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Abstract: The SDH network was built to accommodate the development
of voice service, and has now covered the subscriber base. The SDH
network also carries ATM and IP services, but adopted a unified
network and technology to transport and handle multiple services
to benefit telecommunications operators in all senses.
Key word: SDH; Network Coverage; Multi-service Transport Platform
With the development of telecommunications services and the construction
of network, the SDH network, which once was only used for the transmission
of voice service, is now becoming one of the key integrated service
transport networks of telecommunications operators.
Although compared to other transport technologies, SDH faces inborn
shortcomings. However, in the real world, no matter it is ILECs,
or CLECs and wireless carriers who claim to be forerunners of new
broadband technologies, none of them have been hesitant in building
their own SDH transport networks or integrated networks that are
based on SDH or DWDM/DXC technologies. To be honest, you may find
the scale of such construction is ever on the upward side.
1. The Driving Force behind the Development of the Multi-service
SDH Transport Network
Faced with increasingly fierce competition on the telecommunications
market, operators must consider the issue of survivability and need
to come up with a solution that fully satisfies market demands in
its network construction.
To strike such a solution, the key is to find ways to cut costs
and gain yields.
The cost includes expenses in construction, operation, management
and maintenance.
Once the cost issue is resolved, it would become urgent for operators
to consider how to satisfy customer demands for various telecommunications
services, and duly address the question of the random nature of
customer requirements in terms of time, capacity and location. This
is a determinant factor regarding the viability and prospect of
each and every telco.
As to which technology or equipment is best positioned to resolve
the above question, scholars and experts home and abroad have elaborated
their views in different perspectives, which I would not say more
than is needed.
To resolve the above question, telecos have set up overlapped networks
like SDH, ATM and IP in a row to answer the needs of different market
segments. However, it has been proved that construction of networks
separated from each other has rendered returns and application prospects
not as satisfactory.
Operators feel that the network coverage of the service area is
key to successful service rollouts.
For example, telephony as a universal service should serve areas
where there are residences by the establishment of exchange offices
and the wiring of subscription regions for adequate coverage.
In the initial stage, the SDH network was built only to back the
construction of voice networks, so its coverage is far broader than
networks where other technologies prevail. Along with the urban
development and the shifting of subscription areas, the voice network
also undergo changes, thus requiring the supportive SDH network
to upgrade its capacity respectively.
Basically, the SDH network is growing as POTS network grows, able
to track and cover subscription areas. If the SDH network can carry
ATM and IP services, a unified network and technology would be in
sight to resolve the problem of multi-service transport. As a result,
telcos would become the ultimate beneficiary.
2. The Development Opportunity of Multi-service SDH Transport Network
i. Determination of Network Coverage
The SDH network covers exactly where voice users are located. Given
that data users should first be voice users, the coverage issue
is resolved.
ii. Low Network Management (NM) Cost
By utilizing one unified platform for transport of multiple services,
NM systems in multiple networks are concentrated into one uniform
management system, and the construction cost of NM systems would
also be reduced.
iii. Network capacity may be upgraded as demand grows.
Due to the random nature of user demand, the transport platform
is required to support flexible capacity upgrading and adapt to
variations of service categories. On the multi-service transport
platform, circuit boards may be inserted where needed for capacity
upgrading or modification of service interfaces.
iv. High Reliability Service Transport Capability
With the multi-service SDH transport platform, especially the DXC-integrated
multi-port SDH equipment, the question of service protection may
be well resolved by making full use ring protection and meshed recovery
capabilities.
v. Facilitation of Resource Management
In the actual network operation and management, one headache is
how to manage network resources. As the subscriber base broadens,
network ports and link bandwidths are gradually being exhausted.
Also, the unbalanced distribution of subscribers leads to different
port consumption levels. To prevent the exhaustion of ports, the
node resource in the entire network must be separately monitored.
