Industrial visit to BSNL Exchange

Vaibhav Kate Abhishek Paunikar

One of the exchanges of BSNL is situated at Model Colony, Pune. We visited that Exchange on Saturday, 27 Aug. 2011.
Initially we visited the WLL CDMA Section of BSNL. This section included various kinds of Electronic Devices such as Booster Master (ZXUPS N530), DC Distribution for Inverter etc.

Diagram of CDMA Architecture

In that section, one of the BSNL employees explained us the working of CDMA with the help of diagram as shown above. CDMA is divided into two halves. BSC switches voice calls to MSC and packet call to PDSN. In case of CDMA, if no. customers get increased, then coverage of CDMA is decreased. Basically, CDMA is not designed for roaming purpose. For CDMA, MSC and VLR at located at the same place, but for GSM, this is not the case. Also, for GSM , HLR and VLR are situated at the same place.

Call processing in CDMA:-
Initially, the call from customer is received by BTS. For that call, if code is #777 i.e. if that call is packet call, then that call is switched to PDSN. If that call is voice call, then that call is switched to MSC. MSC forwards that call to authenticate the customer by AUC by checking his/her profile which is located in VLR.
If the call is switched to PDSN, then PDSN switches that call to router, which then routed to AAA/HA for authentication purpose. At last, the call is forwarded to web.
SIM card of CDMA is called RUIM. If we commit any mistake while dialing the number, the announcement is made on the basis of error code. The local call is transferred to local TAX whereas the STD call is transferred to remote TAX. In CDMA, if changing pulse is not defined, then call is only routed, but it cannot be received i.e. if we try to receive such call, then it is automatically ended. Company Converse does the billing for BSNL CDMA.
1 BTS can handle 750 customers’ ongoing voice calls. CDMA handset can control paging rate to save its battery. For CDMA, Signal to noise ratio is zero. It also gives better sound quality. SC i.e. SMS Switching Center handles all the manipulations about the Short Messages sent by customer. MSC is the real time switch whereas SC is Store and Forward type switch. It does maximum two retries, in case if SMS is not delivered. WIN is the WLL Intelligent Network. In CDMA, roaming is done on the basis of TLDN i.e. Temporary Location Directory Number.STP is the Signaling Transfer Point which works as an intermediary between remote VLR and local HLR during roaming.