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1. Introduction
To accommodate the trend of a communications network increasingly
going broadband and packetized, the design of PSTN exchange has
been under continued improvement. This can be seen in the enhancement
of exchanging capacity, the replacement of 2Mbit/s trunking interface
by SDH STM-1 or the integration of data processing module with the
data voice gateway and the dial-in access server with the ADSL module.
It is also no surprise to see the co-existence of PS and CS modules
or an all-round PS layout on the exchanging part. In all, the introduction
of broadband communications has blazed a new trail for the legacy
exchange network. Due to the ever-expanding capacity, it is now
a customary and practical option to employ the STM trunking in the
exchange, which in the long run will substantially impact the structural
evolution of the exchange and transmission networks. With STM, the
network structure is greatly simplified and optimized, the number
of equipment is decreased and the pit footprint is reduced. This
will further cut down on line distribution failures, improve the
network reliability and bring down the O&M cost.
Beijing Telecom's STM-1 experiment could be dated back to the
year 1999 when it was re-organizing its PSTM network. However, STM-1
was too new a technology at that time, and no relevant national
standard was available. So an all-round interoperability testing
between the STM-1 exchange interface and Beijing Telecom's transmission
system only took place in early 2000 with a view to exploring its
technological feasibility and locating its possible problems. Based
on such experiences, the company decided to phase in the deployment
of STM-1 interfaces on its PSTN network and has seen very satisfactory
results ever since. This paper sets out to give you an overview
of Beijing Telecom's STM-1 deployment in its PSTN exchanges in early
2000 and the interoperability test between such interfaces and its
SDH system.
2. Why and What to Test
The test is aimed at:
I. Checking on the compliance of optical and electrical STM-1
interfaces on the tested exchange to the national and ITU-T specifications;
II. Exploring the feasibility of interoperability between the
STM-1 interface on the tested exchange and Beijing Telecom's transmission
system as well as between different tested exchanges, locating lurking
possible problems and coming up with solutions;
III. Basing Beijing Telecom's future exchange type selection
on the test results;
IV Accumulating experiences for Beijing Telecom to enable a full
rollout of STM-1 exchange interfaces and a simplified and optimized
network topology.
The types of exchanges under test then included Shanghai Bell's
P3S, Huawei's C&C08and Ericsson's AXE10, which are all leading
brands dominant in Beijing Telecom's PSTN network. Besides, all
the three brands were able to provide the STM-1 interface in 1999.
Backed by the Telecom Transmission Institute of the Ministry
of Information Industry, Beijing Telecom drew up the specifications
and schemes for the interoperability test between its STM-1 exchange
interface and SDH system. The main tested items included the performance
indicator of the optical STM-1 interface of the transmission part
of the exchange and its testing method; the performance indicator
of the electrical STM-1 interface of the transmission part of the
exchange and its testing method; the method for testing the compliance
of the STM-1 signal overhead byte (including the segment overhead
byte and the channel overhead byte); the method for testing the
synchronization between the exchange and the SDH system; the impact
of 1+1 MSP at the exchange side on SDH; the NM response to the insertion
of error code in the system and the error code performance testing
following the connection of the exchange to the SDH system and so
forth.
The tested items include:
I. Interface parameters. Optical and electrical STM-1 exchange
interfaces were tested respectively to find out the extent to which
they conformed to the national and ITU-T standards and locate possible
problems;
II. Interoperability, which include the STM-1 interoperability
between the exchange and Beijing Telecom's transmission system (including
both Lucent's and Marconi's transmission equipment), direct interoperability
between STM-1 interfaces on different exchanges as well as the interoperability
after trespassing the transmission system. The interoperability
referred to herein involves both basic and high levels. Basic interoperability
test is designed to check alarms on physical connections, the error
rate of information delivery and the 2Mbit/s cross-connect state
when STM-1 signals pass the transmission system. High-level interoperability
test is to check the consistency of generated against readout overhead
bytes, alarms and the performance monitoring as well as the synchronization,
protection rotation, the interworking possibility of NM information
and the O&M capability.
3. Test Results
The following are test results of the aforementioned three exchange
types:
3.1 Huawei Exchange
The test result showed that the STM-1 interface performance indicator
of Huawei exchange completely satisfied the ITU-T standard, and
reported no unusual condition when carrying out 155Mbit/s O/E interface
interworking with Lucent's transmission equipment and 155Mbit/s
electrical interface interworking with GPT's transmission equipment.
The exchange could also synchronize with SDH clock via the STM-1
signal. Except for a full support of frame loss alarm, the exchange
was supportive of all other SDH alarm features. The A1 framing byte
could not generate a frame loss alarm on its own unless the A1 and
A2 bytes were both modified. In addition, the exchange did not support
the protection K1 byte on the multiplexing segment, namely, the
tested version did not offer the 1+1 Multiplexing Segment Protection
(MSP) function. The S1 byte could be manually configured, but could
not be automatically generated.
It was proved that Huawei exchange was capable of interworking
with the serving SDH system, and could support SDH transmission
alarm functions in a satisfactory way. Also it could realize the
call processing function in cooperation with other exchanges via
its STM-1 interface. We suggest that Huawei further improve the
frame synchronization detection feature on its STM-1 exchange interface.
3.2 Ericsson Exchange
The test result showed that the STM-1 interface performance indicator
of Ericsson exchange completely satisfied the ITU-T standard, and
reported no unusual condition when carrying out 155Mbit/s O/E interface
interworking with Lucent's transmission equipment and 155Mbit/s
electrical interface interworking with GPT's transmission equipment.
