|
1. Introduction
The legacy telephone switches all use the 2 Mb/s ports as their
trunk modules. With their Digital Distribution Frames connected
to the transmission equipment, such ports provide trunk paths at
2 Mb/s by means of HF cable.
With the expansion of exchange capacity and network convergence,
quite a lot of manufacturers has installed built-in SDH interfaces
in their switches so as to provide trunk modules of the STM-1 (155
Mb/s) rate, such as Shanghai Bell's P3S exchange and Huawei's C&CO8
exchange.
2. Advantages of the 155 Mb/s Port
1) In terms of cost: For the same transmission capacity, the
average cost of the 155 Mb/s port in switches and transmission equipment
is significantly lower than that of its 2 Mb/s counterpart. Such
a port can also save large amounts of 2 Mb/s connecting cable in
the machine rooms for switching and transmission and reduces maintenance
cost.
2) In terms of machine room space: This port can save the space
occupied by the trunk modules of switches and the DDFs in the transmission
machine room.
3) In terms of the cut-over time of systems: In dispatching 155
Mb/s trunk circuits, the corresponding coaxial cables or tail optical
fibers are connected once for all. No manual connection is necessary
when VC-12 paths are used, thus shortening the cut-over time of
systems.
4) In terms of equipment utilization: Restricted by physical
space, the transmission equipment has limited density of 2 Mb/s
ports, whose equipment utilization rate is evidently lower than
that of the 155 Mb/s. Take Lucent's extensively used ADM16/1 for
example. A 2 Mb/s circuit board can provide 63 2 Mb/s ports, equivalent
to a STM-1 whereas its 155 Mb/s counterparts can provide 4 STM-1
ports. If the ports used are all of 2 Mb/s, it is hard to fully
utilize the line rate of 2.5 Gb/s. Thus the use of the 155 Mb/s
port by switches can overcome the low utilization rate caused by
the limited density of ports in the transmission equipment.
5) In terms of network structure: In some high traffic exchanges,
trunk circuits between exchanges can be reached directly via the
transmission equipment at the rate of 155 Mb/s. This can raise the
utilization rate of the transmission rate, and simplify and optimize
network structure.
6) In terms of development trend: The use of the 155 Mb/s port follows
the trend towards broadband networks and the 155 Mb/s granularity
so as to keep pace with the development of networks.
3. Networking Structure of P3S Tandem Switches
The networking of switches at 155 Mb/s is different from that
at 2 Mb/s. The two issues that need to be taken into consideration
are how to give full play to bandwidth advantages and to coordinate
operation between the switching and transmission networks, while
ensuring network safety and reliability. In networking the four
P3S tandem switches introduced recently, Beijing Telecom considered
the following points.
3.1 Connections between Tandem Switches
3.1.1 2 Mb/s Connections
Owing to the thick line bundle between tandem switches, 126 2
Mb/s trunk circuits are needed. This requires the use of transmission
equipment providing 2 Mb/s ports. This scenario uses ADM4/1 equipment
as the ADM equipment from the 2 Mb/s trunk circuits to the STM-1.
For such connectivity, see Figure 1.
Figure 1
3.1.2 155 Mb/s Connections
Here, the 155 Mb/s ports of the transmission equipment can be
used to achieve direct interconnection. This results in simple and
trenchant structure without 2 Mb/s ADM equipment. To ensure safety,
the TDM 10 G transmission equipment can provide protection of self-healing
rings and two 155 Mb/s coaxial connections share the traffic load.
For such connectivity, see Figure 2.
Figure 2
3.2 Connections between the Tandem Exchange and the Cross-Tandem
Area End Exchange
3.2.1 2 Mb/s Connectivity
The tandem switch must first be connected to the transmission
equipment providing 2 Mb/s ports and to the transmission equipment
in other tandem area via high-speed (e.g. 10 Gb/s) transmission
equipment, and then to digital access and cross-connect system (DACS).
