| Wide
Area Transport of GSM Voice and Data |
Asce Networks Inc.
Introduction
The worldwide market for cellular voice and data
traffic is booming. The instantaneous ability to communicate,
whether for voice or data needs, coupled with advances in cellular
technology are creating huge demand for this technology. As the
build-out of cellular networks increase, the demand for infrastructure
equipment capable of supporting large numbers of subscribers at
low cost is similarly increasing. Locations requiring coverage
may be within a city or municipal area, or may be isolated and
difficult to reach.
Of the various cellular standards, GSM is emerging
worldwide as the most popular, with close to 66% of the worldwide
subscriber base.In addition, next generation cellular technologies
promise to continue to propel GSM as the technology of choice
for supporting wireless voice and data applications.
Wireless Subscribers by Technology
(EMC Worldwide Cellular Database)
This paper discusses a mechanism for enabling wide
area transport of GSM voice and data cost effectively, lowering
the operational cost for GSM service providers in extending their
network coverage.
Anatomy of a GSM Network
GSM Networks consist of several key network
elements:
MSC: The MSC (Mobile Switching Center) is
responsible for connectivity between the PSTN and nodes, which
transmit and receive cellular traffic.
BSC: The BSC (Base Station Controller) is responsible for
connectivity between the MSC and individual cell nodes located
within a small geographic region.
BTS: The BTS (Base Transceiver Station) is the transmitter
and receiver for an individual cell site.
Thus, diagrammatically, a cell phone network looks as follows:
Links between the BSC and the BTS are termed Abis-Interfaces,
while links from the MSC to the BSC are termed A-Interfaces.Call
signaling from the MSC to the BSC is handled by SS7.
The BTS nodes can operate at different frequencies,
usually 900 / 1800 MHz, or 1900 MHz in North America.
GSM Voice and Data Services
GSM provides voice and data services to wireless
users. Voice services are as one would expect, and form the overwhelming
majority of revenue today from wireless services. Voice is compressed
using a 12.2K (Full Rate) CELP Coder Enhanced Full-Rate GSM vocoder.
The compressed voice is typically transcoded into G.711 at the
BSC into 64K channels and sent to the MSC for connection to the
PSTN.
Data Services over GSM are a hot topic currently.
With the emergence of new technologies such as GPRS (Generic Packet
Radio Service), SMS (Short Messaging Service), and WAP (Wireless
Application Protocol), service provider revenue from data services
are expected to explode over the next few years. Customers will
start using wireless devices for leisure (stock quotes, movie
times, sports, news), corporate applications (remote data entry,
e-mail), and e-commerce (banking, shopping).
Currently, SMS is the most widely deployed data service.
It is a store and forward technology, forwarding messages of limited
length to wireless users. It can serve as a vehicle for WAP applications,
but would be somewhat limited due to the non-interactive nature.
SMS is typically used in paging applications to the mobile phones
and its low bandwidth utilization is sufficient for this purpose.
GPRS services are just being rolled out now. These
offer higher bandwidth packetized services, more suited to the
needs of future data applications. Trials are currently taking
place in virtually every country in Europe, as well as some parts
of Asia and the United States.
Finally, WAP is emerging as a thin client for cellular
data services. WAP enables web like applications to run on mobile
devices, with displays being suited for the type of device it
is running on. WAP can run over GPRS services, SMS services, or
circuit switched services.
Enabling the GSM Network Buildout
As GSM networks reach into more and more locations
worldwide, the need to cost effectively interconnect these cellular
nodes exists. Cell locations or mobile themselves may be hard
to reach, (for example wireless nodes aboard ships). In these
types of applications, it can be expensive operationally to provide
GSM coverage.
Carrying full rate GSM data all the way back to the
MSC can be an inefficient way to reduce bandwidth between the
MSC and the BSC. GSM data is quite inefficiently packed into E1
frames, and therefore bandwidth is wasted on the A-Interface.
Generally, between the BSC and the BTS, there is no issue. BTS
units are generally geographically close to one another, and efficient
multiplexing of BTS's can be accomplished using a variety of topological
approaches.