When the network or node resource comes to the critical exhaustion
point, the resource management department must come up with detailed
statistics and generate the resource pre-warning report so as to
apply capacity upgrading in a timely manner. If there are too many
types of networks, then each node in different networks must be
monitored and counted in a holistic way. This would require sophisticated
NM systems and a great deal of manpower. However, by adopting a
unified transport platform, the object of resource management would
turn out to be simple, thus substantially easing the resource management
work.
vi. Shortening of the Provisioning Time of Data Circuit
The multi-service SDH transport platform in supportive of the voice
service has already covered user areas, and is now undertaking the
task of transporting voice traffic. Once there is the demand for
data service, all that is needed is to insert the circuit board
necessitated by data traffic, thus shortening the provisioning time
of data traffic.
vii. Strong Service Transport Capability
The SDH equipment port supports the STM-64 (10Gbit/s) and the upcoming
STM-256 (40Gbit/s). Its transport capability is already far beyond
the ATM network, and is slightly higher than the IP equipment. It
is worth mentioning that it is easy for the SDH network to be integrated
with the DWDM system to form integrated SDH equipment, which may
present a port bandwidth of up to dozens of Tbit/s.
viii. Low CAPEX
Since there are increasingly more and more local vendors getting
involved in the manufacturing of SDH equipment, competition in this
market niche is fierce, particularly in the market of low-end SDH
equipment, the price of equipment is declining sharply. However,
the access part of the multi-service transport platform entails
a large amount of low-end SDH equipment, such as the 155M/622M/2.5G/10Gbit/s
equipment. It should also be noted that local vendors are entering
into the research, development and manufacturing of high-end products.
ix. Simplification of the Network Architecture to Rid Connections
between Equipment
The SDH equipment can integrate DWDM and DXC parts, thus eliminating
the need of installing too much DDF/ODF equipment in the transmission
operating room and reducing manual jumper wire operations. In so
doing, the network architecture is simplified and the network security
is improved.
3. Characteristics of the Multi-service SDH Transport Network
A unified and large-capacity multi-service transport platform should
support multiple protocols and transport mixed traffic, thus necessitating
marked changes in the traditional SDH technology and equipment.
i. Multi-service Mixed Access & Protocol Handling Capability
Besides the general PDH/SDH (E1...STM-16) branch channel access
capability, Ethernet (FE/GE) and ATM service (OC-3 OC-12 etc.) should
also be supported.
Based on the support to transmission transparency, the Ethernet
L2 protocol handling capability needs to be supported, such as point-to-point,
point-to-multi-point, shared rings as well as L3 routing function
(MPLS and BGP).
Based on the support to the above protocols, services like "Transparent
VLAN" and VPN may also be supported.
In addition to the support to ATM traffic transport, VP RING may
also be supported.
ii. Large-capacity, Multi-granularity, Multi-service Cross Capability
VC-12, VC3 and VC4 cross are supported.
The service port should be able to support the following kinds
of signals:
E1, E3, E4, DS3, STM-1, STM-4, STM-16, STM-64, VC-4-Xc, 10/100
Ethernet, Gigabit Ethernet.
For the cross part, data packet and TSS switching matrix may be
adopted according to service demands to obtain high port signal
cross forwarding efficiency.
iii. Support to Multiple Topologies
While building a network, it must be noted that the design and
architecture of the network must cater to actual needs. So multiple
topologies, e.g., linear or ring topology, may be adopted. In different
network scenarios, the add/drop and multiplexing of circuits must
be supported.
iv. Support to Resource Management, Operation Surveillance &
Tiered Service Configuration
When the network is running, a great deal of operation and maintenance
must be carried out. This may require functionally improved network
management and resource data processing systems.
a. Operation Management
To enable the network to be in perfect operation status and provide
superior QoS to subscribers, round-the-clock monitoring of network
operation is needed. Necessary steps may be taken in compliance
with the quality of operation.
b. Service Configuration
Based on the need of service provisioning by the business department,
corresponding network configuration may be conducted with circuits
offered.
c. Resource Management
This operation is done regularly to provide statistics of the network
usage status and the resource consumption condition. Also, it may
be used to identify nodes and networks where resources are being
exhausted and provide a sound basis for capacity upgrading.
d. Tiered Management Capability
The NM system also needs to support tiered network management to
realize tiered management of both core and access layers. In so
doing, the operation and configuration changes of the access layer
will not affect the core layer; correspondingly, configuration changes
in the core layer will not exert too many requirements on the access
layer equipment.