It is further connected to the corresponding SDH sub-ring after
the VC-12 level cross-connection carried out by the DACS and finally
reaches the end exchange. For such connectivity, see Figure 3.
Figure 3
3.2.2 155 Mb/s Connectivity
The tandem switch is directly connected to 10 G transmission
equipment via two 155 Mb/s coaxial cables, saving ADM4/1 or ADM16/1
equipment. As for safety, as all SDH equipment is formed in a ring,
in which it is safe, the location vulnerable to faults is connection
points of the coaxial cables. As a result, at all these connection
points, the two coaxial cables are connected to different transmission
equipment, sharing the traffic load from the tandem switch to a
given end exchange, with half of the respective trunk circuits put
into use, ensuring network safety. For such connectivity, see Figure
4.
Figure 4
3.3 Connections between the Tandem Exchange and the End Exchange
In this case, there is not much difference between 2 Mb/s and
155 Mb/s connections. Here, I am not going to give more details.
4. Issues Worth Attention in Using the 155 Mb/s Interface
4.1 Safety
Since the 155 Mb/s connections of coaxial cables or tail optical
fibers are most vulnerable to faults in channels and are not able
to provide SDH self-healing protection, we should pay special attention
to its safety.
On the one hand, we can use tandem switches in pairs and let
them share traffic loads. On the other hand, in networking trunk
circuits, we should consider using dual coaxial cable connections
as many as possible via different transmission equipment, and enable
the trunk circuits of a given switch to be shared by the two routes.
Thus, in case of any fault over a coaxial cable, three thirds of
the trunk circuits to an end exchange can still be used so that
the safety and robustness of the network can be guaranteed. Furthermore,
the quality of the coaxial products and construction is also critical.
At the same time, card protection should also be used to minimize
the impact made by card faults.
4.2 Consistence of Time Slots
When signals at a nominal rate of 2.048 Mb/s are multiplexed
to AU-4s, first use 3 TU-1s to insert bytes and form a TUG-2, and
then use 7 TUG-2s to insert bytes and form a TUG-3 and finally use
3 TUG-3s to insert bytes plus high-order POH and the inserted bytes
and form net VC-4 loads. Eventually, a nominal rate of 155.52 Mb/s
is achieved for STM-1s. This is also known as the '373' structure.
However, manufacturers have different default mechanisms for
the arrangement of TU-1 time slots in the AU-4. Typically, the arrangement
of the first time slots is consistent. But the second time slot
of Equipment A may be the 22nd in Equipment B. This will cause problems
when the circuit is cut over. Thus we should make the order of all
the time slots consistent.
4.3 Simplification of Network Structure
Full utilization of the 155 Mb/s ports in a switch may save the
number of transmission equipment and simplify the structure of the
trunk transmission network. It is necessary to appropriately combine
and rationally plan the switching trunks in the same direction,
and combine the trunk circuits in different direction into a STM-1
port and determine the way to network the trunk circuits in the
transmission network according to the trunk direction and taking
into account the structure of the transmission network. By doing
so we can ensure network safety as well as improve network efficiency.
4.4 Maintenance
At present, we can track the signaling links in the 155 Mb/s
ports of a switch only by using the signaling of the switch itself
rather than the external analyzer of the CCS 7. This causes inconvenience
to maintenance. It is expected that manufacturers will enhance the
tracking and monitoring functionality of their products and training
switching and maintenance personnel concerning routine maintenance
of the 155 Mb/s port.
4.5 Application
We consider the 155 Mb/s port most suitable for high capacity
switches with thick trunk line bundles, such as tandem switches
and interconnected and interworking gateways. This can bring bandwidth
advantage into full play, enable transmission at 155 Mb/s not via
2 Mb/s level cross connection and facilitate load sharing by dual
routes.
When the 155 Mb/s port is used in end exchanges, these advantages
also exist except that network safety should be paid special attention
to.
Liu Yanxia: She graduated from Beijing University of Posts &
Telecomunication and received a master degree. currently she work
for
Beijing Telecom Network Bureau.
|
