GSM-MAP Evolution

The original standards documents described the three maximum data rates supportable.
Originally the rates were 144 kbps, 384 kbps, and 2048 kbps—equivalent to IDSN
2B + D, ISDN HO, and the lowest entry-level ATM rate. Circuit-switched services
would be supported up to 384 kbps, and higher data rates would be packet-switched;
144 kbps would be available in macrocells; 384 kbps would be available in microcells;
and 2048 kbps would be available in picocells. The original chip spreading rate in the
standard was 4.096 Mcps. This was chosen to support the ATM 2.048 kbps bearer—
that is, 2048 kbps equals 1024 kilosymbols; 1024 kilosymbols times a chip cover of 4
equals 4.096 (1024 × 4).
The chip rate was then reduced. (This was done for political reasons, in the spirit of
bringing the chip rate closer to the CDMA2000 chip rate.) However, as a consequence
of this, adjacent channel performance improved. The cost was that the 2.048 kbps top
user rate was reduced to 1920 kbps equivalent to ISDN H12—that is, 960 kilosymbols
(960 × 4 = 3.84 Mcps), as shown in Table 12.1.
There is less focus now on ISDN partly because of the increased need to support
very variable user data rates. So, for example, we still use the ISDN rates as a maximum
user data throughput but effectively provide an ATM end-to-end wireless and
wireline channel for each individual user’s packet stream (or multiple per-user traffic
streams). Given this shift in emphasis, it was decided to try and improve bandwidth
utilization in the ATM copper access transport layer by maintaining the DTX (discontinuous
transmission) used over the radio layer as traffic moved into the network core.
This is implemented using a protocol known as ATM AAL2.

Our good friend SS7 still stays very much in charge of traffic flow control, but there
are proposals to implement broadband SS7 on the basis that the existing 64 kbps-based
signaling pipes will become too small. This work is being presently undertaken by
3GPP alongside proposals to implement an IP-based signaling bearer known as
SCTP/IP (Signaling Control Transport Plane using IP).
Some network operators and vendors remain unconvinced of the merits of using IP
to replace existing (tried, trusted, and effective) signaling protocols, so progress on
standardization might be rather slower than expected.
The IUB interface between the Node B and the RNC is 2048 kbps ATM (2.048 Mbps),
and the IU interface is 155 Mbps ATM. DTX is implemented using AAL2 across the IUB
and In interface (into the core network). Each individual RNC looks after its own family
of Node Bs except, referring to Figure 12.7, where a mobile is supported by two
Node Bs under separate RNCs. This has to be managed by the IUR interface used by
the RNCs to talk to one another. The RNCs also have to manage load balancing, which
we covered in the previous chapter.
A typical RNC is configured to support several hundred Node Bs. The RNC is
responsible for mobility management, call processing (session setup, session maintenance,
session clear-down), radio resource allocation, link maintenance, and handover
control. Note the RNC needs to make admission control decisions looking out toward
the Node B on the basis of radio layer noise measurements, and admission control
decisions looking inward to the core network on the basis of network congestion.