Mobile Networking
Mobile Network Signaling:
Telecommunications signaling is the transmission of data for purposes of sharing
information for network control and/or call control. The type of signaling used in
a modern network is called Signaling System number Seven (SS7).
SS7 is a critical component of modern telecommunications systems. SS7 is a
communications protocol that provides signaling and control for various network
services and capabilities. Being a layered protocol, SS7 provides various
protocol levels for connection oriented and connectionless (database) signaling
in fixed and mobile networks.
SS7 is comprised of a series of interconnected network elements such as
switches, databases, and routing nodes. Each of these elements is
interconnected with links, each of which has a specific purpose. The routing
nodes are the heart of the SS7 network and are called a Signal Transfer Point
(STP). STPs are connected to Service Switching Points (SSP) that are switches
equipped with SS7 control logic.
SS7 Network Architecture:
Each of these network elements have a specific role in the SS7 network.
• Service Switching Point (SSP): A telecommunications switch that contains the
control logic (software) necessary to send/receive SS7 messages to other
nodes in the network. If a Mobile Switching Center (MSC) has said control
logic, it is by definition considered an SSP.
• Signal Transfer Point (STP): This is the "heart" of the SS7 network. STP
routes messages between other network elements.
• Service Control Point (SCP): This is the "brain" of the SS7 network. The SCP
is nothing more than a database. However, utilization of a SCP offers
profound enhancements for service delivery and network control.
• Service Node (SN): Includes database functionality of the SCP along with
additional capabilities such as voice interaction and control of voice
resources. Generally speaking, SCPs work well with requirements that call
for voluminous data transactions. SNs, on the other hand, are typically not
designed for high volume data processing. Instead, SNs are best suited for
special circumstance call processing involving voice resources and/or
interaction.
SS7 Network Elements:
SS7 involves two different types of signaling:
connection oriented signaling and connectionless oriented signaling. Connection oriented signaling refers to the establishment of switch-to-switch facilities call inter-office trunks. These trunks
carrier voice communications. The ISDN User Part (ISUP) part of the SS7
protocol is utilized to establish trunks between switches. In contrast, the
Transaction Capability Application Part (TCAP) is utilized for connectionless
signaling which typically entails switch-to-database or database-to-database
communications. An example of connectionless signaling is TCAP signaling of
Home Location Register to Visiting Location Register discussed below.
Mobile Networking Standards
There are two major types of inter-system signaling for mobile communications:
GSM Mobile Application Part (MAP) and ANSI-41. GSM MAP is the standard
utilized for GSM and ANSI-41 is the inter-system standard for all other mobile
networks including CDMA, D-AMPS (IS-136), and AMPS.
Although the two standards have their differences, both have certain key things
in common. They support three network elements that are required for mobile
communications: MSCs, HLRs, and VLRs.
Mobile Switching Center:
A Mobile Switching Center (MSC) is a telecommunications switch deployed in
mobile communications networks, to provide call control, processing and access
to the Public Switched Telephone (fixed) Network.
Home Location Register
The Home Location Register (HLR) is a database that is maintained by a user's
home carrier or the mobile operator from whom the user has initiated service.
The HLR stores information about the user, including the user profile
(preferences), account status, features and capabilities.
Visiting Location Register
The Visiting Location Register (VLR) is another database and is used by the
serving carrier system to manage service requests from mobile users who are
away from their home system.
The interaction between the MSC, HLR, and VLR is best understood by walking
through the registration process.
In the above figure, the mobile user's home system is depicted as System A and
the mobile user is currently roaming in System B, the visited or serving system.
Step One: Detecting a Potential User
The first step involves the serving system detecting your mobile phone. This
occurs over the radio interface. Each mobile phone emits a unique identification
that is detected by the radio frequency (RF) equipment of the serving system.
The mere fact that you have your mobile phone "powered on" triggers the serving
system RF equipment to inform the equipment switching of your presence. The
switching equipment in turn queries a database which first determines whether
you are in your "home" area or whether you are a "visitor". In this context, visitor
means that you are not in your normal home location (the city/area where you
signed up for service).
Step Two: Exchanging the Appropriate Information about the User
The second step involves database interaction to determine appropriate handling
of call requests. If you are in your home area, the Home Location Register (HLR)
provides information necessary to handle requests for either call origination
(making a call) or termination (being called). If you are not in your home area, a
Visiting Location Register (VLR) must request information from the HLR so that
the visited (serving) system can process calls appropriately.
Communication between the VLR in the serving system and the HLR of the
home area is facilitated by mobile networking protocols and signaling based on
MSC/VLR
HLR
MSC/VLR
SS7 Signaling Network
System B System A
ReqNot (invoke)
ReqNot (response with profile)
SS7. In GSM networks, GSM MAP mobile networking protocol rides on top of
SS7, allowing VLR to HLR (and HLR to VLR) communications.
This signaling and database communications typically occurs before any call is
either placed or received, allowing the serving system to know exactly how to
handle calls when a call is placed/received.
Step Three: Handling Calls
By the time the user either places or receives a call, this database interaction has
occurred. The serving system now knows whether the mobile phone is
associated with an account in good standing (bills all paid), user feature/service
subscription, and the location of the user so that calls placed to the user may be
delivered.
Placing a call
Upon detecting a request for a mobile phone user to originate a call, the serving
system reviews the VLR record (established in Step Two above) to determine
appropriate treatment. This allows the serving system to determine if the call is
allowed and if any additional features/services should be made available. Once
this investigation occurs (in a fraction of a second), the switch processes the call
attempt as appropriate.
Receiving a call
When someone calls a mobile phone user, the home switch reviews the HLR. If
the user is in the home area, the call is delivered immediately. If the mobile user
is in another serving area (called "roaming"), the HLR record indicates which
VLR is currently maintaining the mobile user's records. The HLR uses SS7 and
the appropriate mobile networking protocol to request delivery instructions from
the VLR. The VLR provides these instructions to the HLR, allowing the home
switch to deliver the call to the serving switch and seamlessly terminate the call
to the mobile phone as if the user were in the home area.
Mobile Networking and Applications:
ANSI-41 and GSM MAP support various applications including basic services
such as call waiting and conference calling. This support is provided in the form
of profile updates between the serving system (VLR) and the home system
(HLR). The HLR updates the VLR regarding subscriber services, allowing the
serving system to provide the appropriate services to the visitor. A discussion of
these applications is beyond the scope and purpose of this white paper.
