Comprehensive GPRS Tutorial

GPRS stands for General Packet Radio Service. It works with a GSM network, mainly for data connections using packet switching. It’s primarily used for browsing the internet on mobile devices. GPRS is a GSM-based packet-switched technology.

To use GPRS, the mobile subscriber (MS) needs to support it, the network operator needs to support it, and GPRS features need to be enabled for the user.

In GSM, a mobile subscriber gets one time slot. With GPRS, users can have multiple time slots allocated. