Below Figure shows the different WAN connection types that can be used to connect your LANs together over a DCE network.
The following list explains the WAN connection types:
Leased lines
Typically referred to as a point-to-point or dedicated connection. It is a pre-established WAN communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting data. It uses synchronous serial lines up to 45Mbps.
Circuit switching
Sets up line like a phone call. No data can transfer before the end-to-end connection is established. Uses dial-up modems and ISDN. It is used for low-bandwidth data transfers.
Packet switching
WAN switching method that allows you to share bandwidth with other companies to save money. Think of packet switching networks as a party line. As long as you are not constantly transmitting data and are instead using bursty data transfers, packet switching can save you a lot of money. However, if you have constant data transfers, then you will need to get a leased line. Frame Relay and X.25 are packetswitching technologies. Speeds can range from 56Kbps to 2.048Mbps.
WAN Support
In this section, we will define the most prominent WAN protocols used today. These are Frame Relay, ISDN, LAPB, HDLC, and PPP. The rest of the chapter will be dedicated to explaining in depth how WAN protocols work and how to configure them with Cisco routers.
Frame Relay
A packet-switched technology that emerged in the early 1990s. Frame Relay is a Data Link and Physical layer specification that provides high performance. Frame Relay assumes that the facilities used are less error prone than when X.25 was used and that they transmit data with less overhead. Frame Relay is more cost-effective than point-to-point links and can typically run at speeds of 64Kbps to 1.544Mbps. Frame Relay provides features for dynamic-bandwidth allocation and congestion control.
ISDN
Integrated Services Digital Network is a set of digital services that transmit voice and data over existing phone lines. ISDN can offer a costeffective solution for remote users who need a higher-speed connection than analog dial-up links offer. ISDN is also a good choice as a backup link for other types of links such as Frame Relay or a T-1 connection.
LAPB
Link Access Procedure, Balanced was created to be used as a connection-oriented protocol at the Data Link layer for use with X.25. It can also be used as a simple Data Link transport. LAPB has a tremendous amount of overhead because of its strict timeout and windowing techniques. You can use LAPB instead of the lower-overhead HDLC if your link is very error prone. However, that typically is not a problem any longer.
HDLC
High-Level Data Link Control was derived from Synchronous Data Link Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC is a connection-oriented protocol at the Data Link layer, but it has very little overhead compared to LAPB. HDLC was not intended to encapsulate multiple Network layer protocols across the same link. The HDLC header carries no identification of the type of protocol being carried inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has their own way of identifying the Network layer protocol, which means that each vendor’s HDLC is proprietary for their equipment.
PPP
Point-to-Point Protocol is an industry-standard protocol. Because many versions of HDLC are proprietary, PPP can be used to create pointto-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol. It allows authentication and multilink connections and can be run over asynchronous and synchronous links.