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1 Background
In 1999 when we discussed the capacity expansion of the tandem
exchanges in the local network of Beijing Telecom, we realized that
the growing concentration of exchanges and increasing capacity demands
on the tandem offices ( 80,000 lines were planned for one tandem
office) had resulted in congestion of 2Mbit/s cables. The 2Mbit/s
cables occupy a large footprint in the limited area of the equipment
rooms of tandem offices, be it switching room or transmission room.
Take a tandem office that accommodates 80,000 lines and 2670 2Mbit/s
as an example, the number of cables from the switching room to the
DDF in the transmission room would be 5340 at most with a section
area of 1400cm2 (Considering the difference among cables provided
by different vendors, we assumed the maximum number of cables and
the maximum footprint in the calculations). As the technology develops,
and for the conveniences of construction and maintenance, it is
a natural trend to use the high-speed data interface. Therefore,
the purchase of 155Mbit/s interfaces were put on the agenda and
a proposal on the number of 155Mbit/s interfaces needed in each
tandem office was put forward based on the fact that many offices
were equipped with more than one exchange system. After the former
Beijing Telecommunications Administration Bureau made a decision,
each of the four tandem offices ordered 12 sets of 155Mbit/s interfaces
for trial.
2 Order of the 155 Mbit/s Interface Equipment
The 155 Mbit/s interface satisfies the requirement of large capacity
networking, helps to distribute routes via transmission equipment,
reduces the number of interconnection equipment needed (DDF wiring)
for the conveniences of deployment and maintenance, cuts networking
cost, decreases the needed area of equipment rooms, including exchange
rooms and transmission rooms, lowers system power consumption, mitigates
electromagnetic interference, raises reliability, etc. Because of
the above advantages of 155Mbit/s interface, the use of 155Mbit/s
interface was expanded from tandem offices to terminal offices during
the trial period. 10 tandem offices ( including gateway offices)
as well as several terminal offices ordered 155Mbit/s interface
equipment.
By the end of November 2001, there had been 115 terminal offices
for local telephony and 191 switching systems (including those in
order) in Bejing Telecom's local network as shown in Table 1 and
Table 2.
Table 1
Note：All these data will be finalized after the project implementation
in 2001, and the temporary offices and the switching equipment to
be dismounted in the Project are excluded in the table.
Table 2 Number of Exchange Systems in Various Regimes
Regime
Note：S12 includes 4 P3S terminal offices，C&C08 includes fifteen
128 k exchange systems
The 155Mbit/s interface equipment used in Beijing Telecom's network
were provided by different exchange vendors with different strength
and were put into commercial use at different time (differing by
2-3 years). Conditions of the equipment ordered for Beijing Telecom's
network are shown in Table 3.
Table 3
In this table, there were no hardware back-up for the 155 Mbit/s
interfaces of C&C08 and AXE10 exchanges, but back-up is made
within the circuit boards or for the cable outlets.
Currently, 96 sets of 155Mbit/s interface equipment have been
ordered for 8 major tandem offices with 12 sets for each office;
64 ssets of 155Mbit/s interface equipment have been ordered for
2 gateway offices with 32 sets for each. Altogether on the tandem
layer, there will be 160 sets of 155 Mbit/s interface equipment.
8 terminal offices in the urban area have ordered 32 sets of
155Mbit/s interface equipment and are expected to increase the order
in 2002.
3 Principle of Using the 155 Mbit/s Interface
The 155 Mbit/s interface is advanced in technology and has such
advantages as large capacity, high concentration, convenient outlet,
small footprint, low investment requirement and easy maintenance.
Currently, 155 Mbit/s interface equipment have been ordered from
3 vendors for the tandem offices ( including gateway offices )and
a few terminal offices of Beijing Telecom's network, and the order
will be increased in the future with the growth of the network.
Therefore, we need to pay full attentions to the proper use of the
155Mbit/s interface.
The 155 Mbit/s interface is applicable to two kinds of routes
from terminal offices to tandem offices( including gateway offices)
and from gateway offices to the network of Beijing Mobile and Beijing
Unicom.
The route between gateway offices and the network of Beijing
Mobile and Beijing Unicom is characterized by concentrated trunks,
large bundle of wires and flexible adjustment, making it suitable
to use and vigorously expand the use of 155Mbit/s interface.
In the current network structure, the tandem office basically
covers all terminal offices, mainly making tandem of outgoing calls,
and the tandem of incoming and outgoing calls is usually seen in
the exchanges with small capacity and low traffic volume. In the
10th five-year plan period, the number of fixed network users will
grow steadily without causing dramatic increase of exchange systems.
Therefore, the network structure could be properly adjusted and
optimized. There will be more tandem offices for both incoming and
outgoing calls , larger traffic volume among tandem offices(including
gateway offices) and more trunk lines. In this case, it would be
more convenient and advantageous to use the 155Mbit/s interface
among tandem offices ( including gateway offices) .
Despite the tandem of outgoing calls between terminal offices
and tandem offices (including gateway offices), the trunk routes
are dispersed and with small bundle of wires。The number of trunk
ports between a switching system and a tandem office is usually
lower than 40-42 2Mbilt/s per 155Mbit/s port. It is suggested that
one 155Mbit/s of the exchange of the terminal office be moved to
ATM (or DXC) in the transmission room and then transferred to related
offices via transmission equipment, or be sent to the tandem office
and then transferred to related offices via the transmission equipment
in the tandem office. The routing security shall be considered in
the movement.
4 Implication of 155Mbit/s Interface among Exchange Offices
for Planning
Among the 115 local telephony offices in Beijing Telecom's network,
43 are equipped with 2 and more sets of exchange systems, and among
the 83 offices in the urban area, 34 are equipped with 2 or more
sets of exchange systems. The largest number of installment in an
office is 7 sets of exchange systems and the largest capacity is
several 10,000 lines. In the existing network structure, the number
of circuits among terminal offices is far less than that between
terminal offices and tandem offices, and there are few exchange
systems added at present. Therefore, the 155Mbit/s interface would
be added to the exchanges of terminal offices only when new exchange
systems are brought in and renovations or expansions are made. When
the number of 155Mbit/s interface of the terminal offices is less
than that of tandem offices, several exchanges in the same office
could be interfaced to the transmission room via the 2Mbit/s interface,
then dispatched to a tandem office via ADM (or DXC) by transmission
equipment and transferred to 155Mbit/s interface directly at the
tandem office.
Among the 115 terminal offices, 83 are in the urban area and
34 are within the third ring road. The tandem offices are usually
located near the busy downtown ( Among the 14 tandem offices, 11
are within the third ring road and 6 are within the second ring
road). In the areas with larger demands, land available to the offices
is in short supply, especially in the prime area. Therefore, to
scale the land area occupied by the offices is the best way to save
investment, and it is important to use 155Mbit/s interface in tandem
offices.
The capacity of tandem offices (including gateway offices) depends
on the capacity of the local network and related networks, the way
of tandem and the determination of tandem threshold. Currently,
the maximum capacity for the 14 tandem offices (including gateway
offices) under construction is 80,000 lines and even more in the
future. The use of 155Mbit/s interface among exchange offices greatly
facilitates the capacity expansion of the tandem offices (including
gateway offices). We shall miss no opportunity to both save investment
and optimize the network to ensure the security and reliability
of the network.
Zhang XinZhu: she graduated in 1969 from Beijing University of
Posts & Telecomunication, engaged in the design of telecom network,
Currently she is deputy Chief
Engineer of Telecom Planning & Design Institute of Beijing.
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