GPRS

General Packet Radio Service

General Packet Radio Service (GPRS) is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications (GSM). GPRS was originally standardized by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).

GPRS usage is typically charged based on volume of data transferred, contrasting with circuit switched data, which is usually billed per minute of connection time. Usage above the bundle cap is charged per megabyte, speed limited, or disallowed.

GPRS is a best-effort service, implying variable throughput and latency that depend on the number of other users sharing the service concurrently, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection. In 2G systems, GPRS provides data rates of 56–114 kbit/second. 2G cellular technology combined with GPRS is sometimes described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate-speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. GPRS is integrated into GSM Release 97 and newer releases.

GPRS Network Architecture

GPRS network architecture

Packet Control Unit(PCU)- This PCU is the core unit to segregate between GSM and GPRS traffic. It separates the circuit switched and packet switched traffic from the user and sends them to the GSM and GPRS networks respectively which is shown in the figure above. In GPRS PCU has following two paths. 

1. PCU-MSC-GMSC-PSTN 
2. PCU-SGSN-GGSN-Internet (packet data network)

Serving GPRS Support Node(SGSN)- It is similar to MSC of GSM network. SGSN functions are outlined below

1. Data compression which helps minimise the size of transmitted data units.
2. Authentication of GPRS subscribers.
3. Routing of data to the corresponding GGSN when a connection to an external network is needed.
4. Mobility management as the subscriber moves from one PLMN area to the another PLMN, and possibly one SGSN to another SGSN.
5. Traffic statistics collections. 

Gateway GPRS Support Node(GGSN)- GGSN is the gateway to external networks such as PDN (packet data network) or IP network. It does two main functions. It is similar to GMSC of GSM network.

1. Routes mobile destined packet coming from external IP networks to the relevant SGSN within the GPRS network.
2. Routes packets originated from a user to the respective external IP network.

Border Gateway(BG)- It is a kind of router which interfaces different operators GPRS networks. The connection between two border gateways is called GPRS tunnel. It is more secure to transfer data between two operators using their own PLMN networks through a direct connection rather than via the public Internet which is less secure. For this both operators need to agree to provide such connectivity and terms and conditions including charging terms.

Charging Gateway(CG)- GPRS users have to be charged for the use of the network, this is taken care by Charging gateway. Charging is done based on Quality of Service or plan user has opted either prepaid or post paid. This charging data generated by all the SGSNs and GGSNs in the network is referred to as Charging Data Records (CDRs). The Charging Gateway (CG) collects all of these CDRs, processes the same and passes it on to the Billing System.

DNS server- Connected at ISP location or at IP network. It converts domain name to IP addresses required to establish internet connection and to deliver web pages on user's terminal screen.

Intra PLMN- An IP based network inter-connecting all the above mentioned GPRS network elements in one PLMN area.

Inter PLMN- Connection between two different PLMN areas. 

GPRS Frame Structure 

Let us examine frame structure as part of GPRS tutorial now. GPRS uses FDMA to divide 25 MHz to 124 channels and each channel is further divided using TDMA into 8 time slots similar to GSM. Each of the frequency and time slot make one physical channel. We have seen that in GSM 26 Frame MF(Multiframe) and 51 Frame MF is used for traffic/SACCH and Signaling channels respectively. Logical channels are time multiplexed on physical channels.In GPRS 52 Frame MF structure is used for traffic as well as signalling. Each time slot follow 52 frame MF structure. Resource allocation here is called radio block or RLC block. 4 consecutive bursts in 4 consecutive TDMA frames in the same time slot is called radio block. In GPRS frame structure of 52 frame MF consists of 12 radio blocks for user data, 2 PTCCH frames for the Timing advance calculation and 2 idle frames used for neighbour cell measurements. Only for few of the initial network entry logical frames such as FCCH,SCH and BCCH 51 Frame MF structure of GSM is used in GPRS. Figure 2. depicts GPRS 52 frame Multiframe structure. PTCCH frames are marked by T and Idle frames by X. As shown it requires four consecutive time slots(carrying bursts) in four consecutive TDMA frames to fill 456 data bits. each burst carry 114 bits.

Different logical frames goes in these 12 blocks both in the downlink and uplink to carry signalling as well as traffic (data).

1 TDMA Frame = 8 TNs(Time Slot Nos)(4.615 ms)

One 52 Frame MF = 4.615 (52) = 240 ms

One super frame = 25.5 (52 Frame MF) = 240 x 25.5 = 6.12 sec

One hyperframe = 2048 (6.12 sec) = 3 h 28 min 53 sec 760 ms

Remember that UEs to BSS link is called uplink and BSS to UEs link is called downlink.Unlike GSM where time slot is dedicated to UE/MS, in GPRS one time slot is used by multiple UEs at different time. UEs are multiplexed using unique USF (Uplink Status Flag) on same time slot. USF helps differentiate UEs on BSS side. On downlink UEs are multiplexed using TFI (Temporary Flow Identity ) which differentiate concurrent TBFs (Temporary Block Flows). 

GPRS Channel types

Let us go through logical channels used in GPRS as part of GPRS tutorial. Logical channels are named and used in GPRS network are PBCCH, PPCH, PAGCH,PNCH,PRACH, PACCH,PTCCH,PDTCH. These channels are divided as mentioned below based on their functions. 

1. Broadcast channel- Packet Broadcast Central Channel (PBCCH) 
2. Common control channels- Packet Paging Channel (PPCH),Packet Access Grant Channel (PAGCH),Packet Notification Channel (PNCH),Packet Random Access Channel (PRACH) 
3. Dedicated control channels-Packet Associated Control Channel (PACCH),Packet Timing Advance Common Control Channel (PTCCH) 
4. Dedicated traffic channel-Packet Data Traffic Channel (PDTCH) 
5. GPRS Logical Channel functions are described below. 
6. PDTCH- Used for data traffic, bidirection between MS(Mobile Subscriber) and BSS(base station subsystem) 
7. PBCCH- Used for Broadcast signalling control, from BSS to MSs 
8. PRACH- Used for random access, from MSs to BSS 
9. PAGCH- Used for Access Grant indication, from BSS to MSs 
10. PPCH-Used for Paging, from BSS to MSs 
11. PNCH- USed for notification purpose, from BSS to MSs 
12. PACCH- Used for Associated control, bidirectional 
13. PTCCH- Used for timing advance control, bidirectional