Bogere Joseph Alfred1, Dr Julianne Sansa-Otim2 and Ronald ...

Part 2: The Feasibility of National Roaming in Highly Competitive Mobile Markets: A Case Study of Uganda 63
Communication between the VLR of the Visited Network “B” and the HLR of
the Home Network “A”.
When a mobile subscriber or roamer from Network “B” approaches the visited Network
“A” or switches on a mobile station (MS) for the fi rst time in a visited network, the VLR
initiates the update location procedure with the roamer’s HLR.
Figure 3: MGT based routing [siddiqui 2006].
At this point the only information available in the VLR of visited Network “A” is the
International Mobile Subscriber Identity (IMSI) of the roamer; this will be used by the
visited Network “A” to build routing information (Signaling Connection Control Part
Message (SCCP) addressing known as the Mobile Global Title (MGT) or E.214 used to
communicate back to the roamers home Network “B”.
Then the roamer’s HLR i.e. Network “B” responds to the VLR of the visited
Network “A” by including its own E.164 address in the Calling Party Address (CgPA)
of the SCCP message. The E.164 part, as defi ned in the ITU-T E.164 recommendation,
is used to identify the country and PLMN (in this particular case, we referred to the
PLMNs as networks “A” and “B”) or PLMN and HLR, where the roamer is registered.
On receiving an initial response from the HLR of Network “B”, the VLR of Network
“A” then builds the routing information for subsequent or future communication with
the HLR of network “B” from the calling party’s address in the received response.
Therefore the VLR of the visited network “A” can address the HLR of the home
network “B” using an E.214 MGT that has been originally built from the roamer’s
IMSI and an E.164 HLR address. An E.214 MGT translation can be done either at the
application level or at the SCCP level in the VLR using a routing table.
The design of both the HLR and VLR depends totally on vendor’s choice, some
vendors have one component performing switching and VLR (all together named
MSC) functions while others have them separated. In most network designs, the HLR
is always kept as a single component.
Engineer’s Perspective
This study considered two network operators in Uganda, which for confi dentiality
purposes are referred to as A and B. Three network engineers were interviewed, one from
A and two from B. The authors had planned to interview at least 5 engineers but due to
their availability only three were interviewed. Based on the study and the results from
the interviews, National Roaming is technically possible, but the operator’s perspective