Page 30 - From GMS to LTE

16 From GSM to LTE-Advanced Pro and 5G

On the radio network side, the A interface continues to be used to connect the radio
network to the MSC‐Ss and MGWs. The connection can be made without any changes
in the radio network over the classic E‐1‐based A interface or over an IP‐based A interface.
In addition, the A interface has been made more flexible and can now be connected to
several media gateways. This adds redundancy toward the radio network as well, as a
geographical region can still be served even if a media gateway fails.
The Nc interface is used to transport voice calls within the core network, for example,
to gateways, to other mobiles or to fixed networks. The protocol used on this interface
is referred to as the BICC protocol and is very similar to the traditional ISUP protocol.
This is specified in ITU Q.1901 [7] and 3GPP TS 29.205 [8]. By using an SGW as shown
in Figure 1.10, the protocol can be converted into ISUP allowing the forwarding of calls
to other core networks that are still based on the classic model. In practice, it can be
observed that despite many networks having moved to an IP‐based architecture, the
gateways between them are still based on the classic architecture.
Virtual speech channels that have been negotiated over the Nc interface are transmit-
ted between MGWs over the Nb interface. The combination of the Nb interface and Nc
interface thus replaces the E interface of the classic network architecture. A voice chan-
nel is transmitted over IP connections either as PCM/G.711, Narrowband‐AMR or
Wideband‐AMR, depending on the type of radio network, the configuration of the
network and the capabilities of the mobile device. At the borders of the core network,
for example, to and from the A interface to the GSM radio network or to and from a
classic fixed‐line PSTN network, MGWs can convert media streams, for example,
between Narrowband‐AMR over IP to G.711/PCM over E‐1. This requires, however,
that an MGW contain both Ethernet ports and E‐1 ports.
Gateways between mobile networks are usually still based on ISUP and circuit‐
switched links, even though most networks are based on IP technology today. In the
future, this is expected to change as advanced speech codecs such as Wideband‐AMR
can only be used over BICN and IP‐based transport links.
Like in classic core networks, the C and D interfaces are used in a BICN network to
communicate with the HLR. Instead of E‐1 links, however, communication is based on
IP links today.

1.6.2 The Visitor Location Register (VLR)
Each MSC has an associated Visitor Location Register (VLR), which holds the record of
each subscriber that is currently served by the MSC (Figure 1.12). These records are only
copies of the original records, which are stored in the HLR (see Section 1.6.3). The VLR is
mainly used to reduce signaling between the MSC and the HLR. If a subscriber roams into
the area of an MSC, the data are copied to the VLR of the MSC and are thus locally avail-
able for every connection establishment. Verification of the subscriber’s record at every
connection establishment is necessary as the record contains information about the ser-
vices that are active and the services from which the subscriber is barred. Thus, it is pos-
sible, for example, to bar outgoing calls while allowing incoming calls, to prevent abuse of
the system. While the standards allow implementation of the VLR as an independent
hardware component, all vendors have implemented the VLR simply as a software com-
ponent in the MSC. This is possible because MSC and VLR use different SCCP SSNs as
shown in Figure 1.12 (see Section 1.4.1) and can thus run on a single physical node.