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After the rapid growth in the 1990s, by the end of 1999 Beijing
Telecom had 3.2 million fixed-line telephones and 1.5 million mobile
phones in operation. Total length of optical cable had exceeded
6,000 kilometers; the PDH equipment had gradually been replaced
by new SDH digital transmission equipment; and there were nearly
100,000 2 Mbit/s transmission circuits in operation throughout the
network.
All these changes took place after the first SPC exchange was
put into service in the capital in 1984, enabling the use of PCM
circuits for local trunks. Before 1984, the great majority of the
trunk circuits in the city were copper cable, with one copper wire
per circuit. After the introduction of PCM, a pair of fiber can
transmit at the rate of 34 Mbit/s, which is equivalent to 480 circuits.
Now, digital trunks have been introduced, with a port rate of 2
Mbit/s, which is equivalent to 30 circuits. As the SPC exchange
develops, in the digital transmission network the basic unit of
64 kbit/s as a circuit has gradually been replaced by that of 30
2 Mbit/s circuits (hereinafter referred to as E1). In end-1999,
the transmission network in Beijing had 60,000 E1s, equivalent to
1,800,000 circuits.
In the second half of the year 1999, we found a trend worth studying
in the development of the basic transfer network. With the emergence
of competition in the market, the transfer network had to provide
circuits to newly emerging operators as well as the fixed network
of Beijing Telecom. The rapid rise of the Internet, the advent of
the IP network, and informatization of businesses have greatly contributed
to the sharp growth of demand for bandwidth. Beijing Telecom had
to make its decision on the basis of the energetic development of
its basic transfer network, instead of rendering it an accessory
to switching. As a result, a basic unit of the transfer network
of 155 Mbit/s (hereinafter referred to as STM-1) is added so that
E1s and STM-1s coexist. The PSTN and the Internet prefer STM-1 connections
at the tandem exchanges, high-capacity end exchanges of users and
backbone convergence points. For the backbone nodes, high-capacity
DACS4-4-1 cross connect equipment with STM-1s as its basic interfaces
is used. Between the various equipment, STM-1 connections are directly
used instead of tens of 2 Mbit/s connections so that large amounts
of HW equipment, power consumption, connecting lines between shelves
and fault rate can be reduced, while network redundancy can be improved.
In a word, the construction and maintenance costs can be lowered.
In the traffic network, the evolution of the telephone exchanges
is a problem, since the technical specifications issued in China
have not stipulated any requirements for the STM-1 interface. However,
in order to meet the needs of the operators and the market, some
manufacturers developed high-capacity switching matrix equipment,
whose access interface uses the STM-1 rate, taking into account
broadband integrated access service. In early 2000, Beijing Telecom
tested interworking of the STM-1 ports of Ericsson's AXE10, Shanghai
Bell's P3S and Huawei's CC08 with the transmission equipment and
their connectivity with the communication network in operation,
and successfully introduced STM-1 equipment between the tandem exchanges
and between some end exchanges and exchanges. Besides, it cut over
dozens of STM-1 paths. The use of these paths simplifies effectively
the network and reduces the number of digital distribution frames
(DDFs), connecting lines and shelves of 2 Mbit/s terminal boards
as well as simplifies maintenance. After more than one year's preparation
and maintenance, the number of the STM-1 trunk circuits put into
operation in 2001 took up 25% of total broadband of the trunk circuits,
and total broadband of the STM-1s in operation exceeded that of
the E1s, thus reducing thousands of pairs of connecting lines and
the fault rate. According to prediction, in 2002 there will be a
demand for 15,000 2 Mbit/s paths and 770 STM-1 paths, with a ratio
in broadband of 1: 3. With the drop of E1s in number, STM-1 paths
will become the most important basic unit in the network. Beijing
Mobile will also introduce in 2002 STM-1 interfaces in MSCs and
GMSCs for the first time, and put into operation 130 155 Mbit/s
paths, equivalent to 8000 E1 paths.
Beijing Telecom's basic transfer network has upgraded from the
analog/digital ratio of 1:30 (see Figures 1 and 2) 16 years ago
to the E1/STM-1 ratio of 1:30 (see Figures 2 and 3). It is going
broadband and digital.
Before the 1980s
Figure 1
After digitalization
Figure 2
Figure 3: A 1:63 tandem exchange (as compared with
Figure 2)
Lin Rong: He graduated from Capital University of Economics and
Business in 1969, now he is deputy Vice General Manager and Professorial
Senior Engineer of China Telecom Group Beijing Corporation.
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