With a unified management system, one may even adopt SDH products
from different vendors at core and access layers. For a management
system like this, the only thing left to do is to realize unified
resource management and monitor the operation quality of the network.
Configuration management may be carried out by NM systems from respective
vendors.
v. Support to Marketing of Network Services & Customer Service
The NM system needs to support the service system of the business
department and link the attribute information of leased line users.
To than end, summary statistics concerning the provisioning, termination
and quality of leased lines may be provided in a QoS report.
Meanwhile, the system can provide users "one-stop service",
which means the business system may directly operate certain attributes
of user services through the service system. The user is also entitled
to get access to the quality report of his leased line service at
any time via the E-commerce interface provided by the operator.
vi. Compact Size, Low Power Consumption & Ease of Installation
in CPN
The access layer equipment of the multi-service transport platform
should be as close to users as possible, e.g., it may be installed
I n the residence building.
The equipment must be compact in size and low in power consumption,
and able to adapt to various power supply conditions.
The equipment should also fit worse operating conditions.
4. Application Examples
The application of multi-service transport platform is one of the
focal points concerned by current telcos. A lot of telcos have already
embarked on building such a network.
In early 2002, AT&T announced the launch of an intelligent
fiber-optic network that runs across the States. See Figure 1.
The network is based on two supporting platforms. One is the Intelligent
Optical Network Platform comprised by the CoreDirector Intelligent
Switch from CIENA, which can accomplish the transport task under
various bitrate conditions (from 51Mbit/s to 1Gbit/s). All circuit
configurations can be put into place through "one click".
At present, there are some 40 sets of such equipment in operation.
The Intelligent Optical Switch can automatically recover the circuit
service and resist multi-point failure through the meshed backbone
in the case of cut fiber links or invalid parts, thus guaranteeing
a high reliability of service transmission.
The Multi-Service Platform (MSP) is the second underlying platform
for the Intelligent Optical Network. It adopts the ONS15454 SDH
multi-service transport equipment from CISCO. ONS15454 can aggregate
multiple low bitrate data traffic (from 1.5 Mbit/s to 2.5 Gbit/s
and GE) to a high bitrate data channel (STM-16 (2.5Gbit/s) or STM-64
(10Gbit/s)), and access the high bitrate traffic to the high-speed
Intelligent Optical Network comprised by Intelligent Optical Switches.
At the same time, AT&T integrated the management over the Intelligent
Optical Switching Backbone Network (CD equipment) and the MSP network
(CISCO ONS15454) by leveraging its own OSS system.
The multi-service platform ONS15454 is characterized by low system
cost, compact form factor and support to capacity upgrading. In
particular, it can provide circuit interfaces for various bitrates
and service categories.
So far, AT&T has installed over 100 sets of access equipment
for the Intelligent Fiber-optic Network, with some of them installed
directly inside the office tower of customers to enable service
access at the closest point.
The Multi-Service Platform (MSP) is a key component of the network.
Presently, AT&T is providing sundry leased-line, Internet, voice,
data and video services in more than 40 cities, which in turn substantially
enhanced the corporate competitiveness in those market niches.
All in all, while selecting network construction plans, telcos
must give thoughts to second points: is it possible for the plan
to bring corporate competitive, and do the supported services have
a prospect of fast return? For that purpose, network coverage and
service diversity may serve as the ultimate differentiators.
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Author Biography
Senior Engineer Feng Yingsheng has been long dedicated to the maintenance,
engineering and construction of transmission networks. Once Vice
Chief Engineer of Beijing Telephone Office, Mr. Feng is currently
Vice Chief Engineer of Engineering & Construction Department
of Beijing Communications Corporation.
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