Design
Links in the network are over connected. The use of redundant links
along with alternative routing ensures that the network will maintain
service event in the event of equipment failures. It must be noted that
packets will be lost when links becomes non-operational. Therefore,
interface computers provide methods of ensuring the quality of service
provided by the network. The links in the network does not implement
a standard bit rate. Each individual link has its own standard bit rate
for data transfer, but the rates varies from link to link. Although
the rates are non-standard, Davies' initial calculations estimate that
the network will average a minimum of 1.5Mbps.
Mark I Networks
The experimental Mark I network was originally designed to have three
packet switching nodes. It was to act as a switching computer which would
make virtual connection between any two devices. But funding constraints
reduced the number to one. With only one node, the NPL team would not
get the opportunity to tackle issues like congestion and routing. The
node computer used was the Honeywell 516. The Mark I had about sixty lines
that provided access to a DEC PDP-8 computer and two mainframes. Through
the network, NPL researchers could have remote access to computers for
writing and running programs, querying a database, sharing files, special
services such as a "desk calculator", and for simple communication between
users. The network provided a throughput of about 150 packets per second
(much less than the couple thousand per second forecasted by Davies early
on). In its first form the Mark I lacked a packet interface; it was just
a byte interface which would produce a communication link between two
terminals, but it wouldn't enable one computer to talk to several terminals
by interleaved packets. The Mark I was only a single character switch
rather than a packet switch. In addition, the Mark I network operating
system proved to be unwieldy and slow, largely due to the aforementioned
hardware implementation.
(Campbell-Kelly)
During the 1970's, additional equipment was added to the network to
attract users. In 1971, the network was fully functional. (Campbell-Kelly)
Mark II Network
The upgraded Mark II system, introduced in 1973, used most of the same
hardware as the Mark I, but software and protocol improvements made it
two to three times faster. In order to eliminate the problem of "packetization
of data that had made certain kinds of terminal interactions impossible"
by the Mark I network, the Mark II used an "intelligent terminal processor"
to act as a packet assembler/disassembler. In addition, the Mark II provided
for both the character switched and the packet switched interface. The
new packet switched interface allowed for user machines, i.e. hosts, to
attach to the network (as planned by Davies in his 1955 proposal). The
Mark II also differed from it's predecessor because it begain to take
a layered approach versus a structured scheme. With all of the improvements
in place, the upgrade network had maximum packet rate of 250-500 packets
per second.
The Mark II remained in service at NPL until 1986- quite an impressive
term of service for an experimental system. (Campbell-Kelly)
