GPRS Call Flow

            GPRS (General Packet Radio Service) is a packet based communication service for mobile devices that allows data to be sent and received across a mobile telephone network. It's a step towards 3G and is often referred to as 2.5G.
            It’s an upgrade to the existing network that sits along side the GSM network. Many of the devices such as the BTS and BSC are still used. Often devices need to be upgraded be it software, hardware or both. When deploying GPRS many of the software changes can be made remotely.
           There are however 2 New Functional Elements which play a major role in how GPRS works - SGSN & GGSN. In simple terms there are in practice two different networks working in parallel, GSM and GPRS.
           In any GSM network there will be several BSC’s. When implementing GPRS a software and hardware upgrade of this unit is required. The hardware upgrade consists of adding a PCU (Packet Control Unit). This extra piece of hardware differentiates data destined for the standard GSM network or Circuit Switched Data and data destined for the GPRS network or Packet Switched Data.
PCU can be a separate entity.

SGSN (Serving GPRS Support Node) - It takes care of some important tasks, including Routing, Handover and IP address assignment. Its a logical connection to the GPRS device. One job of the SGSN is to make sure the connection is not interrupted as you make your journey passing from cell to cell. It works out which BSC to “route” your connection through. If the user moves into a segment of the network that is managed by a different SGSN it will perform a handoff to the new SGSN, this is done extremely quickly and generally the user will not notice this has happened. Any packets that are lost during this process are retransmitted. The SGSN converts mobile data into IP and is connected to the GGSN via a tunneling protocol.

GGSN (Gateway GPRS support node) - It is the “last port of call” in the GPRS network before a connection between an ISP (Internet Service Provider) or corporate network’s router occurs. The GGSN is basically a gateway, router and firewall. It also confirms user details with RADIUS servers for security, which are usually situated in the IP network and outside of the GPRS network.
            The connection between the two GPRS Support Nodes is made with a protocol called GPRS Tunneling Protocol (GTP). GTP sits on top of TCP/IP and is also responsible for the collection of mediation and billing information. GPRS is billed on per megabyte basis.

GPRS Call Scenario :-

• A subscriber accesses the Internet with GPRS mobile phone to set the APN (Access Point Names) & gateway IP address defined on subscription. In fact, APN is a logical name indicating the external data network in GGSN. A subscriber can select different GGSNs via different APNs. Currently, however, only one APN can be activated at a time. The purpose of selecting different APNs is to access the external network via different GGSNs, because without GGSN, a subscriber cannot access the PDN (Public Data Network). An APN consists of a fully qualified DNS (Domain Name Server) name e.g. airtellive.com.cn., which should be parsed by DNS to get the real IP address of GGSN.
• The call reaches the SGSN of the GPRS network. The SGSN triggers the service in the corresponding SCP (Service Control Point) according to subscriber's authentication information on the HLR interconnected to the corresponding home SCP for processing.
• The DNS parses the APN and get the IP address of the GGSN.
• The call is routed to the GGSN according to the IP address.
• The GGSN assigns the IP address to the subscriber.
• After SCP verifies the subscriber, the subscriber begins to transmit data and log in to the external web sites via the gateway whose IP address is set in the mobile phone.
• The subscriber may select the service from the portal web site to connect the SP/CP web site that provides the service, or enter the IP address of the SP/CP in the mobile phone to access the SP/CP web site.

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Major Steps to Improve KPI !!!

Here are some parameters which highly effects network performance like -

• Network Overall ASR (Answer Seizure Ration).
• Location Update Success Rate.
• Paging Success Rate.
• Handover Success Rate.

 & these parameters needs to be monitored continuously for smooth functioning of mobile network.

Network Overall ASR - This is very-very important parameter in telecom industry because it directly relates to Revenue (Money), So we need to keep close monitoring & take certain precautions to keep it higher-n-higher.
Its standard value lies between 35% - 45% & rest % is left considering Subscriber Behaviour i.e. miss call, no answers after long ring, etc.

