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<h2>Frame Relay and Gb Interface</h2>

<p>Gb is the interface between a BSC and a SGSN. It is implemented
using the Frame Relay. The Frame Relay can be either point-to-point (PCU–SGSN),
or there can be a frame relay network located between the BSC and SGSN. The
protocol stack comprises BSSGB, NS, and L1. The Frame Relay as stated in standards
will be part of the Network Service (NS) layer. On top of the physical layer
in the Gb-interface, the direct point-to-point Frame Relay connections or
intermediate Frame Relay network can be used. The physical layer is implemented
as one or several PCM-E1 lines with G.703 interface. The FR network will be
comprised of third-party off-the-shelf products. The following figure displays
examples of Gb interface transmission solutions:</p>


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<img alt="urn:mars:dn0469553:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0469553&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: BSC - SGSN interface</p>
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<p>In the first solution (1) spare capacity of Ater and A
interfaces is used for the Gb. The Gb timeslots are transparently through
connected in the TCSM and in the MSC. If free capacity exists, it is best
to multiplex all Gb traffic to the same physical link to achieve possible
transmission savings. In many cases, the SGSN will be located in the MSC site,
and thus the multiplexing has to take place there as well. Normal cross-connect
equipment, for example, Nokia DN2 can be used for that purpose.</p>


<p>The second solution (2) represents whatever transmission
network provides a point-to-point connection between the BSC and the SGSN.
In the third solution (3) Frame Relay network is used. The Gb interface allows
the exchange of signalling information and user data. The Gb interface allows
many users to be multiplexed over the same physical resources.</p>


<p>At least one timeslot of 64 kbps is needed for each activated
PCU bearer. A PCU can connect to 128 TRX/64 cells maximum. This capacity cannot
be shared with other cells connected to other PCUs in the BSC so there is
no pooling. The PCU has to be installed into every BCSU for redundancy reasons,
but the FR bearer has to be connected only to the active ones. For example,
it is possible to start, for example, with two active PCUs to serve all the
cells in the one 256 TRX, 128 cells BSC2E/A. Considering the transmission
protection, it also needs to be decided whether two Frame Relay bearers are
needed for each PCU using different ETs (external 2Ms) or if the transmission
is protected with cross connection equipment.</p>


<p>It is possible to multiplex more than one Gb interface
directly to the SGSN, or multiplex them on the A interface towards the MSC
and cross-connect them to the SGSN from there. The 2M carrying the Gb timeslots
can be one of the BSC's existing Ets, or an ET can be dedicated to the Gb
interface.</p>


<p>The Gb interface allows the exchange of signalling information
and user data. It also allows many users to be multiplexed over the same physical
resources. The logical structure of the point-to-point Gb interface is presented
in the following figure:</p>


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<img alt="urn:mars:dn0469308:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0469308&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Gb logical structure</p>
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<p>In the BSC, each PCU represents one Network Service Entity
with own Identifier (NSEI). Each PCU can have one to four (ffs) FR bearer
channels. The Access Rate of a FR Bearer Channel can be configured in 64kbit
steps. Each Bearer channel carries one to four Network Service Virtual Connections
(NS-VC). Each BTS has a BSSGP Virtual Connection of its own. The NSE takes
care of the multiplexing of BSSGP Virtual Connections into the NS Virtual
Connections and load sharing between the different NS Virtual Connections
(= Bearer Channels).</p>


<p>The following figure displays the Gb protocol layers:</p>


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<img alt="urn:mars:dn0469359:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0469359&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Gb interface</p>
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