D-Link DI-1750 Reference Manual page 59

The frame relay exchange will be used while the entire flow that received by a DLCI can be sent to a
similar next-skip address by another DLCI. In this condition, it is no need to examine the frame one by
one for confirm the target address, and thereby reduce the load of Router.
5.1.3 LAPB and X.25
X.25 is one of a group of specifications published by the International Telecommunication Union
Telecommunication Standardization Sector (ITU-T); these specifications are international standards
that are formally called Recommendations. The ITU-T Recommendation X.25 defines how connections
between data terminal equipment (DTE) and data communications equipment (DCE) are maintained
for remote terminal access and computer communications. The X.25 specification defines protocols for
two layers of the OSI reference model. The data link layer protocol defined is LAPB. The network layer
is sometimes called the packet level protocol (PLP), but is commonly (although less correctly) referred
to as "the X.25 protocol."
The ITU-T updates its Recommendations periodically. The specifications dated 1980 and 1984 are the
most common versions currently in use. Additionally, the International Standards Organization (ISO)
has published ISO 7776:1986 as an equivalent to the LAPB standard, and ISO 8208:1989 as an
equivalent to the ITU-T 1984 X.25 Recommendation packet layer. D-Link X.25 software follows the
ITU-T 1984 X.25 Recommendation, except for its Defense Data Network (DDN) and Blacker Front End
(BFE) operation, which follow the ITU-T 1980 X.25 Recommendation.
Note:
The ITU-T carries out the functions of the former Consultative Committee for International Telegraph and Telephone (CCITT).
The 1988 X.25 standard was the last published as a CCITT Recommendation. The first ITU-T Recommendation is the 1993
revision.
In addition to providing remote terminal access, our X.25 software provides transport for LAN
protocols—IP transmission.
Briefly, the D-Link X.25 software provides the following capabilities:
♦ LAPB datagram transport--LAPB is a protocol that operates at Level 2 (the data link layer) of the
OSI reference model. It offers a reliable connection service for exchanging data (in units called
frames) with one other host.
♦ X.25 datagram transport--X.25 can establish connections with multiple hosts; these connections
are called virtual circuits. Protocol datagram (IP) is encapsulated inside packets on an X.25 virtual
circuit. Mappings between a host's X.25 address and its datagram protocol addresses allow these
datagrams to be routed through an X.25 network, thereby allowing an X.25 public data network
(PDN) to transport LAN protocols.
♦ X.25 switch--X.25 calls can be routed based on their X.25 addresses either between serial
interfaces on the same router to another router.
♦ PAD--User sessions can be carried across an X.25 network using the Packet Assembly and
Disassembly (PAD) protocols defined by the ITU-T Recommendations X.3 and X.29.
♦
D-Link X.25 implementation does not support fast switching.
5.1.4 HDLC
The High Level Data Link Control (HDLC) protocol is based on IBM's SNA (System Network
Architecture) SDLC (Synchronous Data Link Control) protocol. IBM submited SDLC to ANSI and ISO
after development completed and the protocol separate into U.S. standard and international standard.
ANSI modified the protocol into ADCCP (Advanced Data Communication Control Procedure); ISO
modified the protocol into HDLC (Synchronous Data Link Control). CCITT then adopted and modified
HDLC into Link Access Procedure (LAP) and subsequently become a part of X.25 interface standard.
HDLC is bit-oriented protocol that ensures the transparent of data by bit filling.
HDLC is a protocol of data packet that defined a link encapsulation of IP packet on synchronous line
and runTCP/IP on point-to-point serial line.
Usually, HDLC is use for DDN line with the feature of simplness and high efficiency.
Model Name
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