Network layer passes the data to the data link layer for transmission over the physical layer. The Data Link layer creates frames by adding the necessary checks and controls around the Network layer data. The encapsulation on a router serial interface must be configured to guarantee the correct encapsulation method is used. Different WAN technologies use different encapsulation methods. So, the selection of WAN encapsulation protocol depends on the WAN technology and the communicating equipment.
Each WAN encapsulation protocols usually accompany a certain connection type. The important WAN encapsulation protocols are Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), Frame Relay, Asynchronous Transfer Mode (ATM), X.25, and Serial Line Internet Protocol (SLIP). HDLC is the most common encapsulation protocol type and most framing protocols are based on it. The following are short descriptions of each type of WAN protocol. In the coming articles, we would discuss the protocols in detail.
High-Level Data Link Control (HDLC)
HDLC is the default encapsulation protocol type on point-to-point connections, dedicated links, and circuit-switched links when the link uses two Cisco devices. It is a synchronous Data Link layer bit-oriented protocol originally developed from the Synchronous Data Link Protocol (SDLC) and became standardized by ISO as ISO 13239. It provides both connection-oriented and connectionless service. HDLC also provides flow control and error control by using acknowledgements. Cisco uses a proprietary version of the HDLC protocol known as Cisco HDLC (cHDLC).
Point-to-Point Protocol (PPP)
Point-to-Point Protocol (PPP) is another WAN encapsulation protocol of the data link layer that is used to send and receive multiprotocol data between two directly connected computers or network devices. PPP is used between synchronous and asynchronous circuits. It supports several network layer protocols, such as IPv4 and IPv6. It uses HDLC encapsulation protocol, but also has built-in security mechanisms such as PAP and CHAP. PPP is a byte-oriented protocol widely used in broadband network communications. PPP is also known as RFC 1661.
Frame Relay is a high-performance WAN encapsulation protocol working at the physical layer and data link layers of the OSI reference model. It was originally designed for use across Integrated Services Digital Network (ISDN) interfaces. It is an industry-standard, packet-switched protocol that handles multiple virtual circuits simultaneously. Frame Relay overcomes the time-consuming processes (such as error correction and flow control) employed in previous X.25 protocol. Today, it is used over a variety of network interfaces.
X.25/Link Access Procedure, Balanced (LAPB)
X.25 is a WAN encapsulation protocol suite defined by ITU-T in 1976 for packet-switched communications over WAN. In 1980 it was the most popular WAN encapsulation protocol. It allows several logical channels to use the same physical line and allow data exchange between terminals with different communication speeds. It defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. It is a predecessor to Frame Relay. It is the oldest packet-switching technique available and was commonly used before the Open System Interconnection (OSI) reference model became standard.
Serial Line Internet Protocol (SLIP)
SLIP uses a standard protocol for point-to-point serial connections using TCP/IP. It is an industry-standard protocol developed in 1984 that supports TCP/IP networking over serial transmission lines. The protocol is used for TCP communication between two machines that are previously configured for communication with each other. The older dial-up connection to the server is typically the example of the serial line. It provides TCP/IP hosts with dial-up access to the Internet by using SLIP servers located at Internet service providers (ISPs).
Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) is an international standard for cell relay in which multiple service types are conveyed in fixed-length cells. The cell length is 53 byte and cells processing is occur in hardware; thereby, reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.