Overview

By 'X.25' we mean a protocol recommended by CCITT (International Telegraph and Telephone Consultative Committee), part of the ITU (International Telecommunication Union).

X.25 describes an interface between a DTE (Date Termination Equipment) and a DCE (Data Communication Equipment) and by implication a set of DTEs communicating with each other via their respective DCEs which have some unspecified primitive capability to communicate.

X.25 Network Providers.

Several Public Data Networks exist that provide the DCEs and the primitive communications. Some countries furnish such networks and threaten to establish such networks as a data communications monopoly.

Such networks typically have "X.3 pads" which play the role of a DTE to X.25 and provide dial-up access to the net by start-stop terminals.

Tymnet, Uninet and Telenet all provide X.25 DCEs and X.3 PADs.

The official X.25 definition makes no attempt to distinguish mechanism from function. The reader of that document must glean the purpose of an X.25 circuit from the detailed description of how to use the circuit. We have found that to be a hazardous endevour.

This section provides the precise map between the "symbol stream" terminology given in (symbol-stream) and the terminology used in the X.25 recommendation. This information is necessary to integrate the design of two peers, one one whose specifications are in terms of symbol streams and the other in the X.25 terminology.

In this section technical terminology defined by the X.25 recomendation is between square brackets "[]".

Any valid sequence of X.25 [data packets] may be expressed by some symbol stream and any symbol stream may be expressed by some valid sequence of x.25 [data packets].

Bit streams between other symbols are transmitted as a sequence of zero or more [full data packets with the M=1 and D=0] followed by a [partial data packet]. Each of the [packets has Q=1] iff the bit stream is preceeded by a qualifier. [The M bit of the partial packet is 1] iff the stream is followed by a combo mark. [The D bit of the partial record is 1] iff the bit stream is followed by a delivery mark. {Each packet sequence resulting from such a bit stream is a [complete packet sequence]}.

Bit streams between other symbols are transmitted as a sequence of zero or more [full data packets with the M=1 and D=0] followed by a [data packet] LDP. Each of these [packets has Q=1] iff the bit stream is preceeded by a qualifier. If the bit stream was not empty, LDP isn't empty. {It might be full.} [LDP's M bit is 1] iff the stream is followed by a combo mark. [LDP's D bit is 1] iff the bit stream is followed by a delivery mark. {Each packet sequence resulting from such a bit stream is a [complete packet sequence]}.

As each complete packet sequence arrives it is transformed into a bit stream preceeded by a qualifier if the first packet has Q=1, and followed by bare record mark if M=0 & D=0, by a bare delivery mark if M=0 & D=1 or by a combo mark if M=1 & D=1; where M and D are from the last packet.

Nature of Series/1 Interface

The Series/1 interface implemented by CWA for Key Logic uses two levels of X.25 protocol. This makes for economy of concepts but difficulties in specifications. It is difficult to distinguish which level is under discussion in a particular instance.

Section 3. of KL115-02 refers exclusively to the upper, our outer level. Section 5. tells how a particular X.25 session at the outer level is used to access a real DCE and to support HDLC protocol with that DCE. Being careful to use the official CCITT terminology, the mapping between the levels is as follows:

The Information Field from a inner level HDLC frame is mapped to the User Data field of a outer level data packet. The Q and M bits are not used in the outer level packets.

We propose the following pantheon of objects: Port, Channel, Tandem port, Channel set, Interface and Interface Box.
The channel corresponds to X.25's logical channel. The port is a very light weight object that is mapped to a channel for minutes at a time. The tandem port is a collection of ports which is invented for the great economy it brings. The channel set is a slowly varying set of channels which one application owns. The interface corresponds to the X.25 DCE/DTE interface. The interface box corresponds to the collection of interfaces served by one S1.

The relations between these constructs:
A port is a permanent member of just one tandem port.

A port is breifly mapped to just one channel at a time. Only one port is mapped to a channel at one time.

A channel is a long term member of just one channel set.

A channel is permantly bound to just one interface.

A channel set is permantly bound to just one interface box.

An interface is bound to just one interface box.

Channels

A channel is what is identified by a logical channel number within a given DCE/DTE interface. There are no channel keys, merely port keys that are mapped to channels for a time.

X.25 defines these disjoint channel states:

Design Notes:
It is easy for one domain to serve up to 255 port keys. This provides substatnial economies for some applications. A disadvantage of this is that such port keys may not be revoked except by deleting that domain and thus revoking the other port keys served by that domain.

It is also easy to dynamically map such circuit keys onto channels. This may provide improved abstraction and indirect economies.

There are 255 ports associated with each tandem port.

If TP is a key to a tandem port and 1 <= n <= 255 and P is the key to port n of TP then:

TP(0;P=>n) and TP(n;=>0;P),

TP(65536+n;=>0) or TP(65536;P=>c;) causes P to be un-mapped.

Design Notes:
{The nature of PVCs (Permanent Virtual Circuits) dictates that the logical channel numbers across the interface have long term significance. An application will "own" some set of the logical channels for periods of from days to years.

We define, therefore, a set of logical channels. Such a set may span DCE/DTE interfaces that are served by one S1. It is desirable to be able to modify a channel set while it is "in service". This may be un-smooth.

Different channel sets are disjoint. We may choose to say that all channels belong to some channel set in which case we have a partition.

If CS is a channel set key and TP is a key to a tandem port then CS(n;TP=>c;) waits for inbound packets for some unmapped channel in CS and then causes port n of TP to be mapped to that channel.

Calls to move channels from one set to another or split and combine sets. ...

For each X.25 interface (called a "DTE/DCE interface" in the X.25 Recommendation) there is a key XM. The holder of XM may use it to establish policy at that interface and explore mysteries concerning that interface.

