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Key words: high capacity bandwidth cross management; intelligent
technology; ASTN/ASON; signaling and routing protocols
1. Introduction
Over the transfer network, which serves as a common transfer platform
for different telecommunications services, the growth of services,
including voice, data, multimedia, leased lines and broadband service,
and the development of the support networks such as the signaling
network, the clock transfer network and the NM liaison network have
made demand for transmission circuits. As a major resource of telecom
operators, the transfer network should not only meet its own needs
but also operates bandwidths, network elements and the network as
a commodity. The behavior of the transfer network has a significant
impact on service growth and market competitiveness of telecommu-nications
operators.
SDH, the core technology of the transfer network, started to be
applied in the early 1990s. Now, with its unique advantages, it
has taken an absolutely predominant position. However, as the development
of data and multimedia services represented by the Internet services
continues and the situation of telecommunications operation changes,
the environment of traffic transfer has change a lot. In providing
packetized and new services, the traditional SDH transfer network
has disadvantages such as complex service assignment, low bandwidth
efficiency, high cost and poor network expandability. Fortunately,
As optical-electronic technology advances, driven by market demand,
there have emerged equipment and technologies such as SDH-based
MSTP, high capacity bandwidth cross management and ASTN/ASON. Their
application has given support to high capacity, intelligent and
full service networks.
2. High Capacity Bandwidth Cross-management Equipment and Technology
High capacity bandwidth cross-management equipment is a new type
of network element, which can integrate all the access and transfer
facilities into a single network node. It combines the traditionally
separated network elements such as independent ADM equipment, cross
connect equipment, ATM and IP switch/router. Such integrity can
materially reduce the cost and space occupancy of the equipment
and eliminate the large amount of cable connection between the equipment
and faults that cable connection may cause. The core technology
of such equipment has the cross connecting capability of the high
capacity VC-4/VC-4-nc or VC-4/3/12/VC-12-nc and provides large numbers
of optical line interfaces of different rates and perfect network
protection and recovery functions. Therefore, networking by this
equipment enables flexible dispatch and dispersion of the traffic
and easy capacity and network expansion, without having any impact
on the existing network structure. Figure 1 shows the basic system
structure of the equipment.
Control/maintenance/synchronous timing/power system
ATM switching module IP routing switching module
Core cross matrix matrix with cross connection
Figure 1: Basic system structure of high capacity bandwidth cross
management
The high capacity bandwidth cross-management equipment is located
at the core, or the hub of the transfer network, connecting different
layers of the network and dispatching and managing the traffic flexibly.
With its extra-high capacity cross-connecting capability and features
of the optical line interfaces of different rates, it plays an important
role in the transfer network in the following ways:
* To flexibly dispatch and disperse traffics, especially to set
up and delete flexibly emergency and burst traffics;
* To disperse, merge, distribute and combine traffics, increasing
channel utilization and forming a clear structure of the network
channels;
* To perform traffic restoration throughout the network, improving
the operating capacity of the network;
* To perform all kinds of protection functions;
* To connect the different layers of the network, resulting in a
clear and orderly network structure, and to reduce the layers of
the network by means of its integrity;
* To build up channels gradually according to demands, and expand
the capacity and extend the network quickly and easily without having
impact on the existing network structure and services;
* To reduce materially the cost and space occupancy of the equipment
and eliminate the large amount of cable connection between the equipment
and faults that cable connection causes.
With the advancement of optical-electronic technology and driven
by market demand, broadband cross management equipment trends toward
high capacity, intelligentalization and full service. To be specific,
high capacity means cross connection capacity increasing from the
present hundreds of Gbit/s to some Tbit/s and tens of Tbit/s while
integrity is becoming increasingly higher and size smaller. Intelligentalization
is discussed below. Full service means realization of full service
processing capability by combining optical cross connection, ATM/IP
and Tbit/s processing capability and linking realtime and broadband
data services.
3. Intelligentalization of the Transfer Network
In order to adapt to the new traffic transfer environment and enhance
market competitiveness of telecommunications operators, there came
up intelligent technology of the transfer network, which is represented
by the automatic switching transfer network (ASTN) and the automatic
switching optical network (ASON). It may well be said that ASTN
technology represents a qualitative transition in the development
of the transfer network. In essence, the intelligentalization of
the transfer network is the realization of automatic and dynamic
configuration of network nodes and links, dynamic configuration
of bandwidths of the links and automatic routing, and the provision
of reliable network protection and recovery, by introducing signaling
and routing protocols in the transfer network.
The following is a discussion about some issues concerning the
ASTN.
* Network Management
In the traditional transfer network, network management is centralized,
with the network elements having no intelligence and information
about the entire network. This of course does not comply with dynamics
of the network. In contrast, in the ASTN a new multi-layer management
plane based on the transfer plane, the control plane and the signaling
network substitutes for the NM system of the transfer network, forming
a management mechanism that links centralized management and distributed
intelligence. The main functions of the centralized management system
are service provision management, such as virtual private network
(VPN), service level agreement (SLA), traffic engineering (TE),
billing and network resources management, and network auxiliary
planning. A major feature of the ASTN is the realization of distributed
intelligence, i.e., the intelligent network elements, which can
enable network topology discovery, routing calculation, automatic
link configuration, path management and control and traffic protection
and restoration. Thus many functions can be performed automatically
rather than manually.
