The interface computer and the node computer can be the same computer.
It is the single point through which all network traffic must enter.
Access to the network and accounting information for the users is controlled
in the interface computer. The interface computer is also responsible
for reconciling protocol of the high level networks for the various
peripherals because they are all attached to the network through it.(Davies).
This is the point where the users connect to the network.
The interface computer is responsible for assembling data in to the
fixed packet formats. Additional formatting information such as the
source and destination information is included into the packet. The
interface computer's control protocol can include serial numbers for
the packets in a fixed position of the message area for reordering.
Multiplexing and demultiplexing also takes place at this point of the
network. Although the network itself provided methods for error control
and flow control, the interface computer also proves some form of these
The interface computer was originally designed to be implemented using
a Plessey XL12 computer. The computer was chosen because of it's "unique
multiplexing arrangement" that allowed a large number of devices to
be connected. Although this scheme can included a "tree of demand sorters
which could be infinitely deep," the NPL project proposed a depth of
3 which allowed at most 512 device to be connected. But in late 1968,
Plessey announced that the XL12 will be cancelled and the group decided
to replace it with a Honeywell DDP-516. This implied that the responsibility
of the demand multiplexer was left to the group, mainly Barlett and
Scantlebury who were in charged of the interface computers. (Campbell-Kelly
In the implemented networks, the Mark I & Mark II Networks, a Honeywell
DDP-516 computer is used.
- Flow control
- Error detection
- Error recovery
- Congestion control