| 1. Overview
Since 2000, the construction of a broadband information network
has gradually become a hot point in the development of data communications
network, resulting in many network carriers setting feet on this
area. Broadband access network as a means to break the "last
mile" access bottleneck has become a de facto key concern in
the broadband field. Currently leading broadband access technologies
include DSL, CABLE, LAN and the follow-up WIRELESS technology, etc.
With the IP technology being continuously improved, most carriers
are coming to take IP as a dominating transmission technology for
the data network. Consequently, a multitude of Ethernet-based access
technologies such as LAN and IP DSL followed suit. After two years
of technological preparations and investigations, Shanghai Telecom
rolled out its broadband network in 2001 on a large scale, which
included both backbone and broadband access networks. The network
through one year of operation contributed certain experiences and
inevitably ran across some challenges, which mainly arose from network
architecture, certification and billing, network management and
maintenance, etc. Around the problems stated above, this paper intends
to explore them into depth and give some personal views.
2. Network Architecture
The importance of network architecture is self-evident, given
its underpinning role in traffic transmission, network development
and network manageability. Taking into consideration of such factors
as customer management, topology simplicity and the saving of IP
addresses, the current network in Shanghai adopts a three-layer
architecture, which breaks down to the Corridor Layer, the Community
Aggregation Layer and the DSLAM Layer (see Figure 1).
Although Layer 2 switch is used at the corridor layer, which implies
simplicity in equipment deployment, it has also brought about an
outstanding wiring problem. As the transmission distance of the
equipment is subject to a 100-meter restriction, the coverage of
corridor equipment is small, thus requiring more pieces of equipment
to be deployed. There are also other lurking difficulties. On the
one hand, Class-5 lines are only fit for outdoor usage. On the other
hand no electrical cables of enough number of pairs are available.
All this has limited the placement of equipment and added to the
wiring complexity.
The Community Aggregation Layer is mainly large-capacity Layer
2 or Layer 3 equipment, which is basically placed in the existing
community telecom operating rooms.
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The DSLAM Layer is where different Ethernet solutions differ from
each other. Competitive broadband carriers generally adopt generic
Layer 3 switches or routers, primarily for the sake of slashing
investment at the expense of user management. Legacy telecom carriers,
out of their long history of operating large-capacity network, tend
to attach much importance to user management. Therefore they often
adopt equipment with certain user management features (e.g., BAS)
at this layer.
3. Certification & Billing
In the early stage, broadband access will be mainly charged at
a monthly flat rate, but with the continuous development of the
service, it will turn out to be particularly important for incumbent
carriers like China Telecom to perform unified management and provide
differentiated services. Therefore a complete billing solution and
certification solution is in much need. Actually this would constitute
a supporting point for the generation of services.
In the present stage, leading certification methods include VLAN+WEB,
PPPOE and 802.1 x, etc. In the case of VLAN+WEB, the user data is
delivered transparently. However, the user has to get the address
via DHCP. So to realize certification at the application layer,
the only way a user can leverage to get the address will be broadcasting.
Then there is inevitably the problem of address substitution or
the availability of public network address as soon as the user turns
on the machine. The substitution of private network address by public
network address will unavoidably disable the user to perform concentrated
management and give rise to problems like operating system relevance
at the user end. The availability of the public network address
when the machine is turned on will lead to a huge body of public
addresses being squatted by non-existent users, thus leading to
a considerable waste of addresses. Between the WEB certification
server and the CPE equipment, at present only private vendor protocols
may be applied, resulting in a lack of coherence in user management.
If a user is logged off in an exceptional case, the access server
may need to determine the user status based on the longest idle
time configured for the user. If it has exceeded the longest idle
time, the server will deem that the user has logged off. This would
possibly render a billing period longer than the actual online period
of the user. To resolve this problem, it may be necessary to configure
a shorter period of time for the longest idle time parameter. However,
if the configured time is too short, it will be quite possible for
the user to be required multiple re-certifications when he is on
line, thus leading to much inconvenience.
PPPOE has been widely applied in ADSL access. Its major weakness
is large DSLAM overhead, leading to possible bottleneck at that
side. There is certain overhead in the data packet. After the user
certification is passed, the broadband access server (BAS) sends
start-of-billing packet to the back-end RADIUS server. After the
user logs off (e.g., active user hang-up, unusual outage and network
breakdown), BAS will send end-of-billing packet to the back-end
RADIUS server. Then the back-end billing system may conduct real-time
time or traffic-based billing according to the start-of-billing
and end-of-billing packets to produce very accurate billing data.
Because the first stage of PPP protocol certification process is
the Discovery stage, the broadcasting can only discover BAS at layer
2 network. This has decided that it impossible to place a router
or Layer 3 switch between the user host and BAS. Moreover, due to
the point-to-point nature of PPPoE, the existence of multicasting
protocols is restricted between the user host and BAS. This will
in turn impact future development of video services to a certain
degree.
802.1 X is a new standard recently proposed by IEEE, and is gaining
support from more and more vendors. It is mainly designed to realize
user certification on the two layers of switches so as to bring
down the cost of the overall network. Its main weakness is the re-certification
problem. Since the protocol itself has no KEEP ALIVE contents, it
needs to send certification information on a regular basis to judge
the user's status. Inevitably this manner will lead to inaccurate
billing information and add to the burden of the certification system.