The exchange could also synchronize with SDH clock via the STM-1
signal, and could support the 1+1 multiplexing segment protection
function as well as most of SDH alarming functions. However, the
A1 framing byte could not generate a frame loss alarm on its own
unless an A2 byte was modified. Outgoing S1 bytes could be configured
on the exchange, but no received S1 byte was shown on the exchange
NM. In addition, the exchange was configured with no M1 byte, and
could not support far-end error indication on the multiplexing segment
and low-order and high-order channels. It could not support far-end
out-of-work indication on the low-order channel either.
It was proved that Huawei exchange was capable of interworking
with the serving SDH system, and could support most of the SDH transmission
alarm functions. Also it could realize the call processing function
in cooperation with other exchanges via its STM-1 interface. We
suggest that Ericsson further improve the frame synchronization
detection feature and the S1 byte display feature on its STM-1 exchange
interface, and add far-end error indication on the multiplexing
segment and low-order and high-order channels and far-end out-of-work
indication on the low-order channel.
3.3 Shanghai Bell's P3S Exchange
The test result showed that the STM-1 interface performance indicator
of Shanghai Bell's P3S exchange completely satisfied the ITU-T standard,
and reported no unusual condition when carrying out 155Mbit/s O/E
interface interworking with Lucent's transmission equipment. The
exchange could also synchronize with SDH clock via the STM-1 signal.
However, only by modifying both the first A1 byte and the first
A2 byte in the framing bytes or the second A1 byte and the first
A2 byte or the third A1 byte and the first A2 byte could a frame
loss alarm be generated. Also, with a value always being "00",
S1 could be neither manually configured nor automatically generated.
The exchange could not terminate M1 bytes, high-order channel far-end
error indication signals, N1 bytes, low-order far-end error indication
signals, N2 bytes, K4 bytes and low-order channel far-end out-of-work
indication. Instead, it could only send those bytes back to the
originating side. At the same time, the exchange could not support
far-end error indication on the multiplexing segment and low-order
and high-order channels. It could not support far-end out-of-work
indication on the low-order channel either.
It was proved that Shanghai Bell's P3S exchange could not terminate
some alarm overhead bytes. Instead, it sent back those bytes to
mistake local errors for far-end faults. We suggest that Shanghai
Bell terminate such bytes, improve the frame synchronization detection
function and the processing of S1 bytes, and add far-end error indication
on the multiplexing segment and low-order and high-order channels
and far-end out-of-work indication on the low-order channel.
In addition, we conducted call tests on Shanghai Bell's P3S exchange
and Huawei's C&C08 exchange. The two exchanges were connected
via Lucent's SDH transmission equipment. The test ended up with
successful completion of calls.
4. Conclusion
The test done in early 2000 showed that the STM-1 interfaces
on all tested exchanges completely satisfied the ITU-T standard,
and there appeared no problem when they interworked with Beijing
Telecom's transmission equipment in realizing O/E STM-1 interface
connection at lower levels. So all the exchanges could meet basic
communications demands. Therefore the test demonstrated the feasibility
of employing the STM-1 exchange interface to take the place of the
2Mbit/s trunking interface in the network organization work. The
major problems found in the test was that some overhead bytes of
the STM-1 signal processed on those exchanges were inconsistent
with those in the transmission system. This would to some extent
undermine the effective cooperation of the NM function between the
exchange and the transmission system, obstruct the alarm judgement
and locating and impede the realization of the protection rotation
function. Based on the results, Beijing Telecom has forwarded its
suggestions to relevant vendors, who in turn have made a number
of improvements in their later product versions.
With the STM-1 link as the inter-office trunk, the number of
DDFs will be markedly reduced, thus saving floor space in the pit
and wiping off distribution-related failures. Besides, the number
of outgoing trunks will be brought down, thus demanding less maintenance
staff and improving the maintenance efficiency. In that sense, it
represents a major trend in the exchange network to replace the
current 2Mbit/s trunks by STM-1 links or even faster links. As there
is expected to be a huge traffic to be carried on one STM-1 link,
the possibility of failures is running high. Therefore, when STM-1
interfaces are implemented on the exchange, the backup and protection
rotation of circuits and the detour of routes must be given adequate
stress. The STM-1 interface will first be led in through tandem
points where large volume of traffic is assembled, then gradually
applied to large nodes that require over 30 2Mbit/s trunks. The
application of STM-1 on the exchange network also implies substantive
influence on the development of the whole transmission network,
which used to provide only 2Mbit/s circuits and seldom put in use
155Mbit/s ones. So if too many 155Mbit/s circuits were applied,
it would be difficult for the current SDH network to fit in. That
is why a 155Mbit/s-granularity broadband SDH platform is in need.
Author Autobiography
Mr. Tang Xiongyan is now the deputy chief engineer of China Telecom
Group Beijing Corporation, and a guest professor of Beijing University
of Posts and Telecommunications (BUPT). He received a Ph.D. degree
from BUPT in 1994. He was a post-doctoral fellow at Nanyang Technological
University, Singapore from 1994 to 1996, and worked at the Technical
University of Berlin, Germany as an Alexander von Humboldt research
fellow from 1996 to 1997. His professional fields include optical
communications, IP/ATM networks, access networks and network planning.
Mr. Tang has published more than 50 technical papers.
Xiao Jianguo: Mr Xiao obtained a Docter degree from Tsinghua University
in 1998. He work for the Technical Department of Beijing Telecom.
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