WAN Frame Encapsulation Format

to ensure the proper protocol is used, the router must be configured with the appropriate layer 2nd encapsulation. The choice of the protocol needs to be determined based on the WAN technology and communication equipment used.

The router encapsulates the packet in two-layer frame format and then transmits it to the WAN link. Although there are several different WAN packages, most have the same principle. This is because most WAN packages evolve from high-level Data link Control (HDLC) and synchronous Data Link Control (SDLC). Although they have similar structures, each data link protocol specifies its own special frame type, and the different types are incompatible.

By default, the Cisco router's serial port encapsulation uses the HDLC protocol. To use a different package, you must configure it manually. The choice of encapsulation protocol relies on the WAN technology and communication devices used. The usual WAN protocols are as follows:

(1) Point-to-Point Protocol (PPP): PPP is a standard protocol that specifies a router-to-router, host-to-network connection on a synchronous or asynchronous circuit.

(2) Serial Line Interconnect protocol (Serial lines Internet protocol,slip): SLIP is the predecessor of PPP and is used for point-to-point serial connections using TCP/IP. Slip has been largely replaced by PPP.

(3) The HDLC:HDLC standard is private, which is the default package type for point-to-point, dedicated links, and circuit-switched connections. HDLC is a bitwise-accessed Synchronous Data Link layer protocol that defines a data encapsulation method that uses frame identification and checksum on a synchronous serial link. PPP Encapsulation (standards-based) is used when connecting routers to different device vendors. HDLC supports both point-to-point and point-to-multipoint connections.

(4) x.25/Balanced Link Access Program (LAPB): X. A is a prototype of frame relay that specifies LAPB as a data Link layer protocol. LAPB is the ITU-T standard that defines how DTE and DCE are connected, maintaining remote terminal access to computer communication on a public data network. The LAPB is used in packet-switched networks to encapsulate packets in the second tier of X. X. * provides extended error detection and sliding window features, because the X. "is implemented on an analog copper circuit with high error rates.

(5) Frame Relay: Frame Relay is a high-performance packet-switched WAN protocol that can be applied to various types of network interfaces. Frame relay is used for higher reliability on digital transmission devices.

(6) ATM:ATM is an international standard for cell exchange, which enables the transmission of a wide variety of service types (such as voice, audio, data) in a fixed-length (53-byte) cell. ATMs are suitable for use in high-speed transmission media such as SONET.

(7) CISCO/IETF: Used for sealing and binding relay traffic. Cisco-defined exclusive options, which can only be used between Cisco routers.

(8) Integrated Services Digital Network (ISDN): A set of digital services that can transmit voice and data through existing telephone lines.  The most commonly used two WAN protocols are HDLC and PPP, all serial line packages share a common frame format, and the frame format has been introduced in the 4th chapter of the Data link layer. Each WAN connection type uses a second-level protocol to encapsulate the data for the WAN link. To ensure that the correct encapsulation protocol is used, the second-tier package type must be configured for each serial interface of the router.

WAN Frame Encapsulation Format