21 Call Routing in GSM Network

Suchit Purohit

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Learning Objectives

  • Recap of previous module
  • Mechanism of Call Terminating Call
  • Mechanism of Call Originating Call
  • Call Routing from fixed network to mobile device and vice versa
  • Call Routing from mobile phone to mobile phone

Introduction

 

In the previous modules, we learnt about architecture, security mechanisms and handover mechanisms in GSM. This is the last module of GSM standard. In this module we will learn about how a call is made to and from GSM network and the principles underlying it.

 

Types of Calls

 

There can be two types of calls. Either a call is originated from a mobile device or it is terminated on a mobile phone.

  • Mobile Terminating Call: A Call from Fixed landline/PLMN network to mobile device in GSM network
  • Mobile Originating Call: A Call from Mobile phone to fixed landline/ Mobile phone in GSM network

In the next section we will learn about the details of mechanisms which takes place during these calls.

Mobile Terminating Call

 

Let us see the steps which are performed when a call is made from a landline network to a mobile phone in GSM network which may or may not be in its home network.

 

Step 1: GMSC is identified and call is forwarded to GMSC

 

The number is dialed from a fixed landline number; it goes to the PSTN network. Looking at the MSISDN number, PSTN identifies that number belongs to GSM network. The call now should be forwarded to GMSC because GMSC is the entity which is intermediate between PSTN and GSM network. Now it is to be identified to which GMSC call is to be forwarded. This information is coded in NDC of MSISDN number. Fig. 1 and Fig 2 show these sequences.

 

Figure 1: Identification of GMSC

 

Figure 2: GMSC identified and call routed to GMSC

 

Step 2: Identify the MSC area where mobile phone is residing

 

GSMC identifies HLR of the subscriber which is coded in the number. HLR is used to identify the status of the mobile subscriber. The following information about called subscriber is extracted from HLR (Fig. 3)

  • Mobile phone is not switched off
  • Mobile phone is in the network?
  • Dialed number is valid number?
  • Account information stating bill not paid?

Figure 3: HLR identified and status of subscriber identified

 

All this information can be extracted from HLR. If the mobile phone is disconnected from GSM network, an IMSI detach field is updated in HLR. This field is updated explicitly or implicitly. If the mobile phone is switched off, IMSI detach is explicit and if it is out of coverage area, an implicit IMSI detach is set. If no IMSI entry for the MSISDN number dialed exists in HLR, it is found that number is not a valid GSM subscriber. HLR also contains information about balance of the dialed number.

 

Figure 4: Status identification from HLR

 

Step 3: After identifying status of subscriber, GMSC routes the call to the subscriber. So it should know in which MSC area mobile phone is residing.

 

HLR identifies current serving MSC VLR which is updated on regular basis when the mobile device moves from one MSC to another. HLR requests MSRN number from current MSC/VLR and forwards the MSRN number to GMSC (Fig. 5). MSRN (Mobile Station Roaming Number) is a temporary address that hides the identity and location of subscriber when it is roaming to other MSC. It is a temporary location dependent ISDN number assign to the subscriber when it is roaming in other network. It has same structure as MSISDN but contains current visitor Country Code (VCC), Visitor National Destination Code (VNDC), Identification of current MSC together with subscriber number. It is assigned by locally responsible VLR. Used by HLR to find the so that the call can be routed there. Looking at sender of MSRN number, GMSC identifies the current MSC in which mobile is residing. Mobile station is then assigned a TMSI number to hide its IMSI number. The different identifiers and their flow is depicted in Fig 6.

 

Figure 5: HLR asks for MSRN number from VLR

 

Different identifiers used and their flow during mobile terminating call

 

 

Figure 6: Different identifiers used and their flow during mobile terminating call

 

Localization of mobile phone

 

GSM is a hierarchical organization. Highest administrative area is MSC. Each MSC controls many BSCs. A group of BSCs are divided into logical groups called location areas. Each BSC controls many base station areas (Fig. 7). Once MSC is identified, it is important to find where in this MSC area the mobile phone is. This is done via Paging.

 

Figure 7: Organization of GSM network

Paging

 

Paging is a mechanism to search the mobile phone and alert it that it is getting a call. Number of mobile phone is broadcasted in all the BSC belonging to the sought MSC which in turn broadcasts to all BTS in its area. All base stations then broadcast the number on their downlink. Mobile phones continuously listening to the downlink channels. It compares TMSI on the downlink with its own TMSI. If matched, it responds (Fig. 8).

 

TMSI is broadcasted instead of IMSI.

 

Figure 8: Paging

The question is

FROM WHERE TO START PAGING??????

 

Global paging: Start from the center of the network and search nearby cells. This would be overhead on signaling process and time consuming.

 

Last location Paging: Start from the location where mobile station was last present as recorded in HLR because the probability of finding the phone is maximum there

 

Message exchanges for Call setup

Once the mobile phone is found, call is set up with the help of exchange of messages which includes channel allocation, authentication, encryption etc. The sequences of messages are as follows:

  • Mobile station sends channel request message to the network
  • BTS sends this request to BSC, If free channel available, BSC allocates the channel and sends channel activation message to the BTS.
  • When channel is activated, BSC sends immediate assignment message
  • Channel can be traffic channel or signalling channel
  • Traffic channel may or may not be allocated at this stage but signalling channel is allocated now
  • Mobile phone tunes to the assigned  channel and transmits page response and initiates the session
  • Mobile station request for the services
  • BTS replies with the authentication of MS
  • MS responds the authentication request
  • Authentication and ciphering processes takes place
  • Call setup initiated
  • If the traffic channel is not assigned previously it is assigned now. The advantage of allocating  channel  here  is  that  if  the  call  set  up  fails,  the  traffic  channel  won’t  be blocked. Advantage of previously allocating traffic channel is that it speeds up the call setup process
  • Call is setup by call management protocol by MS and BTS management protocol by BS
  • Call setup is confirmed by BTS to the mobile station
  • In MTC, MS answers with an alerting message to indicate it is ringing
  • BTS answers with a connect message to indicate user has pressed the connect button.
  • After connection acknowledgement, both the parties can exchange voice or data

Refer Fig. 9 for representation of exchanges

 

Mobile originating callsCall can be made to a PSTN network or a PLMN network. Mobile Caller dials the number. Network checks whether the subscriber is authorized for the use of the services. Network after authorising activates the requested services to the subscriber then routes the call. The following exchange of messages takes place:

  • Mobile station asks for a dedicated channel (signalling / traffic channel ) from the base station
  • BSC allocates signalling channel
  • Request of call setup is forwarded to current MSC in which mobile phone is residing
  • It sends its TMSI and last location area identifier to the VLR
  • VLR assigns mobile to be busy
  • VLR authenticates the caller
  • VLR starts ciphering and mobile phone is marked as active in the current VLR
  • It is checked whether mobile phone is not blocked by referring to the EIR (Equipment Identity register)
  • VLR reallocates TMSI
  • Mobile phone provides information about called subscriber and requested services to MSC
  • VLR checks whether the requested service can be provided to the mobile phone or not
  • MSC allocates traffic channel
  • MS and BTS tunes to the channels
  • MSC connects to the called party via GMSC
  • An alert message is sent to the MS indicating that a ringing tone has been generated on the other side. The ringing tone generated in the exchange on the via the group switch in the MSC.
  • When the B subscriber answers, the  network  sends  a  connect  message  to  the MS indicating that the call is accepted. The MS returns a connect acknowledgement which completes the call set-up.

Refer Fig. 10 for representation of exchanges

 

you can view video on Call Routing in GSM Network

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.