Prior in the past article, I have talked about that HDLC is the default embodiment convention type right on track to-point associations, devoted connections, and circuit-exchanged connections when the connection utilizes two Cisco gadgets. It is a coordinated Data Link layer bit-situated convention initially created from the Synchronous Data Link Protocol (SDLC) and became normalized by ISO as ISO 13239. It gives both association situated and connectionless help. HDLC additionally gives stream control and mistake control by utilizing affirmations. Cisco utilizes a restrictive rendition of the HDLC convention known as Cisco HDLC (cHDLC).

HDLC utilizes simultaneous sequential transmission between two focuses to give mistake free correspondence. It characterizes a Layer 2 outlining structure for stream control and blunder control through utilizing the affirmations. Each HDLC outline has a similar design and organization, regardless of whether it is an information outline or a control outline. For the start and end of edges, HDLC has no technique, so therefore, HDLC utilizes a casing delimiter, or banner, to check the start and the finish of each edge.

Figure 1 outlines standard HDLC outline construction and organization and figure represents cHDLC outline configuration and design. The standard convention support just a single convention, while the Cisco HDLC support multi-convention conditions. Multi-convention is upheld is conceivable because of the convention field in the header, which distinguishes the various conventions.

HDLC Frame Types

As displayed in figure 1 and Figure 2, each casing in HDLC might contain up to six and seven fields, a beginning banner field, address field, control field, data field, outline really look at succession (FCS) field, and a completion banner field. In various edge transmissions, the closure banner of one edge can fill in as the beginning banner of the following casing. The Cisco HDLC (cHDLC) outline contains an additional a convention field.


The banner field of a HDLC outline is a 8-digit grouping with the piece design 01111110 that distinguishing both the beginning and the finish of a casing and fills in as synchronization design for the beneficiary. This casing field starts and ends mistake checking. The 01111110 example happens in the real information, and the sending HDLC framework generally embeds a 0 digit after each five continuous 1s in the information field, so practically speaking, the banner arrangement can happen at the casing closes. The less than desirable end strips out the embedded pieces. At the point when casings are sent over and over, the end banner of the principal outline is utilized as the beginning banner of the following edge.


The location field can be 1 byte or a few bytes in length contain the HDLC address of the optional station. The location can be a gathering address or a transmission address. An essential location is either a correspondence source or an objective, which kills the need to incorporate the location of the essential. Assuming an essential station made the edge, it contains a “to address”. In the event that an optional makes the casing, it contains a “from address”. In the event that the location field is one byte long, it can distinguish up to 128 stations. Bigger organizations require numerous byte address fields.

Control Field

The control field is a 1-or 2-byte long field recognizes the sort of edge including its usefulness, stream control and mistake control. The control field utilizes three distinct organizations, contingent upon the sort of HDLC outline utilized:

Data (I) Frame – I-outlines convey the client information from the upper layer including a few control data like stream control and mistake control. This casing sends and gets grouping numbers, and the survey last piece. The send succession number is the quantity of the edge to be sent straightaway and the get grouping number gives the quantity of the casing close to be gotten. Both source and objective keep up with send and get succession numbers. An essential station utilizes the Poll Final piece to tell the auxiliary whether it requires a prompt reaction.

An optional station utilizes the Poll Final piece to tell the essential whether the current casing is the toward the end in its present reaction. The originally piece of the control field characterizes the kind of the control field. In the event that the principal piece of the control field is 0, this implies the edge is an I-outline. Figure 3 outlines the control field structure. The following 3 pieces known as N(S), distinguish the grouping number of the casing. The last 3 pieces, known as N(R), match to the affirmation number while piggybacking is utilized. The single-piece among N(S) and N(R) is known as the survey Final piece.

Administrative (S) Frame – S-outlines or administrative casing give control data. A S-casing can ask for and suspend transmission, report on status, and recognize receipt of I-outlines. S-outlines don’t have a data field. While piggybacking is either incomprehensible or unseemly, an administrative casing is utilized for stream control and blunder control. Assuming the initial two pieces of the control fields are set to “10”, this implies the casing is a S-outline. The last three pieces known as N(R), characterizes the ACK or NACK, contingent upon the kind of S-outline. The 2 pieces characterize the sort of S-edge and 1 digit is for survey last. The figure-5 outlines the administrative (S) outline.

Unnumbered (U) Frame U-outlines are utilized for connect the executives and including some edge are utilized for the exchange of client information. Contingent upon the capacity of the U-outline, its control field is 1 or 2 bytes. U-outlines trade meeting the board and control data between associated gadgets. The initial 2 pieces (11) characterizes that the casing is a U-outline. The 2 code bits before P/F digit and 3 cycle after P/F bit make various sorts of U-outline. In a couple of cases, a similar encoding is utilized for various things as an order and a reaction.


This casing characterizes that the edge has a place with cHDLC and indicates the convention type embodied inside the casing (for example 0x0800 for IP).


The information outline conveys information from the organization layer. Its length might differ starting with one organization then onto the next.

Outline Check Sequence (FCS)

The casing check grouping is the HDLC mistake recognition field. It can contain be 2 or 4-byte long. It goes before the closure banner delimiter and is by and large a Cyclic Redundancy Check (CRC) estimation leftover portion. The CRC computation is reevaluated in the getting side. In the event that the consequence of review is not quite the same as the worth in the first casing, a blunder is expected.

Designing HDLC Encapsulation

Cisco gadgets utilized cHDLC is the default embodiment technique for simultaneous sequential lines. If interfacing non-Cisco gadgets, utilize coordinated PPP. We can change the default epitome strategy. Assuming that it is changed to some other exemplification type and presently need to change back to HDLC epitome, the accompanying orders can be utilized

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