[scrap] WAN Layer 2 Encapsulation

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Interconnecting WANs

> Establishing Serial Point-to-Point Connections

> WAN Layer 2 Encapsulation

WAN Layer 2 Encapsulation

As you move up from the physical layer of the OSI model, serial devices must encapsulate data in a frame format at the data link layer (Layer 2). Different services can use different framing formats.

To ensure that the correct protocol is used, you need to configure the appropriate data link layer encapsulation type. The choice of protocol depends on the WAN technology and communicating equipment. Figure 7-5 shows the protocols that are associated with the three WAN connectivity options.

Figure 7-5. Data Link Layer: WAN Encapsulation Types Based on Connection Type

Typical WAN encapsulation types include the following:

• Cisco High-Level Data Link Control (HDLC or cHDLC)— cHDLC is the default encapsulation type for Cisco routers and is used on point-to-point dedicated links and circuit-switched connections. Cisco HDLC is a proprietary synchronous data link layer protocol typically used when communicating between two Cisco devices. HDLC is covered in more detail later in this chapter.

• Point-to-Point Protocol (PPP)— PPP is a standard protocol that provides router-to-router and host-to-network connections over many physical standards, including synchronous and asynchronous circuits. PPP was designed to work with several network layer protocols, such as IP. It also has built-in security mechanisms such as Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). PPP is covered in more detail later in this chapter.

• Serial Line Internet Protocol (SLIP)— SLIP is a standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.

• X.25/Link Access Procedure, Balanced (LAPB)— LAPB is an International Telecommunications Union Telecommunications Standardization Sector (ITU-T) standard that defines how connections between DTE and DCE are established and maintained for remote terminal access and computer communications over unreliable links. The X.25 specification defines LAPB as its data link layer protocol.

• Frame Relay— Frame Relay is an industry-standard switched data link layer protocol based on ISDN framing technology that handles multiple virtual circuits. Frame Relay is viewed as a successor to X.25, streamlined to eliminate some of the time-consuming processes that were employed in X.25, such as error correction and flow control, that were employed to compensate for older, less-reliable communications links. Frame Relay is covered in more detail in Chapter 8, "Establishing a Frame Relay PVC Connection."

• Asynchronous Transfer Mode (ATM)— ATM is the international standard for cell relay in which multiple service types (such as voice, video, and data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as T3, E3, and Synchronous Optical Network (SONET).