PPPOE is an extension of the traditional PSTN narrowband dial-up
technology over the Ethernet. Its main advantages are reflected
in its consistency with the user certification system of the original
narrowband network and the convenience it brings in both user and
service management. So it is well adapted to the current operation
and maintenance system of telcos and has been widely adopted in
Shanghai.
4. User Management & Security
A unified management of broadband users has become a prerequisite
for the large-scale development of broadband network. At the same
time, an effective management of line and equipment resources also
provides a vital support for network maintenance and management.
Currently there is a multitude of access means, so is the case with
certification and billing methods. Therefore the realization of
a unified certification and billing method constitutes the foundation
for a unified user management. At present, the certification of
Shanghai broadband users have already realized a unified RADIUS
management, and differentiated services based on user characteristics
are also in place.
To realize a unified resource management and a unified business
interface, Shanghai Telecom has gradually realized unified broadband
service handling through its "97 business system". A unified
management of the assignment of labors and numbers has also been
realized via this "97 business system". Because the Ethernet
access is quite different from PSTN and ADSL in structure, traditional
management methods are still yet to fully satisfy the requirement
of Ethernet access. So the management process must be gradually
improved.
At present, user isolation and security mechanism can only be
realized through the port-based VLAN and 802.1 QVLAN. The port-based
VLAN is short of exclusive user identifications and lacks a security
mechanism, so it cannot tell users from each other and does no good
to the service development. 802.1Q VLAN has certain requirement
for the equipment, and is often subject to the number of VLAN IDs.
At the same time, the planning of VLAN has also become very important.
Given that Ethernet switches are widely and densely distributed,
the configuration of equipment has become quite difficult, which
in turn has add to the complexity in engineering, maintenance and
management.
5. Network Management (NM) & Maintenance
Operation and maintenance serves as a guarantee for network carriers
to serve users, and is also related to the service quality and corporate
image of telecom enterprises. Shanghai Telecom has long been attaching
great importance to the development of the NM system, which is not
only a key component of network management, but also an indispensable
support for network maintenance and operational analysis. The Ethernet
access originated from the computer network, therefore its capability
in management and maintenance is poor. That is also why it is still
slow in development pace.
With the continuous growth of broadband services and given its
particular technical nature, the number of NM layers is being gradually
decreased. But poor manageability prevails in the current data products,
especially switches, which are scattered in large number and multiple
types and thus constitute a more salient bottleneck in the NM respect.
The SNMP protocol is simple, but it is hard to realize a unified
configuration and management of the equipment.
The Ethernet access network features complex layers, and most
of the equipment is close to the user. Also its equipment environment
is poor, and the power supply and grounding conditions cannot even
begin to compare with those in the machine room. In the network,
the failure rate of equipment is high, and multiple network layers
lead to added failure points, making them hard to be identified.
At the same time, the equipment cannot support the corresponding
failure detection feature. So the whole maintenance process needs
an all new model. Compared to the traditional maintenance process,
the Ethernet is more complex and poses more strict requirements
on the equipment.
6. Engineering & Construction
The difficulty in engineering and construction has become an
impeding factor for the Ethernet access. First of all the Class
5 wire per se is limited to outdoor usage, and lacks cables of big
number of pairs. This has restricted the placement of equipment
and requires the deployment of communication pipes in the building,
thus adding to the wiring complexity. For communities where wiring
has already been completed, this will more often than not influence
the inhabitants and increase the construction difficulty. At the
same time, optical cables are needed between the corridor equipment
and the community center equipment, so a large quantity of optical
cables is to be laid in the community. Meanwhile, the power supply
and grounding of corridor equipment also implies certain problems.
At present, lighting mains is adopted in most cases.
7. Investment & Services
At this point of time, the poor utilization ratio of the Ethernet
access equipment has become the biggest headache for carriers. The
small coverage accounts largely for the low installation rate. At
the same time, the insufficient service offering is also is an important
factor for the sluggish market. From the perspective of per port
price, the Ethernet access equipment owns an evident price advantage.
However, taking into account the wiring cost and the equipment utilization
ratio, its total investment would be far more expensive than other
access means.
Most of the services from telecom carriers are based on tariff
and service, and content-based service is never an advantage of
carriers. So the forces of other public sectors need to be combined.
Shanghai Telecom used to consider adopting the Ethernet to access
communities and providing contents on community basis. But it soon
found that the limited technical level of properties affected the
service quality, and too scattered information sources were also
not conducive to the update of contents. Therefore in the end it
decided to provide information services on the MAN.
8. Summary
Based on the above discussion, we may conclude that in order
to apply the current Ethernet access on a massive operational basis,
more work needs to be done to improve its standard protocols, technical
solutions and management methods. An extensive application of it
requires a combined effort of technical bodies, equipment vendors
and carriers. Thus far, VDSL has been regarded as an improvement
to the Ethernet access technology, and Shanghai Telecom is keeping
a close eye on its development.
Author Biography:
Ma Ning : A graduate of Xi'an Technology
University, Currently he is engineer and working for the Planning
Division of the Planning and Development Department of Shanghai
Telecom Co., engaging in the planning and construction of data communications
network and broadband access network.
Mao Xiaoling : A graduate of Shanghai Jiaotong
University, Currently he is engineer and working for the Planning
Division of the Planning and Development Department of Shanghai
Telecom Co., engaging in the planning and construction of broadband
access network.
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