A WAN uses data links, such as ISDN and Frame Relay that are provided by carrier services to access bandwidth over wide-area geographies.
WAN technologies function at the three lowest layers of the OSI reference model.
An overview of the WAN cloud organizes WAN provider services into three main types:
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, it avoids congested facilities and variable delays. Basic telephone service and ISDN use these circuits.
Frame relay -Information contained in frames shares bandwidth with other WAN subscribers. This is statistical multiplexed service, unlike TDM, which uses Layer 2 identifiers and permanent virtual circuits. In addition, it uses Layer 3 routing with sender and receiver addressing contained in the packet.
The following are the most commonly used terms associated with the main parts of WAN services:
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.
A key interface in the customer site occurs between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DTE is the router. The DTE is the end of the user’s device on the WAN link.
The DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility. The DCE is an attached modem, channel service unit/data service unit (CSU/DSU), or terminal adapter/network termination 1 (TA/NT1). The DCE is the end of the WAN service’s side of the communication facility. The DTE/DCE interface acts as a boundary where responsibility for the traffic passes between the WAN subscriber and the WAN provider.
A virtual circuit is a logical circuit, as opposed to a point-to-point circuit, created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over this circuit consists of three phases: circuit establishment, data transfer, and circuit termination. It is used in situations where data transmission between devices is sporadic.
A PVC is a permanently established virtual circuit that consists of one mode: data transfer. These are used in situations where data transfer between devices is constant.
WAN links can be ordered from the WAN provider at various speeds that are stated in bits per second capacity.
WANs use numerous types of devices, including the following:
Routers, which offer many services, including LAN and WAN interface ports.
Modems, which interface voice-grade services. These include CSUs/ DSUs and TA/NT1 devices that interface ISDN services.
Communication servers, which concentrate dial-in and dial-out user communication.
WANs use the OSI reference model layered approach to encapsulation, just as LANs do, but they are mainly focused on the physical and data link layers. WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services. The WAN physical layer also describes the interface between the DTE and the DCE.
The WAN data link layer defines how data is encapsulated for transmission to remote sites. WAN data-link protocols describe how frames are carried between systems on a single data path. The common data-link encapsulations associated with WAN lines are:
Frame relay -- By using simplified encapsulation with no error correction mechanisms over high-quality digital facilities, it can transmit data very rapidly compared to the other WAN protocols.
Point-to-Point -- Described by RFC 1661, it was developed by the IETF. It contains a protocol field to identify the network-layer protocol.
ISDN -- A set of digital services that transmits voice and data over existing phone lines.
Link Access Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. It can also be used over a point-to-point link if the link is unreliable or there is an inherent delay associated with the link, such as in a satellite link. LAPB provides reliability and flow control on a point-to-point basis.
Cisco/IETF -- Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers.
High-Level Data-Link Control (HDLC) -- An ISO standard, HDLC might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations.
The two most common point-to-point WAN encapsulations are HDLC and PPP.
PPP can check for link quality during connection establishment. In addition, there is support for authentication through Password Authentication Protocol and Challenge Handshake Authentication Protocol.
HDLC is 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, also called leased lines, provide full-time service. Dedicated lines typically are used to carry data, voice, and occasionally video. In data network design, dedicated lines generally provide core or backbone connectivity between major sites or campuses, as well as LAN-toLAN connectivity. Dedicated lines generally are considered reasonable design options for WANs.
These are required for dedicated line connections.
a router port
CSU / DSU
actual circuit from the service provider.
Dedicated lines of this type are ideal for high-volume environments with a steady-rate traffic pattern. Dedicated lines also are referred to as point-to-point links because their established path is permanent and fixed for each remote network reached through the carrier facilities.
Packet switching is a WAN switching method in which network devices share a permanent virtual circuit , which is like a point-to-point link to transport packets from a source to a destination across a carrier network.
The following are all examples of packet-switched WAN technologies.
Frame Relay was designed to be used over high-speed, high quality digital facilities.
As a result, Frame Relay does not offer much error checking or reliability, but expects upper-layer protocols to attend to these issues. A data-link connection identifier (DLCI) identifies a PVC. The DLCI number is a local identifier between the DTE and the DCE that identifies the logical circuit between the source and destination devices.
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. ISDN is an example of a circuit-switched WAN technology.
Dial-on-demand routing (DDR) is a technique in which a router can dynamically initiate and close circuit-switched sessions when transmitting end stations need them. When the router receives traffic destined for a remote network, a circuit is established, and the traffic is transmitted normally. DDR enables you to make a standard telephone connection or an ISDN connection only when required by the volume of network traffic. DDR can be used to provide backup load sharing and interface backup. Compared to LAN or campus-based networking, the traffic that uses DDR is typically low volume and periodic.
ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels. Two of the channels, called B (bearer) channels, operate at 64 kbps and are used to carry voice or data traffic. The third channel, the D (delta) channel, is a 16-kbps signaling channel used to carry instructions that tell the telephone network how to handle each of the B channels. ISDN BRI often is referred to as 2B+D.