5/4/12 Roaming Management under SS7 It mainly addresses the interactions between the PCS network and the PSTN Signaling – Exchange of control information Network – to – Network(NNI) Signa ling System Channel Associated Signaling System(CAS) Common Channel Signaling System#7(CCS#7) In-band Signaling – Control Information and userinformation sent on a single line Out-band Signaling - Control Information and userinformation sent separately into logically orh sicall se arate aths
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CCSS#7 / Signaling System No.7(SS7)CCSS#7 is an advanced digital signaling and control system,that is a set of telephony signaling protocols which are used to
set up most of the world’s telephone calls (Also called asSS7)
SS7 being a high speed and high-performance packet-basedcommunication protocol, can communicate significantamounts of information when setting up a call, during a calland at the end of the call
Signal Transfer Point (STP) A switch that relays SS7 messages between
network switches and databases.
Based on the address fields of the SS7 messages,the STPs route the messages to the correct out-going signaling links.
To meet the stringent reliability requirements,STPs are provisioned in mated pairs
STPs maintain routing tables for the purposes of directing messages to their intended destinationsi.e. based on the address fields of the SS7messages the STPs route the messages to the
The MS enters the area controlled by MSC2.MSC2 launches a registration query to its VLR through
STP2, assuming that VLR2 and MSC2 are not co-located
Step 2.VLR2 sends a registration message to the MS's HLR
(HLR4 in Figure 2.8).
VLR2 may not know the actual address of HLR. Instead,
VLR2 sends the message containing the MS identity,called the Mobile Identification Number (MIN), to anSTP (STP3 in our example) that can translate the MINinto the HLR address.
In steps 2, 3, 4, and 5, the messages may visitseveral STPs before arriving at their destinations,and the registration process may generateconsiderable traffic in the SS7 network.
Thus, it is desirable to reduce the registrationtraffic.
Two approaches have been proposed to reducethe "cost" of deregistration at step 5 in Figure2.8:
Periodic re-registrationthe MS periodically reregisters to the VLR.
If the VLR does not receive the re-registration messagewithin a timeout period, the record is deleted.
This approach only creates local message traffic betweenthe MSC and the VLR . Furthermore, no SS7 signalingmessages are generated if the VLR is co-located with theMSC.
Pointer Forwarding Scheme
To reduce the registration traffic at steps 2 and 3 a
pointer forwarding scheme was proposed, whichconsists of two operations:
Depending on the memory capacities of theVLRs, the pointers in the obsolete chain may or may not be deleted.
To limit the pointer traversal time in the findoperation, the registration procedure in Figure 2.8may be performed for every k move operations.
In other words, the number of pointers visited inthe find operation will be limited by k. The pointer forwarding scheme should not be considered whenthe net cost of pointer creation and pointer
traversal is higher than the cost of accessing theHLR.
As performance studies indicate, the pointer forwarding scheme significantly reduces the
DiscussionAnother heuristic determines the obsoleteness of an
entry based on the period that an MS resides in a VLR as
indicated in the cache entry.
If the cache entry indicates that the MS has stayed in a
VLR for a period longer than a specified threshold, theentry is assumed to be obsolete.
The threshold can be adjusted in real time based oncache hit statistics. If case 3 is more likely to occur thancase 2, then the cache scheme should not be considered.
Performance studies indicate that the cache scheme
significantly reduces the call delivery cost in many