For the interworking with other networks, the first thing to consider is the service interworking. Presently, the services provided by the operators include fixed telephone service, IP access service, IP toll service and mobile phone service. In addition, the multimedia video phone service may be a key service of the 3G networks, for which the interworking should be considered.
For
the interworking with other networks, the second thing to consider is the
allocation of key resources to services, which affects the QoS. For the voice
communication, it is required to consider the proper configuration of EC
resources while preventing other services from wasting the EC resources.
For the
interworking with other networks, it is required to consider the security and
reliability of the whole network. It is better for the gateway exchange
construction to follow the principles of large capacity and paired
construction.
For
the interworking with other networks, it is better to access the nearby network
to affectively avoid the alternative routes of the mobile subscribers. The
remote network may also be accessed for this purpose, which is useful for the
charging settlement and the full utilization of EC functions.
10.3.1 Numbering
Plan
Now,
the numbering plans adopted by the mobile communication include the network
numbering plan and the PSTN numbering plan.
The
net numbering plan is the numbering plan adopted in current China
The
PSTN numbering plan is to number the mobile subscribers and equipment with the
original PSTN coding scheme. The plan is consistent with the PSTN as to the
dialing scheme with a short number and easy availability of the number.
Advantages
of the PSTN numbering plan:
l The types of the numbers are the same, so such original equipment of
PSTN as the tandem exchange and the toll exchange may be still adopted by only
adding relevant data configurations.
l Compared with the network numbering plan, its number length is
shorter for the convenience of dialing by fixed subscribers.
However,
the following problems may occur in this kind of numbering plan:
l Alternative routes may occur when the called is roaming.
l After the separation of North Telecom and South Telecom, this plan
may obtain different number resources from different regions, or be different
from the dialing habits of subscribers. It is not certain whether subscribers
can accept such a plan.
The
situation of severe competitions among the operators in the China
With
MAP routing analysis and call routing analysis, the adoption of the net
numbering plan is helpful for the network selection with simple data
configurations. It is better to adopt this numbering plan for the call among
some international gateway exchanges or some operators, while the PSTN numbering
plan is useful to select the geographical position of the call or MAP signaling
routes.
10.3.2 Gateway Exchange Solution
1. Requirements
for gateway exchange construction
1) Have such features as large-capacity trunks, powerful signaling
processing capacity and abundant voice resources, thus satisfying the
development trend and construction requirements of the gateway exchange for
“large capacity & few offices”.
2) Have a variety of interfaces (at least E1 interfaces available) to
support diverse services and access modes; and provide ISP with the PRI
interface to adapt to the fast development of the Internet.
3) Have echo cancellation function to reduce the echo resulting from
the two-wire/four-wire transformation between mobile calls and fixed calls with
the relevant echo canceller configured in the gateway exchange.
4) Have the built-in SDH to provide the 155M interfaces for
implementing the centralized traffic, few office directions and large trunk
groups in the gateway exchange, so as to reduce the networking cost, the utilization
area of equipment room and the system power consumption.
5) Have the ability to analyze the calling number of the incoming
trunk.
6) Support flexible networking: For the local network without LSTP or
enough LSTP capacity, the gateway exchange may be served also as LSTP; for the
mobile network providing abundant services, it is required to support 2M
signaling or multi-signaling point technology to satisfy the high capacity
requirement of the signaling links; For the exchange with over 4096 circuits in
a single office direction, the switching equipment must support multi-signaling
point technologies.
7) Have the SSP/IP function, the CAMEL and upgrading capabilities for
the convenience of CAP capability expansion according to the service
requirements.
2. Gateway
exchange construction solution
The
networking of the GMSC is very simple. The GMSC is usually the external
interface exchange of an operator. For the outgoing calls, it is to transfer
the voice channels out directly; for the incoming calls, it needs to inquire
the MSRN of the called, and then connect the call according to the MSRN. At the
early stage of construction, generally the GMSC is also served as the VMSC.
With the increase of VMSCs, independent GMSC construction gradually begins (if
there are more than three VMSCs) to reduce alternative voice channels between
these VMSCs.
For
the former mobile operators, construction of the mobile gateway exchange is to
build new ones and upgrade old gateway exchanges. It is required to consider
the networking setting problems of the gateway exchange, that is, the planning
of the gateway exchange.