Points to be closely monitoring for improvement of ASR :-

1. POI Utilization (whether more E1's are required or not).
2. Routing of Levels.
3. Selection of Routes.
4. CIC (Circuit) Hunting e.g. Odd-Even Selection or Sequential Routing.
5. Unallocated Numbers, e.g. subscriber which are churn (De-active), delete those numbers on regular basis.
6. Proper Announcements, so that Subscriber won't re-attempts again n again.
7. CIC matching should be there with other operator.
8. Network Equipment like MSC, etc. should not be congested n Many more......

Location Update Success Rate - It is Number of Successful Location Updates w.r.t. Total Number of Location Updating Attempts. This parameter is calculated for 24 Hrs. Its standard value >= 95%.

LUSR = 100*(Number of successful location updates) / (Total number of location updating attempts)
Where above both paramteres are considered for Non-Registered Mobile Subscribers & Already Registered Mobile Subscribers.

Major contributor for decreasing LUSR -

1. Congestion in C7 Signaling.
2. Incorrect IMSI definition of IMSI analysis in Switch.
3. Incorrect roaming subscriber definition in Switch.
4. SDDCH Congestion.
5. LU timers setting.
6. Network Synchronization problem.

Improvement Plan -

1. Continuous Monitor of C7 Signaling utilization and it should be optimize as much as possible.
2. Correct definition of IMSI and Roaming Subscriber.
3. For Narrow Band Signaling the utilization should not go above 0.3 Erl. & for High Speed Signaling the utilization must be kept below 0.4 Erl(Per time slot).

Paging Success Rate - It is rate of successful page responses to First and Repeated Page Attempts to a location area w.r.t. Number of Initial and Repeated Page Attempts to a location area. This parameter is calculated for 24 Hours. Its standard value >= 92%.

LSR = (Number of Page responses to first page to an LA + Number of Page responses to repeated page to an LA) / Number of Page Attempts to an LA (Location Area).
                                          OR
LSR = (first paging response+ repeated paging response)*100/first paging request).

Major contributor for decreasing PSR -

1. Improper Paging / LU (Location Update) related parameter setting.
2. O&M issue i.e Outages.
3. Lower RACH success rate.
4. Air Interface Issues like Interference, SDCCH Congestion, etc.
5. Footprints.
6. Paging overload on BSC i.e. paging capacity of BSC compared with the actual paging.
7. Congestion on A-bis interface i.e. Paging command from BSC is delivered to BTS via A-bis.

Improvement Plan -

1. Paging / LU timers setting, like Paging Timers in MSC must longer than Paging Timer in BSC (prolonging 1st and repeated page) and also paging strategy (local vs global), or repeated page on/off.
2. LAC optimization.
3. Paging / LU related parameter setting like increasing paging capacity through uncombined BCCH, changing Access grant and MFRMS (multiframe) parameters.
4. Address Coverage issues.
5. Check Discard/Paging queue on cell level.

HandOver Success Rate - It is the mechanism that transfers an ongoing call from one cell to another as a user moves through the coverage area of a cellular system.
             The handover success rate shows the percentage of successful handovers of all handover attempts. A handover attempt is when a handover command is sent to the mobile.

HOSR = (Successful Incoming Inter-Cell Handover + Successful Outgoing Inter-Cell Handover) / (Incoming Inter-Cell Handover + Outgoing Inter-Cell Handover)

Major contributor for decreasing HOSR -

1. C/I Ratio (Carrier-to-Interference ratio), Lower value gives Worst Connection Quality.
2. High Interference, Co-Channel or adjacent i.e., High Bit-Error Ratio.
3. Bad Antenna Installation.
4. Bad Radio Coverage.
5. Incorrect Locating Parameter Settings.
6. Insufficient Planning in Certain Areas.
7. Repeated Handover between two base stations, caused by rapid fluctuations in the received signal strengths from both base stations.
8. Un-Necessary Handover often leads to Increased Signaling Traffic.

Improvement Plan -

1. Updating & Optimising Neighbours List.
2. Removing Neighbours which have fewer no of HOs and cells having poor HOSR,
3. Avoid same BCCH+BSIC Combination.

More Information from Readers are Expected !!!


Thanks
telecomtigers@gmail.com
http://homepageforu.webs.com/