The Econet-X25 Gateway
http://acorn.chriswhy.co.uk/docs/Acorn/Misc/Acorn_TheEconet-X25Gateway.pdf

Introduction
------------
The Econet-X25 gateway (hereafter referred to as the gateway) is a specific
item of hardware connected between an Econet and X25 packet switching
network with appropriate software for supporting calls between the two
networks. This will give Econet users remote terminal access to host
computers attached to the X25 network, and provide the necessary link
between the two types of networks for file transfer. Any other applications
requiring connections between the two networks should be able to use the
gateway. The gateway will equally well connect to a host computer with an
X25 connection as to an X25 network.

Use of the Gateway
------------------
It is not envisaged that the initial gateway hardware will perform any other
task than providing a gateway between Econet and X25 networks. In other
words, no application can run in the gateway, with the exception of any
configuration, monitoring and control utilities that may be found desirable.
This does not preclude a re-engineering of the gateway to provide a
selfcontained BBC micro with X25 access. It is not likely that more than one
call will be in progress at any one time between the gateway and an Econet
user, but there is no specific ban on this.

Hardware
--------
This is almost finalised, and looks like consisting of a BBC 6502 micro
connected via a Tube interface to a new board with Z80 and SIO chips. The
micro will not have a keyboard, and any operator control will have to come
from the Econet. However, a video output may well be useful for monitoring
and debugging. The new board has two distinct 6Mhz Z80 processors, one for
driving the X25 and the other driving the Tube. The Tube interface may be
driven over the 1Mhz bus rather than the dedicated Tube port. The two
processors will communicate via shared memory (16K of dual ported RAM) and
an interrupt path will exist to the X25 Z80 from the other one. The X25 Z80
will have 32K (RAM?) for code+local data, and the Tube Z80 will have 16K
PROM and 32K RAM for its own use.

The gateway will support a single HDLC line over a V24 interface operating
at speeds up to 19.2Kb.

Software
--------
The essential items of software that comprise the Econet-X25 gateway are:

(a) a network service module, by which the Econet is accessed,

(b) an X25 module, by which the X25 line is accessed, and

(c) a gateway module which interfaces to (a) and (b)

The network service module runs in the gateway 6502 BBC machine just as it
does in a standard 6502 machine. The X25 module runs in a dedicated Z80
processor on the new board, as does the gateway module. The interface to the
network service from the gateway is via OSWORD calls over the Tube, which
means that no specific gateway software is required in the 6502. At least 16
simultaneous connections will be supported by the gateway. In order to
establish a connection, it will be necessary to access the new network
service interface within a BBC micro attached to the Econet. It is possible
to both initiate and receive calls at this interface. The network service
interface at the user is replicated within the gateway. The gateway provides
sufficient protocol and address mapping to relay incoming X25 calls across
the Econet, and incoming Econet calls across the X25 network.

Network Service Module
----------------------
This is designed to support an ISO network service over the Econet and other
communications links. It is accessed via OSWORD calls, with the user
encouraged to poll the network service for incoming items rather than use
events. It will support 29 simultaneous calls over the Econet.

X25 Module
----------
This is a self-contained module designed by Symicron and designated STS
(Symicron Telematics Software) ported onto the Acorn purpose-built gateway
board. The interface is well documented and the software proven. The X25
facilities and limitations are:

  * Window sizes negotiable to 7
  * Maximum data packet size fixed at 128
  * Closed User Group requests passed transparently
  * Reverse charge requests passed transparently
  * Extended formats supported
  * Fast Select supported (incoming and outgoing)
  * Call statistics from the network passed transparently

It is possible that a maximum data packet size of 256 can be permitted in
the future if space allows. It is certainly the intention to establish a
buffer size sufficient for a fast select call request packet with 128 bytes
of user data. Currently Symicron estimates the program code to reside in 24K
bytes, with as much RAM as necessary to support 16 simultaneous calls. 1K
bytes per call is likely to be adequate with a packet size of 128 bytes.

Gateway Module
--------------
This is the module that routes on calls from either network (Econet or X25)
to the other. One major task is to map primitives received from one
interface into appropriate primitives to the other. The other task is to
make appropriate protocol changes when required.

Gateway conversions
-------------------
The gateway is presented with two interfaces: one to the network service
(found in the 6502) and one to the X25 module (found in the Z80 board).
These are not dissimilar, but have some definite differences which prevent
simple relaying. Likely known uses of the gateway are:

(a) Simple X25 calls
    Here the gateway is almost an end-point of the X25 network and a remote
    X25 user attempts to access a BBC micro or server on the Econet using a
    simple X25 subaddress (0 to 99) to distinguish the Econet subscriber/
    application. The gateway will need a look-up table to do this extremely
    simple mapping. The network service can mirror most of the requests from
    the X25 user and vice-versa. Calls made from the Econet to X25 are more
    easily handled since the network service called address may easily
    include the X25 called address.

    This use of the gateway is likely to be valuable when testing the
    gateway, and for interworking with existing X25 applications on host
    computers which do not use a transport service.

(b) Yellow Book Transport Service calls (YBTS)
    Here the gateway behaves as a transport service relay between two
    networks. The major difficulty is involved in sorting out addressing
    across the networks. The transport service addresses used over the X25
    network are "horizontal addresses" while the transport service addresses
    used over the econet are mapped directly into network service addresses.
    The gateway will be used in this mode when any remote X25 subscriber
    wishes to communicate with any econet user. There are none of the
    addressing restrictions of (a) applicable to destination addressing.
    Equally, this is a very flexible way for Econet users to set up calls
    through the gateway to X25 or beyond into other networks. In particular,
    a terminal user on the Econet with a PAD TS29 interface can call up TS29
    hosts through the gateway. Applications such as file transfer, job
    transfer and mail which are built to use the YBTS will be able to
    operate equally well over the Econet and X25 network together as they
    can over the Econet on its own.

(c) Econet PAD to X25 host.
    The specific protocol for terminal access over X25 is X29, and the
    Network Service must be used in the best way to support an equivalent
    protocol. In practice this is not particularly difficult.

    The gateway will be used in this mode when an Econet user with the PAD
    PROM wishes to call up a remote X25 host which does not support the YBTS
    (and many do not, particularly in the commercial environment).

Management
----------
The management of the gateway will be accomplished via an X29 internal
interface which in the first place will only be accessible from the Econet.
It is recognised that making this facility available to a PAD user across
the X25 network could facilitate support in the field.

The configuration tables required by this unit (X25 parameters, title to
address translations) etc. will be held on file across the Econet