relay, and ATM. 2003, Cisco Systems, Inc. All rights reserved. ICND v References For additional information, refer to these resources: Cisco IOS Configuration Guides Cisco IOS Command Reference Guides 5-12 Interconnecting Cisco Network Devices (ICND) v2.1 Copyright 2003, Cisco Systems, Inc.