• High-Level Data Link Control (HDLC) – HDLC is the default encapsulation type on point-to-point, 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.
• Point-to-Point Protocol (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, and IPX. PPP also has built in security mechanisms such as PAP and CHAP
• 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) – ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data line layer protocol. X.25 is a predecessor to Frame Relay.
• Frame Relay – Industry standard, switched data link layer protocol that handles multiple virtual circuits. It is a next-generation 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.
• Asynchronous Transfer Mode (ATM) – ATM is 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. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, Synchronous Optical Network (SONET), and T3.

Configuring HDLC and PPP Encapsulation

• HDLC is a protocol that encapsulates data on synchronous serial data links. While HDLC supports only a single protocol, Cisco&qt;&qt;s proprietary HDLC has a data field to support multiprotocol environments.
• PPP encapsulates network-layer protocol information over point-to-point links. It can be configured on the following physical interface types: asynchronous serial, HSSI, ASDN, and synchronous serial.
• LCP configuration options for PPP include authentication, requiring passwords and challenge handshakes,compression which reduces the amount of data in a frame, error detection which monitors dropped data and fault conditions, and multilink which provides load balancing.
• PPP session establishment has three possible phases:

1. 1.Link Establishment Phase – each PPP device sends LCP packets to configure and test the data link
2. 2.Optional Authentication Phase – authentication of peer
3. 3.Network-Layer Protocol Phase – PPP devices send NCP packets to choose and configure network-layer protocols.

• The two types of PPP authentication protocols are PAP and CHAP.
  PAP provides a simple method for a remote node to establish its identity using a two-way handshake. It is done only upon initial establishment.
  CHAP is used at both the startup of a link and periodically to verify the identity of a remote node using a three-way handshake.
• The four steps in enabling PPP authentication include:

1. Enable PPP encapsulation
2. Assign a hostname
3. Define user name and password
4. Enable PPP authentication

• The commands used to enable PPP encapsulation and authentication include ppp encapsulation, username name password password, and ppp authentication