A scalabe solution to connect ip-based BSS to legacy GSM networks

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Dmitri Soloviev dmi3sol at gmail.com
Sat Mar 23 09:09:39 UTC 2013


Hi

we've got several requests to extend existing GSM networks with our radio
equipment, where ip transport is appreciated.

Please take a look at my vision of interfacing A-over-ip to a legacy GSM
A-interface.

Building blocks:

1. OpenBSC with ip.access-compatible A-over-ip interface

2. MGCP-controlled transcoder, built with Sangoma D-series cards. Can be
assumed as a shared resource, available for several BSC instances.
http://www.sangoma.com/assets/docs/datasheets/en/d500.pdf

3. TelscaleSS7card - a stand-alone signaling and media gateway, with 2 E1
and 2 Ethernet interfaces, running embedded linux on board.
It converts A-over-ip to A-interface, acts as MGCP call agent for BTSs,
transcoders and other telscaleSS7cards. A card is implemented in PCI form
factor, but bus is used just to provide power supply.
In other words, a card can act either as a combined SG+MG, converting
A-over-ip into SS7 and RTP into E1 timeslot, or just a MG, converting RTP
into E1, without any impact on the host system.


When a [signaling gateway + call agent] detects ASSIGNMENT_REQUEST or
HANDOVER_REQUEST, it stores cic value. As soon as ASSIGNMENT_COMPLETE or
HO_REQ_ACK is detected, it builds RTP chain that unites
BTS<->transcoder<->media_gateway. The chain is broken after the call is
finished.

Depending on the chosen cross plane, 4U 19" industrial PC can carry several
BSC instances with transcoding, serving about 20 E1 interfaces (ea about 80
ARFCNs). Performance requirements for industrial PC cards are also
negligible: computer will run OpenBSC and configure transcoder cards, being
controlled by means of MGCP.



Best Regards,
Dmitri
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