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The year 2001 witnessed an increase in demand for 155 Mbit/s
transmission paths. One reason for this increase is that the traffic
on the 155 Mbit/s dedicated lines increased; the other reason is
the use of 155 Mbit/s trunk ports in the data networks (DDN, FR,
etc.) and switches.
1. Background
The issue of unifying numbering time slots of 155 Mbit/s signals
in the Beijing telecom network came up after the use of 155 Mbit/s
trunk ports in the switches, which resulted in the change of the
usage of the transmission network. Figures 1 and 2 show the typical
connection modes after the use of such trunk ports.
SDH ring SDH ring SDH backbone layer
Figure 1: Type 1 interworking between interexchange 155 Mbit/s
ports
SDH ring SDH ring SDH backbone layer
Figure 2: Type 2 interworking between interexchange 155 Mbit/s
ports
From these figures we can see that the use of 155 Mbit/s trunk
ports in the switches simplifies the interface between the switching
and transmission networks, reduces lots of multiplex equipment of
2 Mbit/s tributary channels in the transmission network. Besides,
the 2 Mbit/s paths of different trunks are totally carried by digital
cross connection (DXC4/1 cross connect system). All this contributes
to the reduced workload and investment, expedited installation and
simplified capacity expansion and maintenance.
However, while bringing us a lot of benefits, it raises requirements
for network interworking. As is shown in Figures 1 and 2, if the
order of the 2 Mbit/s time slots is inconsistent on the 155 Mbit/s
paths, disorder will take place when these paths are interconnected,
thus making impossible the functioning of the point-to-point 2 Mbit/s
links. Hence the unification of the arrangement of the time slots
of 155 Mbit/s signals throughout the network.
2. The Establishment of Unified Rules of Numbering Time Slots
of 155 Mbit/s Signals throughout the Network
The establishment of unified rules of numbering time slots of
155 Mbit/s signals throughout the network should follow four principles.
2.1 The unified rules of numbering time slots must comply with
the requirements of the MII's YDN099-1998 Standards. Section 5.2.5.4
of MII's YDN099-1998 Standards stipulates that numbering time slots
of 155 Mbit/s paths of transmission equipment should comply with
the plan in Table 10 (see pp. 26 and 27). In order to achieve interworking
between multi-vendor equipment, the corresponding relationship between
time slot numbering and tributary channel numbering can be changed
at least through software in the VC-4. If it can not be changed,
the ports of the tributary channels should be numbered according
to Table 10.
2.2 The time slot numbering rules of 155 Mbit/s signals in the
various SDH transmission equipment throughout the network must be
consistent. If the corresponding relationship between time slot
and tributary channel numbering of 155 Mbit/s signals in the various
SDH transmission equipment is not consistent, time slot disorder
will occur when paths between multi-vendor equipment are to be set
up, thus bringing trouble on the provision of services and maintenance.
2.3 The time slot numbering rules of 155 Mbit/s trunk port signals
in the switches throughout the network must be consistent. If not,
the time slots will be displaced and the in and out time slots of
the trunks can not correspond, making interworking impossible.
2.4 The time slot numbering rules of 155 Mbit/s signals in the
switches and the transmission network throughout the network must
be consistent. If not, the time slots of the switches and those
transmitted will be different. This will cause difference in interpretation
between the operators of the switching and the transmission networks,
and also bring trouble on fault processing.
3. Strategy of Unifying Time Slot Numbering rules of 155 Mbit/S
Signals in the Transmission Network
Of all the SDH transmission equipment used in the Beijing telecom
network, the default values of the time slots of 155 Mbit/s signals
in Lucent's transmission equipment, which has the lion's share in
the transmission network in Beijing, are set according to YDN099-1998
Table 10. Marconi's SDH equipment has changed its time slot numbering
rules settings when it accesses the network so as to interwork with
Lucent's counterparts. Huawei's equipment has different default
values from Table 10, however, the company has changed them accordingly
in network level NM (in versions above RMS473) so that operators
can switch between the two numbering modes by setting an option.
After the switching of the numbering mode, services can be displayed
and configured on the basis of Lucent's numbering mode. But at that
time since there was no network-level NM in Beijing Telecom's transmission
network and the LCT (PC-based element-level NM) could display and
configure services only on the basis of Huawei's mode, the configuration
mode needed to be modified manually when implementing interconnection
to Lucent's equipment and each adjustment had to be made according
to Lucent's numbering mode through the built-in cross connection
function while configuring services. The change of the configuration
mode in the existing Huawei's transmission equipment would involve
a lot of work and lead to interruption of service. Thus a transitional
strategy was adopted, that is, time slot configuration was done
totally according to YDN099-1998 Table 10 and the equipment already
in operation was chiefly used in the access layer, not connected
to Lucent's transmission network. Now, since numbers can be displayed
in Lucent's numbering mode on the T2100/T2000 NM platform, with
only irregular arrangement, no reconfiguration will be done to most
of the existing equipment in the near future. As to the limited
number of sub-networks that have relatively numerous connections,
reconfiguration of services will be done. Adjustment of equipment
configuration in the network will not be taken into consideration
until the completion of Huawei's central NM system, T2100/T2000.
3.2. Strategy of Unifying the Time slot numbering rules of 155
Mbit/S Trunk Port Signals in the Switches
This issue does not exist in the switches that do not use 155
Mbit/s trunk ports. All the switches that use 155 Mbit/s trunk ports
are switching systems newly installed. Since they have not been
put into operation, it is easy to unify the time slot configuration
rules. The time slot configuration rules of 155 Mbit/s trunk port
signals can remain unchanged since they comply with the requirements
in YDN099-1998 Table 10. Equipment of other companies such as Huawei
and Ericsson needs to change its configuration according to those
requirements by modifying software so as to achieve the unification
of time slot numbering and configuration of 155 Mbit/s throughout
the network.
From the above, it can be seen that the unification of time slot
numbering and configuration of 155 Mbit/s in the switching and transmission
equipment throughout the network will bring great benefit to service
and maintenance. Therefore, before new transmission equipment and
switches access the network, an additional stipulation should be
written in technical specifications to require the default values
of time slot numbers of 155 Mbit/s port signals to comply with YDN099-1998
Table 10.
Ren Guangwei: He graduated from Northern Jiao Tong University
in 1988,he is currently deputy Chief Engineer of Beijing Telecom
Network Bureau.
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