HP X.25/9000 User's Guide HP-UX 11i v3 (5900-1523, August 2011)

The X.25 link provides a BSD to PLP (Packet Level Protocol) translator to allow access to PLP at level
3 for user-written application programs via BSD IPC.
The X.25 link also provides an OTS to PLP translator to allow access to PLP at level 3 for OSI
Services via Xport OSI.
Packet/Network level (L3)
At the packet/network level (L3), X.25 link provides direct X.25 programmatic access via BSD IPC.
For full details on X.25 programmatic access, refer to the X.25/9000 Programmer’s guide (part
number: J2793-90065).
The X.25 link also provides IP access to TCP or UDP Transport protocols for programs such as
Internet/Berkeley Services and NS, allowing communication over X.25 in accordance with RFC
877.
Data link (L2) and Physical (L1) levels
At the data link level (level 2), the X.25 link provides the LAP-B (Link Access Procedure-Balanced)
protocol. LAP-B is a data link protocol, specified by the 1980 CCITT X.25 recommendations, that
determines frame exchange procedures.
At the physical level (level 1), the X.25 link provides support for X.21, X.21bis, and V.35 interfaces
(depending on your particular communications hardware). These sets of recommendations define
the standards for X.25 at the physical level and apply specifically to connections to a packet
switching network. X.21bis (equivalent to V.24 and RS-232) is applicable to X.25 physical interfaces
with transmission speeds up to 64 kb/s. V.35, RS-449, X.21, and RS-530 supports transmission
speeds up to 2 Mb/s. Other standards may also be supported as they become available.
Packet/Network level (L3) 9