X.25 Protocol Support
X.25 Packet Switched networks allow
remote devices to communicate with each other across high speed digital links without the
expense of individual leased lines. Packet Switching is a technique whereby the network
routes individual packets of HDLC data between different destinations
based on addressing within each packet.
X.25 Overview
The protocol known as X.25
encompasses the first three layers of the OSI 7-layered architecture as
defined by the International Standards Organization (ISO) as follows:
- Layer 1: The Physical Layer is concerned with electrical
or signaling. It includes several standards such as V.35,
RS232 and X.21.
- Layer 2: The Data Link Layer, which is an implementation of the
ISO HDLC standard called Link Access Procedure
Balanced (LAPB) and provides an error free link between two connected devices.
- Layer 3: The Network Layer which provides communications between devices connected to a
common network. In the case of X.25, this layer is referred to as the X.25
Packet Layer Protocol (PLP) and is primarily concerned with network routing
functions and the muliplexing of simultaneous logical connections over a single
physical connection.
The user end of the
network is known as Data Terminal Equipment (DTE) and the carrier's
equipment is Data Circuit-terminating Equipment (DCE). The X.25 PLP
permits a DTE user on an X.25 network to communicate with a number of remote
DTEs simultaneously. Connections occur on logical channels of two types:
- Switched virtual
circuits (SVCs) - SVCs are very much like telephone calls; a connection
is established, data are transferred and then the connection is released. Each
DTE on the network is given a unique DTE address which can be used much like a
telephone number.
- Permanent virtual circuits (PVCs) - a PVC is similar to a leased
line in that the connection is always present. The logical connection is established
permanently by the Packet Switched Network administration. Therefore, data may always
be sent, without any call setup.
To establish a connection on an
SVC, the calling DTE sends a Call Request Packet, which includes the address
of the remote DTE to be contacted.
The destination DTE decides whether
or not to accept the call (the Call Request packet includes the sender's DTE address,
as well as other information that the called DTE can use to decide whether or not to
accept the call). A call is accepted by issuing a Call Accepted packet, or
cleared by issuing a Clear Request packet.
Once the originating DTE receives
the Call Accepted packet, the virtual circuit is established and data transfer may
take place. When either DTE wishes to terminate the call, a Clear Request packet
is sent to the remote DTE, which responds with a Clear Confirmation packet.
The destination for each packet is
identified by means of the Logical Channel Identifier (LCI) or Logical Channel
Number (LCN). This allows the PSN to route the each packet to its intended DTE.
X.25 relies on the underlying
robustness of HDLC LAPB to get data from node to node through
the X.25 network. An X.25 packet makes up the data field of an HDLC frame. Additional
flow control and windowing are provided for each Logical Channel at the X.25 level.
Maximum packet sizes vary from
64 bytes to 4096 bytes, with 128 bytes being a default on most networks. Both maximum
packet size and packet level windowing may be negotiated between DTEs on call set up.
X.25 gives you a virtual high
quality digital network at low cost. It is economical for the same reason that it is
usually cheaper to use the mail than to run your own postal service: there are tremendous
savings to be made if multiple parties share the same infrastructure.
In most parts of the world,
X.25 is paid for by a monthly connect fee plus packet charges. There is usually no
holding charge, making X.25 ideal for organizations that need to be on line all the time.
Another useful feature is speed matching: because of the store-and-forward nature of
Packet Switching, plus excellent flow control, DTEs do not have to use the same line speed.
So you can have, for instance, a host connected at 56kbps communicating with numerous remote
sites connected with cheaper 19.2kbps lines.
X.25 has been around since the mid
1970's and so is pretty well debugged and stable. There are literally no data
errors on modern X.25 networks.
X.25 does have some drawbacks.
There is an inherent delay caused by the store-and-forward mechanism. On most
single networks the turn-around delay is about 0.6 seconds. This has no effect on
large block transfers, but in flip-flop types of transmissions the delay can
be very noticeable. Frame Relay (also called Fast
Packet Switching) does not store and forward, but simply switches to the
destination part way through the frame, reducing the transmission delay considerably.
Another problem for the
networks is a large requirement for buffering to support the store-and-forward data
transfer. One of the reasons that Frame Relay is so cost effective is that storage
requirements are minimal.
X.25 is a data pump: there
has to be some higher level that is making sense of the bits. There are standards
for allowing certain applications to make use of X.25. Among them is IBM's
QLLC protocol that defines how SNA traffic can be carried over X.25 networks.
Another is the asynchronous X.25 PAD. The Gateway 1000
supports asynchronous virtual PAD implementations.
X.25 and TCP/IP are similar
in that they are both packet switched protocols. However, they differ in a
number of areas:
- TCP/IP has only end-to
end error checking and flow control, while X.25 is error checked from node to node.
- TCP/IP has a much more complicated flow control and window mechanism than X.25, to
compensate for the fact that a TCP/IP netork is completely passive.
- The electrical and link levels are tightly specified in the X.25
specifications, while TCP/IP is designed to travel over many different kinds
of media, with many different types of link service (e.g. Ethernet, Frame
relay, X.25, ATM, FDDI etc.).
JBM Electronics X.25 Support
Our units offer the following X.25 support:
- ISO 7776 and ISO 8208 network certified.
- 255 Logical Channels (PVCs and SVCs).
- ITU 1980, 1984 and 1988 implementations.
- Packet size to 4096 bytes.
- Automatic flow control and packet negotiation.
- Higher Level APIs:
- X.3/X.28/X.29 PAD support
- TCP/IP and IPX interfaces
- STREAMS interfaces
- QLLC support
If you need further information
on X.25 protocol or our implementation and support for this protocol,
please e-mail us.
Information in this
document is provided by Sangoma Technologies.
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