Thursday, August 6, 2009

Cisco HDLC (Why called Cisco HDLC ?)

High-level Data Link Control, a layer-2 (data-link layer) transmission protocol. The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors. For any HDLC communications session, one station is designated primary and the other secondary. A session can use one of the following connection modes, which determine how the primary and secondary stations interact.

Normal unbalanced: The secondary station responds only to the primary station.

Asynchronous: The secondary station can initiate a message.

Asynchronous balanced: Both stations send and receive over its part of a duplex line. This mode is used for X.25 packet-switching networks.

The Link Access Procedure-Balanced (LAP-B) and Link Access Procedure D-channel (LAP-D) protocols are subsets of HDLC.

Q: Now question comes why it is called cisco HDLC? because cisco added some very important features like:

1. Keepalive Mechanism

2. Added serial link address resolution protocol (SLARP), It is used for the purpose of auto-installation like when router boot up so to fine the TFTP server, get the configuration including IP address etc.

I hope it will be informative for you!

3. Added STAC compression, Its a good one but very processor heavy.

4.
Performance Increase (7 - 12 Byte), The header size is increase and this is the beauty of HDLC and works efficiently across WAN.

I Hope this will be informative for You!

4 comments:

nayyares said...

i am not sure if HDLC is still valid in market ?

Sohail Akhtar said...

Yes u r right?
The Industry standard is PPP and yet very cool!
And supported on most connection synchronous (pt-to-pt) and Asynchronous (modem) and is quite feature rictic (Authentication, compression, Multiprotocol encapsulation, Multilink, call-back etc

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