* Control Model of Network Interconnection
There are two control models of network interconnection for the
ASTN, one the overlay model and the other the peer model.
In the overlay model, the addressing scheme, the routing protocol
and the signaling protocol at the customer layer run independently
from those at the transfer layer. The two layers exchange information
about the circuits, bandwidth requests, etc. via the user network
interface (UNI). Its advantages are that the transfer layer can
provide multi-service transfer as a common transparent platform;
the development of the transfer network is not restricted by the
customer layer; and the security of information in the transfer
network is guaranteed. Its disadvantages are that the functions
overlap and protocol conversion is complex.
In the peer Model, both the customer and transfer layers use the
common control plane and the general multi-protocol label switching
(GMPLS). Since they use the same addressing scheme, routing protocol
and signaling protocol, there is no UNI protocol. The advantages
of this model are that it enables protocol re-use, thus avoiding
the development of a set of protocols for the control plane of the
transfer network; it simplifies network management; and it promotes
the convergence of control of the data and transfer networks. However,
it is difficult to unify the standards of protocols of the customer
and transfer layers, and information safety remains to be addressed.
Based on the above analysis, taking into account the present situation
of the telecommunications operators and the characteristics of the
traffic, it is desirable to adopt the overlay model in the near
term. When IP-based service becomes the mainstream, the peer model
will be the direction.
* Standards of Protocols
The key to the application of the ASTN is standardization. At present
there are several international standard bodies which are engaged
in the standardization of the ASTN, such as ITU-T, IETF,OIF and
ODSI, each having its own focal point in the development of the
ASTN protocol standards.
In ITU-T G.807/Y.1302, in the control plane, three very important
interfaces are defined, depending on their positions and functions.
They are UNI (User Network Interface), I-NNI (Internal Network Network
Interface) and E-NNI (External Network Network Interface). The signaling
and routing information is exchange via these interfaces. See Figure
2.
Figure 2: Relationship and interfaces between entities of ASTN
UNI is a bi-directional signaling interface between control entities
of the service requesters (customers) and the service provider.
The main functions it supports are user request access, call control,
resources discovery, connection control and connection selection
as well as call safety and authentication management.
I-NNI is a bi-directional signaling interface in an autonomous
domain or between several autonomous domains having trusted relations
with each other. The main functions it supports are resources discovery,
connection control, connection selection and route hunting.
E-NNI is a bi-directional signaling interface between the control
entities of different autonomous domains, used to set up cross-domain
connections. The functions it supports include call control, resources
discovery, connection control, connection selection and route hunting.
ITU-T's G.ASTN pays particular attention to the overall architecture
and adopts the overlay model. Its related standards of protocols
include, among others:
* G.8070 (needs of the ASTN) and G.8080 (architecture of the ASON),
relating to the framework structure;
* G.7712, relating to the architecture and specifications of DCN
(used to carry management information and control signaling);
* G.7713, relating to the signaling (distributed calling and connection
management);
* G.7714, relating to automatic topology discovery;
* G. 7715, relating to routing; and
* G.7716, relating to link resources management.
By improving the existing protocols of the IP network, the IETF
developed the GMPLS protocol, which can be used in various networks
including the ASTN. This protocol pays particular attention to routing
and signaling protocols and adopts the peer model. Its related standards
of protocols include, among others:
* RSVP-TE (Resources Reservation Protocol-Traffic Engineering)
and CR-LDP (Constrained Routing-Label Distribution Protocol), relating
to signaling;
* OSPF (Open Shorted Path First) and IS-IS (Inter-domain System-Inter-domain
System), relating to routing; and
* LMP, relating to link management.
The OIF standard adopts the overlay model. Its related standards
of protocols include, among others, O-UNI and O-NNI. The establishment
of the O-UNI standard (Version 1.0) has finished and that of Version
2.0 is underway. The establishment of the O-NNI standard is expected
to finish at end-2002.
4. Conclusion
Thanks to the introduction into the transfer network of high capacity
bandwidth management equipment with high integrity, high capacity
and multi-service processing capability, intelligent network element-based
distributed network management, automatic dynamically configured
network nodes and links, dynamically configured link bandwidth,
automatic routing and intelligent technology that provides highly
reliable network protection and recovery, the telecommunications
operators has got much benefit in cost, flexibility and efficiency
in many respects such as network structure, traffic dispatch, network
operation and maintenance, network management and optimization.
Thus, their competitiveness in the telecommunications arena will
be greatly enhanced.
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Du Wei: Graduated from the BUPT in
1988, he is currently vice chief engineer of Beijing Institute of
Telecommunications Planning and Design. Having long been engaged
in the planning, networking and design of large-scale local transfer
networks, he is experienced both practically and theoretically.
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