For
the operators that own fixed networks, the following solutions are available
for the construction of their mobile gateway exchange:
1) Solution 1: The fixed gateway exchange is integrated with the mobile GMSC by upgrading the fixed network gateway exchange
The
following functional upgrading should be carried out for the fixed gateway
exchange:
l Increase MAP functional modules for route enquiry.
l Increase EC functional modules.
l Increase the functions of CAMEL3 SSPs.
l Increase relevant billing functions.
l Enhance the number analysis function.
If
only partial upgrading can be achieved for the current fixed gateway exchange,
some local networks should be set separately and some may be integrated. For
the office that can be upgraded, it is also necessary to check the current
traffic load of the gateway exchange. If the traffic load is heavy, the switch
should be checked to find whether capacity expansion can solve the load
problem. For this reason, the following requirements should be satisfied before
carrying out the upgrading:
l The current traffic load is light.
l The current traffic load is heavy but it may be solved through
capacity expansion.
l The equipment vendors ensure the exchange can be upgraded smoothly.
Advantages
of the solution:
l The GW and GMSC integration is in accordance with the network
construction trend of large capacity and few offices. Especially at the early
stage of networking, the existing resources of the fixed gateway exchange are
utilized to carry out efficient networking, thus shortening the construction
cycle and reducing the construction cost.
Disadvantages
of the solution:
l It requires to upgrade the fixed gateway exchange, which may affect
the existing network.
l Part of the gateway exchange cannot be upgraded, which makes the
networking structure unclear.
2) Solution 2: Build a new 3G mobile gateway exchange for the fixed
network
This
solution is to build a new 3G mobile gateway exchange separated from the fixed
gateway exchange.
Advantages
of the solution:
l There is no change to the old gateway exchange, no risks from the
upgrading, and no impact on the existing fixed network from the traffic load.
l The networking structure is clear, which is helpful for future network optimization and upgrading.
l It is helpful for the integrated operators to carry out independent
operation management and independent accounting of the mobile and fixed
networks.
l The EC allocation and utilization is clear, which is helpful to
reduce the EC costs.
Disadvantages
of the solution:
l At the early stage of construction, it causes the trunk waste; and
compared with the integrated gateway exchange solution, it increases the
constructions costs and operation costs of the office and its auxiliary
equipment.
l Although the integrated operators have fixed telephone networks,
they treat their mobile phone networks the same as those of other mobile
operators with two GWs passed, which increases the links of the call and
affects the network quality.
Generally,
independent accounting is required for each internal network of the integrated
operators. At present, the interconnection and accounting of the internal
networks are carried out by building their respective gateway exchanges. Thus,
each private network needs its own independent gateway exchange, which results
in repeated investment and does no good to the unified internal accounting.
10.3.3 Echo Canceller (EC) Configuration Solution
The
EC configuration is critical for the interconnection between the 3G mobile
network and the PSTN. The EC has the automatic detection function to detect the
modern signals and can disable the EC function automatically, so it will not
affect the data service functions. However, the equipment cost is fairly high.
To configure EC resources where unnecessary will increase the costs of network
construction.
EC
should be configured as near as possible to the fixed telephone. Generally, the
EC can cancel 64 ms echo of the toll calls within about 6500 km. Echo problems
occur if the distance is more than this figure.
1. Access mode
of the EC
MSC
provides two types of echo cancellers: Embedded echo canceller and independent
echo canceller.
The
embedded echo canceller is based on the concept of exclusive use of resources,
that is, one trunk circuit exclusively occupies one echo canceller. When the E1
signals access the MSC, the signal echo cancellation function is implemented.
The
independent echo canceller is based on the concept of resources sharing. All
the echo cancellers are placed in the ECPOOL (Echo Cancellation Resources
Pool). The echo canceller is occupied upon request and then released after use
for reoccupation by other connections. The number of independent echo
cancellers may be configured flexibly according to the traffic and the number
of trunk equipment generating the echo to maximize the resources sharing.
