TXE2 - PO Journal 1974
Progress
withTXE2
DAE Carter
and AJ Palmer
Please excuse the layout and spelling of this page, more work needs to be done on it.
RAPID STRIDES have been made in providing small-to-medium sized electronic exchanges, known as TXE2, under the Post Office programme to modernise Britain's telephone network. Since the first production electronic exchange was introduced in 1966 at Arnbergate in Derbyshire more than 500 TXE2 exchanges have been brought into service.
These exchanges currently provide sufficient capacity for about half-a-million telephone customers' lines and the numbers continue to grow. An annual order rate of about 100 new exchanges and 100 extensions to existing equipment is expected over the next five years, involving a capital outlay of some £40 million.
When the TXE2 was introduced it was intended to be the standard installation for local exchanges with approximately 600 to 2,000 lines. Following detailed economic studies the capacity has since been extended to cater for local exchanges serving 400 or more lines provided that the ultimate capacity of a single unit will not exceed 7,000 lines and the traffic to be handled is within certain limits related to the switching capacity of the equipment. A second exchange unit with identical capacities can be provided if required to meet growth. This meets the requirements for local exchanges in most small towns and rural areas where there is a high proportion of residential customers.
Another important development of TXE2 has taken place to meet situations where it is necessary to provide telephone service quickly. A mobile version of the exchange, designated MXE, has been introduced which can be moved around the country to provide service here it is urgently required, as part of the Post Office's drive to reduce the waiting list for telephones. The MXE can also be used to meet emergencies, for example, in the event of interruption to normal operation at an exchange due to fire or flooding. Electronic equipment is well suited for mobile exchange applications because it is lighter in weight, more compact, has fewer moving parts and is more reliable than existing mobile exchange equipment. A 1000-line MXE consists of two trailer units, one containing the control and power equipment and the other switching equipment, providing more than twice the capacity of existing mobile exchanges. Increased capacity can be provided by the addition of a third trailer to cater for a maximum of 2,000 lines. The first MXE came into service last year at Padgate, Lancashire, to provide lines for new telephone customers and to cater for growth in Warrington new town. Believed to be
the first of its type in the world to go into operation, this MXE is providing service pending the provision of a new permanent exchange at Padgate. In the light of practical experience gained in the field, the original design of TXE2 equipment has proved to be basically satisfactory. Some minor changes have been necessary to meet service requirements and to overcome some capacity limitations.
One significant change to meet current Post Office requirements concerns the register which receives the digits dialled by a caller and 'controls the setting up of a call. The original register was designed for an exchange with a four-digit self-contained numbering scheme.
Where required it permitted the direct routing of calls to nearby exchanges following the examination of up to three digits of the number dialled. The current register provides for a local numbering range containing four, five or six digits and for the examination of up to four· digits of the number dialled. The latter facility allows access to adjacent switching centres over direct junction routes to be obtained by dialling the national or ·local code as appropriate. These improved facilities now permit TXE2 exchanges to be included in linked numbering schemes with minimal additional cost because the number of switching stages is constant, irrespective of the length of the customer's telephone number.
The integration of all exchanges into numbering group linked numbering schemes - that is, where all exchanges in a group share a common national code - and the use of national code dialling procedures for access to exchanges outside the home numbering group (as a long-term objective) will
eliminate the need for local code dialling and individual exchange dialling
instructions. With so many TXE2 exchanges now in service, the advantages of electronic equipment compared with Strowger electro-mechanical equipment are being realised in practice. For the Post Office one of the major advantages is that the planning, design and installation of new exchanges and extensions to existing equipment are easier and quicker to carry out. Basically this is because the equipment is of modular design and, being more compact than Strowger, it occupies less space.
The key components of the TXE2 are miniature reed relays which are used to switch speech paths through the exchange. The precious metal contacts of a reed relay are enclosed in an hermetically sealed glass envelope to prevent contamination and ensure long life with
A Post Office engineer makes final wiring connections at the Taff Wells TXE2 exchange near Cardiff. The equipment was manufactured at STC's factory only one mile away.
minimum maintenance effort. With the reduction in maintenance requirements TXE2 equipment lends itself to eventual
part-time maintenance by specialised staff from a central control point. Other benefits of TXE2 include a
simplified procedure for tracing calls and in-built facilities for measuring exchange performance. It also allows
complete flexibility in the allocation of customers' telephone numbers, enabling full use to be made of the fitted number
capacity of the exchange. For example, it is not necessary to reserve spare numbers for a private branch exchange (PBX) to cater for expected growth. Additional PBX lines can be provided without the need to change the customer's number as is often the case with Strowger exchanges.
Another important customer facility is the ability of the TXE2 to initiate automatically a second attempt if the original path through the exchange proves unsuccessful. Electronic switching equipment operates much faster than Strowger thus allowing the second
attempt, using an alternative path through the equipment, to be carried out without any additional action by the caller or delaying completion of the call. The alternative path gives a greater chance of effecting a successful connection than at Strowger exchanges where, particularly in less busy periods, the same path through the equipment
is often encountered on successive calls.
The likelihood of encountering back- ground noise while a call is in progress is also substantially reduced with TXE2
exchanges. This is because the enclosed contacts of the reed relay provide a quieter speech path than the exposed
contacts in Strowger exchanges. The present service being given by the TXE2 has fully justified the decision by the Post Office to go ahead with the provision of electronic switching equipment for local exchanges. It will soon be followed by the introduction of the
larger version of the electronic exchange, known as TXE4. (See the article on page 24 of this issue.)
This step forward in telecommunications is a tribute to the co-operation and development work carried out by both the Post Office and the tele- communications industry.
Mr D. A. E. Carter is head of a group in the Local Exchange Systems Planning and Design Division at Telecommunications Headquarters.
Mr A. J. Palmer is a Senior Telecommunications Superintendent in the same group.
PO Telecommunications Journal, Autumn 1974