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1. Introduction
In the times of fierce competition in the telecom market, operators
are facing rigorous challenges. To keep an invincible position in
the arena, they must improve their networks continuously, enabling
themselves to have the capability of providing services quickly
and flexibly while reducing the costs of network operation, maintenance
and management. In particular, they must simplify network structure
to make the networks easy to expand and plan traffic development.
To maximize the utilization of network resources is a key way to
bring down the operational cost of the networks and thus is an issue
of major interest to operators.
The next generation network (NGN) is an integrated and open network
architecture that can provide voice, data and media services. It
is a packetized network based on unified protocols as well as a
service-driven network so that it can undoubtedly realize the above
purpose ideally. However, for traditional operators, who already
own a large network, it is not realistic to give up their existing
networks to build a brand new one. Therefore, emphasis should be
placed on transformation of the existing networks while developing
the NGN. Only by doing so can we lay a solid foundation for the
development of future networks as well as increase the economic
benefit of telecom carriers.
Since the early 1980s, China's telecom industry has experienced
an unprecedented growth. As of the end of September 2001, it had
317,000,000 telephone exchange lines. However, compared with developed
nations, we still have large room for development in voice service.
Thus, it is of realistic significance for us to transform the legacy
switches by replacing the 2 Mbit/s trunk interfaces with STM-1 trunk
interfaces (155 Mbit/s) in the legacy circuit-switched switches.
From the technical viewpoint, it is possible.
2. Background for the Use of 2 Mbit/s Trunks in Switches
The PSTN has historical reasons for the formation of its present
structure. Before the 1990s, the digital transmission equipment
was based on the technical standards of Plesiochronous Digital Hierarchy
(PDH). In order to connect the switch to PDH transmission equipment,
the switch side needs lots of terminal equipment of the primary
rate, i.e., 2048 kbit/s (2M) switching terminal circuits in compliance
with ITU-T standards (e.g. S1240's DTM trunk module and AXE10's
ETC module) and ANSI-standard-based 1544 kbit/s ones. At that time
such connection was reasonable and necessary. The connection of
the switch with the transmission is as shown in Figure 1.
Figure 1: The connection of the switch with the
transmission network via the PDH 2M link
Initially, PDH was introduced to improve and replace the obsolete
analog frequency division multiplex (FDM) system. However, it was
soon found that PDH had shortcomings such as difficulty in management,
high construction cost, limited applications and lack of connectivity
standards at the optical layer. Thus, in the early 1990s, Synchronous
Digital Hierarchy (SDH) was introduced in the transmission system.
But then 2 Mbit/s PDH links were still be used in connections between
the networks and the telephone switches. See Figure 2.
Figure 2: The connection of the switch with the
SDH transmission network via 2 Mbit/s trunks
3. Comparison between the 2 Mbit/s and 155 Mbit/s Trunks
No matter whether in the PDH or SDH transmission network, the
use of 2 Mbit/s trunks to connect to the switch needs large amounts
of HW equipment, including cable and DDFs. The advent of SDH technology
has created conditions for the improvement of the switching network.
That is why switch manufacturers developed SDH interfaces at the
switch side, i.e., 155 Mbit/s trunk modules, such as ET155 ETSI
for Ericsson's AXE10, SLD for Alcatel's S-1240 and STU for Huawei's
C&C08. The use of the 155 Mbit/s trunk module has the following
advantages:
a) The network structure can be greatly simplified so that the
switch can be connection to the transmission network directly.
b) The 63 2 Mbit/s terminal circuits and their frames and shelves
in the switch can be replaced by a single 155 Mbit/s terminal circuit.
c) The 252 2 Mbit/s cables are replaced by only 4 STM-1 optical
fibers or cables.
d) The drop of cables in number reduces the number of DDFs significantly.
e) Low-end synchronous multiplex equipment is no longer required.
f) The reduction of HW equipment means the reduction of machine
room space and power consumption.
