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HDLC Encapsulation Protocol

HDLC Encapsulation Protocol

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Earlier in the previous article, I have discussed that 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).

HDLC uses synchronous serial transmission between two points to provide error-free communication. It defines a Layer 2 framing structure for flow control and error control through using the acknowledgements. Every HDLC frame has the same structure and format, whether it is a data frame or a control frame. For the beginning and end of frames, HDLC has no method, so for this reason, HDLC uses a frame delimiter, or flag, to mark the beginning and the end of each frame.

Figure 1 illustrates standard HDLC frame structure and format and figure illustrates cHDLC frame format and structure. The standard protocol support only one protocol, while the Cisco HDLC support multi-protocol environments. Multi-protocol is supported is possible due to the protocol field in the header, which identifies the different protocols.

HDLC Frame Types

As shown in figure 1 and Figure 2, each frame in HDLC may contain up to six and seven fields, a starting flag field, address field, control field, information field, frame check sequence (FCS) field, and an ending flag field. In multiple-frame transmissions, the ending flag of one frame can serve as the starting flag of the next frame. The Cisco HDLC (cHDLC) frame contains an extra protocol field.

HDLC Frame

Flag

The flag field of an HDLC frame is an 8-bit sequence with the bit pattern 01111110 that identifying both the starting and the end of a frame and serves as synchronization pattern for the receiver. This frame field initiates and terminates error checking. The 01111110 pattern occurs in the actual data, and the sending HDLC system always inserts a 0 bit after every five consecutive 1s in the data field, so in practice, the flag sequence can only occur at the frame ends. The receiving end strips out the inserted bits. When frames are transmitted repeatedly, the end flag of the first frame is used as the start flag of the next frame.

Address

The address field can be 1 byte or several bytes long contain the HDLC address of the secondary station. The address can be a group address or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary. If a primary station created the frame, it contains a “to address”. If a secondary creates the frame, it contains a “from address”. If the address field is one byte long, it can identify up to 128 stations. Larger networks require multiple-byte address fields.

Control Field

The control field is a 1- or 2-byte long field identifies the type of frame including its functionality, flow control and error control. The control field uses three different formats, depending on the type of HDLC frame used:

  • Information (I) Frame – I-frames carry the user data from the upper layer including some control information like flow control and error control. This frame sends and receives sequence numbers, and the poll final bit. The send sequence number is the number of the frame to be sent next and the receive sequence number provides the number of the frame next to be received. Both source and destination maintain send and receive sequence numbers. A primary station uses the Poll Final bit to tell the secondary whether it requires an immediate response.
  • A secondary station uses the Poll Final bit to tell the primary whether the current frame is the last in its current response. The first bit of the control field defines the type of the control field. If the first bit of the control field is 0, this means the frame is an I-frame. Figure 3 illustrates the control field structure. The next 3 bits known as N(S), identify the sequence number of the frame. The last 3 bits, known as N(R), match to the acknowledgement number when piggybacking is used. The single-bit between N(S) and N(R) is called the poll Final bit.
  • Supervisory (S) Frame –S-frames or supervisory frame provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames. S-frames do not have an information field. Whenever piggybacking is either impossible or inappropriate, a supervisory frame is used for flow control and error control. If the first two bits of the control fields are set to “10”, this means the frame is an S-frame. The last three bits known as N(R), defines the ACK or NACK, depending on the type of S-frame. The 2 bits define the type of S-frame and 1 bit is for poll final. The figure-5 illustrates the supervisory (S) frame.
  • Unnumbered (U) Frame U-frames are used for link management and including some frame are used for the transfer of user data. Depending on the function of the U-frame, its control field is 1 or 2 bytes. U-frames exchange session management and control information between connected devices. The first 2 bits (11) defines that the frame is a U-frame. The 2 code bits before P/F bit and 3 bit after P/F bit create different types of U-frame. In a few cases, the same encoding is used for different things as a command and a response.

Protocol

This frame defines that the frame belongs to cHDLC and specifies the protocol type encapsulated within the frame (e.g. 0x0800 for IP).

Data

The data frame carries data from the network layer. Its length may vary from one network to another.

Frame Check Sequence (FCS)

The frame check sequence is the HDLC error detection field. It can contain be 2 or 4-byte long. It precedes the ending flag delimiter and is generally a Cyclic Redundancy Check (CRC) calculation remainder. The CRC calculation is rechecked in the receiving side. If the result of recheck is different from the value in the original frame, an error is assumed.

Configuring HDLC Encapsulation

Cisco devices used cHDLC is the default encapsulation method for synchronous serial lines. If connecting non-Cisco devices, use synchronous PPP. We can change the default encapsulation method. If it is changed to some other encapsulation type and now want to change back to HDLC encapsulation, the following commands can be used.

  • Router>enable
  • Router#configure terminal
  • Router(config)#interface serial 0/0/0
  • Router(config-if)encapsulation hdlc
  • Router(config-if)exit
  • Router(config)exit
  • Rotuer#wr
WAN Encapsulation Protocols

WAN Encapsulation Protocols

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The 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 will 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 initially developed from the Synchronous Data Link Protocol (SDLC) and became standardized by ISO as ISO 13239. It provides both connection-oriented and connectionless services. HDLC also provides flow control and error control by using acknowledgments. Cisco uses a proprietary version of the HDLC protocol called Cisco HDLC (cHDLC).

Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is another WAN encapsulation protocol of the data link layer that sends and receives 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 and 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

Frame Relay is a high-performance WAN encapsulation protocol working at the OSI reference model’s physical layer and data link layers. It was originally designed 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 the 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 became the most popular WAN encapsulation protocol. It allows several logical channels to use the same physical line and allows 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 previously configured machines. 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 bytes and cell processing occurs in hardware, thereby reducing transit delays. ATM uses high-speed transmission media such as E3, SONET, and T3.