X.25 is the oldest Packet-Switched service, created even earlier than OSI model. It is designed for low data rate and high error-rate transmission facilities, and provides error correction and orderly delivery on hop-by-hop basis. X.25 specification defines the interface between DTE and DCE, while X.75 defines the interface between two x.25 networks.
- X.25 Packet Layer Protocol (PLP)
X.25 PLP manages packet exchange between 2 DTEs across virtual circuits.
Packet layer error handling focuses on resynchronizing the information flow in calls, as well as clearing calls that have gone into unrecoverable states.
General Format Identifier (GFI) – Identifies packet parameters, such as whether the packet carries user data or control information, what kind of windowing is being used, and whether delivery confirmation is required.
Logical Channel Identifier (LCI) – Identifies the virtual circuit across the local DTE/DCE interface.
Packet Type Identifier (PTI) – Identifies the packet as one of 17 different PLP packet types.
Reference URL: http://docwiki.cisco.com/wiki/X.25, https://en.wikipedia.org/wiki/X.25
- X.25 Link Access Layer Details
Both LAPB and LAPD are all originated from HDLC, which has three different operation modes (refer to right picture). LAPB is essentially HDLC in Asynchronous Balanced Mode (ABM), while LAPD is almost exactly the same as LAPB with Control Field (8 Bits) stripped.
(Possibly not correct, need further confirm)
- LAPB PDU:
Packet Data Unit of LAPB is as below, it provides error correction and orderly delivery on per hop basis.
- Addressing of Permanent and Virtual circuit
Virtual circuit is established on as-needed basis, however, permanent circuit is preconfigured into the network. On DTE-DCE interface of X.25 interface, each packet contains 12-bit logical channel identifier which is made up of 8-bit logical channel number and 4-bit logical group number. Logical channel number is only local significant and assigned to a virtual circuit for the duration of the connection.
The range of possible logical channels is split into 4 groups: channels assigned to permanent virtual circuits, assigned to incoming virtual calls, two-way (incoming or outgoing) virtual calls, and outgoing virtual calls. All International networks are required to implement support for permanent virtual circuits, two-way logical channels and one-way logical channels outgoing; one-way logical channels incoming is an additional optional facility.
The logical channel identifier of zero is used for packets which don’t relate to a specific virtual circuit (e.g. packet layer restart, registration, and diagnostic packets).
To be Continued