From
the perspective of system resources occupation, the ECPOOL should occupy the
interface frame slot. For the call connection that requires echo cancellation,
if an independent echo canceller is adopted, then four switching timeslots will
be occupied in the central switching network: One is connected to the incoming
E1 timeslot, one to the outgoing E1 timeslot, and the other two to the echo
canceller as shown in the following figure. And if an embedded echo canceller
is adopted, then the connection is the same as the common call connection with
only two switching timeslots occupied: One is connected to the incoming E1
timeslot, and the other to the outgoing E1 timeslot shown in the following
figure. That is, the independent echo canceller realizes the global sharing of
echo cancellation resources by occupying more system resources than the
embedded echo canceller. In particular, the EC is usually configured on the
GMSC equipment with great capacity. The shared EC will greatly reduce the
number of E1s that can access the GMSC.
Although
the embedded EC can save the switching resources, sometimes the ECs are wasted.
For example, the data services (including dial-up access and multimedia calls)
are converted with IWF equipment and need no EC to cancel echo any more. In
such a case, EC will not affect these services, but it is a waste of the EC
processing resources. The embedded EC is suitable for the GSM network, in which
there are few such services and little waste of EC resources.
In
the other case, PSTN calls the mobile subscriber and then forwards the call to
another PSTN subscriber, thus forming an end-to-end PSTN-to-PSTN call and
causing a waste of the embedded EC resources.
The
adoption of ECPOOL may cause a waste of EC resources, because the TUP protocol
and ISUP protocol on the network do not support the EC processing. In this way,
the GMSC with ECPOOL resources does not know whether the exchanges in the call
route have applied for EC. Incorrect judgment will cause excessive or
insufficient EC resources to be requested, thus lowering the communication
quality.
2. Configuration
of data service channels
The
data service, multimedia service and fax service need no EC equipment. PSTN
calls the mobile network, and the call is forwarded to the PSTN again without
passing the EC equipment. When interconnecting with the PSTN, the data service
and the voice service may have their respective trunk groups or office
directions, with one for speech call and one for data service or multimedia
service. This requires that the GMSC should have the ability to select
different routes for the data service and the voice service.
If the data or
multimedia traffic is not large, then data services and voice services may not
be separated. In the current GSM network, they are usually not separated. However,
the 3G multimedia service is considered as a key service with wide application,
so it may be separated to some extent.
10.3.4 Routing
Mode
The
routing mode is considered generally from the perspective of the caller.
1. Access to
the nearby network is recommended for internal calls of the operator
Considering
the fact of internal calls and all revenues to the same operator, technically
the mode of accessing the nearby network is recommended.
The
advantage of this mode is that it may effectively reduce the alternative routes
of the mobile services. Its disadvantage is that it does no good to the fully
play of the EC function when the mobile network calls the toll fixed network.
2. Access to
the remote network is recommended for the calls among operators (a subscriber
of the local network calls a subscriber of other networks)
In this mode,
the toll service adopts its own toll network, thus achieving the maximum
benefits during internetwork settlement.
10.3.5 R4
Interworking
The
3G R4 network is characterized by the separation of the MSC server from the MGW
and the use of ATM/IP as the transmission bearer. Just like R99, the R4 network
needs the interworking of the bearer connections and the conversion of the user
plane media stream formats, as well as the echo control and the interworking of
control layer protocols. As to the service interworking mode, R99 and R4 are
almost the same.
On
the whole, the interaction between the 3G R4 core network and the PSTN is
completed via the ISUP signaling and trunk signaling. Just like the traditional
core network, 3G R4 should be configured with a dedicated gateway exchange to
fulfill the interworking requirements as described above, so as to simplify the
internetwork interactive connections and reduce the number of interactive
points for the convenience of internetwork settlement and interworking resources
management. Based on the architecture of separating the 3G R4 core network
bearer from control, it is clear that the GMSC server undertakes the signaling
interworking in the application layer of the control plane. The signaling
bearer interworking is completed by the independent signaling gateway SG
equipment or GMSC server, while the interworking function between the bearer
connection and the multimedia layer depends on the MGW equipment.
In
the traditional mobile network and the PSTN interworking model, the
interworking between the signaling control plane and the bearer plane is
implemented by the centralized GMSC with large capacity. In the 3G R4 model
where bearer is separate from control, the same MSC server with large capacity
may control several MGWs, so the interworking may be more flexible according to
the MGW capability and the peripheral networking conditions, for instance, the
independent gateway MSC server or the integrated MSC server (that combines the
local exchange and gateway exchange functions) controls the interworking
between the multiple local MGWs that provide TDM interfaces and the local PSTN
network.
