| A | B |
|---|
| A WAN | a data communications network that operates beyond a LAN's geographic scope. |
| 3 types of WAN provider services | Call setup; Time-division multiplexing (TDM); Frame Relay |
| Call setup | sets up and clears calls between telephone users. Also called signaling, call setup uses a separate telephone channel not used for other traffic. The most commonly used call setup is Signaling System 7 (SS7), which uses telephone control messages and signals between the transfer points along the way to the called destination. |
| Time-division multiplexing (TDM) | Information from many sources has bandwidth allocation on a single medium. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, TDM avoids congested facilities and variable delays. Basic telephone service and ISDN use TDM circuits. |
| Frame Relay | Information contained in frames shares bandwidth with other WAN Frame Relay subscribers. Frame Relay is statistical multiplexed service, unlike TDM, which uses Layer 2 identifiers and permanent virtual circuits. In addition, Frame Relay packet switching uses Layer 3 routing with sender and receiver addressing contained in the packet. |
| WAN Service Providers | Customer premises equipment (CPE); Demarcation (or demarc); Local loop (or "last-mile"); CO switch; Toll network; |
| Customer premises equipment (CPE) | Devices physically located on the subscriber's premises. Includes both devices owned by the subscriber and devices leased to the subscriber by the service provider. |
| Demarcation (or demarc) | The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building. |
| Local loop (or "last-mile") | Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office. |
| CO switch | A switching facility that provides the nearest point of presence for the provider's WAN service. |
| Toll network | The collective switches and facilities (called trunks) inside the WAN provider's cloud. The caller's traffic may cross a trunk to a primary center, then to a sectional center, and then to a regional- or international-carrier center as the call travels the long distance to its destination. |
| (DTE) | data terminal equipment. Typically, the DTE is the router. |
| (DCE) | data circuit-terminating equipment. the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility |
| Two types of virtual circuits | switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). |
| switched virtual circuits (SVCs) | SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. |
| permanent virtual circuits (PVCs) | A PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations where data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but increase costs due to constant virtual-circuit availability. |
| Fundamental WAN devices | 1) Routers, which offer many services, including LAN and WAN interface ports. 2) WAN switches, which connect to WAN bandwidth for voice, data, and video communication. 4) Modems, which interface voice-grade services. Modems include CSUs/ DSUs and TA/NT1 devices that interface ISDN services. 5) Communication servers, which concentrate dial-in and dial-out user communication. |
| WAN switch | a multiport networking device, which typically switches such traffic as Frame Relay, X.25, and Switched Multimegabit Data Service (SMDS). WAN switches typically operate at the data link layer of the OSI reference model. |
| modem | a device that interprets digital and analog signals by modulating and demodulating the signal, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form. |
| CSU/DSU | a digital-interface device -- or sometimes two separate digital devices -- that adapts the physical interface on a DTE device (such as a terminal) to the interface of a DCE device (such as a switch) in a switched-carrier network. |
| ISDN Terminal Adapter (TA) | a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces. |
| six WAN data link encapsulations | Frame Relay; Point-to-Point Protocol (PPP); ISDN; Link Access Procedure, Balanced (LAPB);Cisco/IETF; High-Level Data Link Control (HDLC) |
| two most common point-to-point WAN encapsulations | HDLC and PPP |
| PPP | a standard serial-line encapsulation method (described in RFC 1332 and RFC 1661). This protocol can, among other things, check for link quality during connection establishment. In addition, there is support for authentication through Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). |
| HDLC | a data link-layer protocol derived from the Synchronous Data Link Control (SDLC) encapsulation protocol. HDLC is Cisco's default encapsulation for serial lines. |
| Dedicated lines | Dedicated lines, also called leased lines, provide full-time service. |
| Packet-switched connections | Packet switching is a WAN switching method in which network devices share a permanent virtual circuit (PVC), which is like a point-to-point link to transport packets from a source to a destination across a carrier network, as shown in the Figure. Frame Relay, SMDS, and X.25 are all examples of packet-switched WAN technologies. Frame Relay is a packet-switching data communications technology that can connect multiple network devices on a multipoint WAN |
| Circuit-switched connections | Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Used extensively in telephone company networks, circuit switching operates much like a normal telephone call. ISDN is an example of a circuit-switched WAN technology. |
| DDR | Dial-on-demand routing. Technique whereby a Cisco router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem. |
| ISDN devices | Terminal Equipment 1 (TE1); Terminal Equipment 2 (TE2); TA; NT Type 1 (NT1); NT Type 2 (NT2) |
| Terminal Equipment 1 (TE1) | Designates a device that is compatible with the ISDN network. A TE1 connects to an NT of either Type 1 or Type 2. |
| Terminal Equipment 2 (TE2) | Designates a device that is not compatible with ISDN and requires a TA. |
| TA | Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network. |
| NT Type 1 (NT1) | Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility. |
| NT Type 2 (NT2) | Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating. |