Take Switch AXE10 as an example. From a comparison between the
2 Mbit/s (ETC5) and the 155 Mbit/s (ETC155) trunk modules, we can
see that for the same cabinet, the equivalent number of E1s of the
ETC155 is four times that of the ETC5 while power consumption of
the former is merely twice that of the latter. With the 2 Mbit/s
trunk, a full-capacity 128 k tandem switch needs 45 shelves while
with the 155 Mbit/s trunk, it only needs 21 shelves. That is to
say, a 100,000-line tandem exchange needs machine room space less
than twice as it needs 32 2 Mbit/s trunk shelves but only 8 155
Mbit/s trunk shelves. Besides, power consumption decreases by 30%
and the trunk cables from 3,334 to 53. We are not going to mention
the cost of the cable for the moment. Only look at the great deal
of cables laid under the floor. They necessitate the increase of
cooling volume to meet the environmental requirement. What is more,
these cables run from the switching room to the transmission room,
occupying a great deal of channel space between stories, which is
even more precious than machine room space. To solve this problem,
2.5G transmission equipment has to be used as bridge cable between
stories. Moreover, look at the transmission machine room. The use
of the 2 Mbit/s interface requires a complete set of DDFs whereas
the 155 Mbit/s interface only requires a coaxial cabinet without
the use of 2 Mbit/s multiplex equipment, greatly reducing the occupancy
of machine room space.
The use of the 155 Mbit/s trunk can also decrease the workload
of installing HW and make network structure more trenchant and maintenance
and management easier. For network connection, see Figure 3, from
which we can see network structure is much more trenchant than that
in the previous figures when capacity remains the same.
Figure 3: The connection of the switch with the
transmission network via the STM-1 (155 Mbit/s) interface
4. Aspects Worth Attention in Implementing 155 Mbit/s Trunks
1) Network Safety and Reliability
Since a 155 Mbit/s trunk contains 1,890 trunk lines at most,
it makes a greater impact than a 2 Mbit/s one in case of a fault.
Thus the guarantee of network safety is a primary consideration
in using the 155 Mbit/s trunk. Switching equipment vendors adopt
different safety mechanisms between the link and the equipment layers
in developing 155 Mbit/s trunk modules. For some switches, N+1 redundancy
is used, enabling automatic switchover to the spare equipment in
case of a fault on one of the modules. Other switches use MSP 1+1
protection. Besides the protection of the modules themselves, dispersion
is another factor we should take into account in organizing a network.
For instance, the trunk lines of a trunk should be dispersed to
different 155 Mbit/s trunk modules and each 155 Mbit/s trunk modules
to different physical routes. That is to say, safety should be adequately
guaranteed in organizing a network.
2) Flexibility in Dispatching the Transmission Network
When the 155 Mbit/s trunk is used, the dispatch of different trunks
depends greatly on the transmission network. Thus, when planning
and designing a transmission network (e.g. determining the network
structure, the capacity and number of ADCs), the types and the number
of interfaces should be determined, taking into account the usage
of 155 Mbit/s trunks in the switch.
3) Time Slot Arrangement and Tributary Channel Numbering
For the convenience of operation, it is necessary to number 63 E1s.
Since there are no definite speculations about the corresponding
relationship between the E1 numbers and the positions of the tributary
channels in the optical circuit, vendors number the E1s on the basis
of before and after multiplexing. Hence two E1 numbering methods.
While difference in numbering will not cause problems in interconnectivity
of equipment, it will cause inconvenience of maintenance. In case
of any man-made error in distributing the circuits, it is very hard
to correct. That is why it is critical to unify the methods to number
the switches and the transmission networks before the implementation
of 155 Mbit/s trunks.
4) Rigorous Management and Service Scheduling
Owing to fact that at present 155 Mbit/s and 2 Mbit/s trunks coexist
in the switching network and the latter has a larger share, and
traffic has to go through several SDH rings in the transmission
network, it is necessary to develop stringent management procedures
in configuring the circuits and carry out all-length tests.
The transformation of networks is an evolutionary process. The
transmission network is moving towards an intelligent platform and
an interface providing several services. The evolution of transmission
technology has promoted the transformation of switching technology
and the latter, in return, has changed the structure of the transmission
network. It is under such an interaction that the telecom network
moves forward.
Hao Xiufang: She graduated in 1975 from Beijing University of
Posts & Telecomunication, She is currently deputy general manager
and senior engineer in the Engineering and Construction Department
of China Telecom Group Beijing Telecomunication.
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