There may be several operations in progress at once on XM.

{Interrogate interface state}XM(0=>c)

c is 1, 2 or 3 respectively for interface states r1, r2 or r3 as described in X.25 Annex B. This is the state as determined by actions taken by and observed by the program holding the device key to the S1. It assumes the correct operation of the channel interface, the S1 and the line terminating equipment. It is of use in exploring mysteries at the X.25 interface.
What about states p1 through p7 within r1, and d1 through d3 within p4?

{reset interface}XM(1,((1,Restarting cause),(1,Diagnostic code));=>0)
issues the restart request to the interface. The DTE is left in the r2 state. This operation is never required in normal operation except, perhaps, to clear existing calls so that logical channel limit numbers may be established by the registration process.

The key to this box may be used to initiate the interface to the S1, and re IPLing the S1 and, perhaps configuration.

A shared segment XDB. -- A master domain holding the device key to the X.25 interface. -- A class of server domains that each serve one logical channel for at least minutes at a time. -- A time-out domain.

These domains share read-write assess to XDB. The key called 'X' above is a start key to a server domain. A server domain is devoted to a single logical channel for minutes at a time. Also shared is some structure, perhaps a super node, that stores resume keys to holders of X keys whose invocations are delayed.

I think one server domain could serve many logical channels using databytes.

In XDB there is an array of blocks XB indexed by logical channel number. In an XB are:
a state code,

3 bit out transmission counter,

3 bit in transmission counter,

pointer to newest inbound packet frame,

pointer to oldest inbound packet frame,

pointer to newest outbound packet frame,

pointer to oldest outbound packet frame.

In cooler storage somewhere there is other storage concerned with each logical channel.

The state code serves as as compare and swap interlock. The state code determins:

Whether there are inbound packets,

Whether there are outbound packets,

The various DCE interface states such as: r1, r2, r3; p1 : p7; d1, d2, d3.

States r2, p2, d2, & p6 each indicate an active timeout.
The recommended values are multiples of 20 sec and have a maximum value of 300 sec. There is a timeout domain that runs each 20 seconds and serivces each XCB whose time-out has expired. These are found on a chain that links XCBs expiring together. Thus there an array of 15 time-out chain heads. This array is indexed by CEIL(time/(20 sec)) MOD 15.

Lock Collisions
The only locks are one per XDB. For a particular XDB just the master domain and a particular server will reference the lock.

When a server domain fails to get its lock it is because the master holds it. The server then invokes the master with a NOP operation after which the lock has been freed and the server can try again.

When the master fails to get a lock it is because some server holds it. The master then leaves the lock in a special state to alert the server that the master is waiting. The master then calls the lock domain which merely stores the resume key and becomes ready. When the server releases the lock it calls the lock domain to restart the master.

The master domain holds no locks while it does I/O. The various domains hold locks only for brief times.

Each packet frame has from one to 128 bytes of user data and a pointer to the next packet frame.

The X.25 Interface Partitioner
An X.25 interface partitioner holds the only key to a device key that addresses an X.25 interface. It provides several keys that each behave as a key to an X.25 interface. It also provides a control key that:
controls the partitioning dynamically,

establishes policy concerning call set-up and clearing signals that may traverse the logical channels,

process packets that are not associated with particular logical channels. These are interface reset, diagnostics and registration.

The X.25 Interface Multiplexor
This is an object that holds a device key to an X.25 interface and provides a logical channel interface for each channel thereupon.

Programming the "Signalman LIGHTMAN 24" modem

This is a cheap flexible modem that supports synchronous dialup access. The manual for this modem is very hard to understand. I record here some discoveries on how to use this modem to support dialup synchronous access.

Global Modem States

There are three complete "modem states" stored in the modem: F(Factory), A(Active) and N(Non-Volatile). The active state governs the operation of the modem.

Power up and command ATZ performs the function A:=N (i.e. change the active state to the non-volatile state). Command AT&F performs A:=F. Command AT&W performs N:=A.

This information comes from page 30 of the modem manual.

States for dialup synchronous access
Command ATN0T9226660 stores the string "T9226660" in slot 0 of the directory array whereupon command ATDN0 acts as ATDT9226660. This comes from page 45 of the manual. You can guess as well as I how this generalizes.

After commands AT&M2 (page 28) followed by AT&W the modem is programmed (in its non-volatile state N) to respond to ON transition of DTR by dialing 9226660 as if one had typed ATDT9226660 and then enter synchronous operation upon connection.

The modem can be removed from &M2 mode by turning on the modem when DTR is already on and then commanding AT&M0.

The CTS signal
The CTS (Clear To Send) signal causes the DTE to transmit the SABM (the initial HDLC frame after a reset or new connection).

This modem seems to want to make this signal true when is should be false such as when there is no connection to another modem.

DIP switch 2 must be on lest DCD remain on which, in turn, seems to make CTS true (or least provides a facsimily of an excuese therefore).

Then mode caused by command AT&C1 (page 25) may also be necessary for the same reason.

Even with all of these spells CTS remains true almost all of the time. I thus decided to try wiring DCD (data carrier detect, pin 8) from the modem to CTS (clrear to send, pin 5) of the 370 since that signal seems to come true at about the right time.

At this point the modem fails to produce DSR. I don't know why. This blocks progress. I wired the DTE's DTR to its own DSR. This way when DTR comes up (as the application begins) the DTE sees DSR as an immediate response.

At this point the system behaves as follows as seen by the protocol analyser at the DTE side:

DSR comes on, RTS comes on, a few seconds pass, RTS goes off, then DSR goes off.

This is compatible with the fact that the dialed number is not answering.

The jack nearest the computer cable attaches to the phone company line.