In addition, SS7 signaling is required between the
MSC and the BSC over the A-Interface. When no signaling (that
is, no calls) are required, SS7 continuously transmits idle frames
(called FISU - Frame Idle Signaling Units) over the link. A 64K
signaling channel is completely utilized, whether signaling is
required or not. Thus, the signaling is bandwidth inefficient.
In order to reduce the operational cost of supporting
these remote GSM nodes, it is important to reduce the bandwidth
associated with maintaining these nodes.This will of course, dramatically
reduce the operational costs associated with GSM network buildouts.
There are several techniques that may be applied
to reduce the operational bandwidth of GSM remote nodes:
Voice compression: Voice compression techniques may
be applied to dramatically reduce voice bandwidth while maintaining
high quality voice. Voice compression techniques may reduce bandwidth
for voice conversations by a factor of 6:1. This can enable a
topology where bandwidth between the BSC and the MSC is much more
efficient, where there are no wasted bits in a compressed GSM
frame, and there is no need to carry G.711 64K voice all the way
out to a remote BSC.
Data compression: Compression of circuit switched data can dramatically
reduce bandwidth when applications are sending wireless data or
fax information. This can achieve bandwidth savings of over 4:1.
If a cell phone makes a data call, once the call reaches the BSC
is it rate adapted to fill an entire channel. Therefore, a 9.6Kbps
call is padded to fill an entire 64K channel using the V.110 standard.
All the padding information is wasted bandwidth and can be removed.
Signaling compression: Signaling associated with cellular traffic
can be quite bandwidth intensive. SS7 is a high bandwidth protocol,
and techniques may be applied to dramatically reduce the bandwidth
associated with this signaling.
While applying these techniques, it is important that the user
notices no loss of performance or applications.These techniques
must be completely transparent, yet support all of the voice and
data applications discussed earlier.
Finally, to provide maximum flexibility for providers
to provision remote nodes, all of these techniques must be available
independent of the interconnect mechanisms. These techniques should
work over any data backbone (IP based networks, Frame Relay based
networks, ATM networks, VSAT Satellite networks) and provide the
same benefit.
Asce Networks' Solution
Asce Networks offers a product called the Climax-960,
which provides for compression of voice and data traffic specifically
targeted at cellular network providers. Each unit is capable of
handling up to 60 channels of voice or data traffic, and compressing
it over any type of network infrastructure (Frame Relay, IP, ATM,
or VSAT). The Climax-960 focuses on reducing bandwidth over the
A-Interface in a cellular network. This enables low cost provisioning
of remote BSC nodes in a network. In particular, it offers:
G.729 based compression of voice traffic: Compression
of voice traffic, coupled with silence suppression techniques,
can reduce the bandwidth required without a significant compromise
to voice quality.
Automatic Detection of Circuit Switched Data: Circuit switched
data carried over V.110 is automatically detected by the Climax-960,
and different compression techniques are applied to eliminate
redundant information. Using this technique, bandwidth is reduced
up to 4:1 or more. When voice traffic returns on a channel, the
channel reverts to standard voice compression mode.
Spoofing of SS7 traffic: The Climax-960 is capable of spoofing
unimportant SS7 traffic, reducing bandwidth associated with call
signaling. This does not interfere with normal SS7 call signaling,
call roaming, or SS7 based data applications such as SMS.
Applications
These techniques may be applied to GSM network buildouts,
or to mobile GSM network nodes where the cost of bandwidth is
expensive. Examples include:
Extension of GSM service to rural or remote regions.
Rural or remote regions may be difficult or expensive to reach,
and bandwidth limitations may exist in trying to access them.
Service may be limited via landline circuits, or reachable only
via satellite. Deployment of the Climax-960 enables transparent
bandwidth reduction in these situations.
Emergency or Supplemental GSM service. Vans may be equipped with
GSM gear and deployed in situations where there are temporary
outages or where supplemental service is required. For example,
large sporting events may cause local cellular areas to be overwhelmed,
limiting the service available to customers and revenue to the
carrier. Supplemental service may be deployed over satellite networks
to temporarily increase coverage in an area.
Mobile Platforms. Providing GSM service to mobile platforms, for
example, cruise ships, is another application where bandwidth
is expensive and techniques discussed here may be beneficial.
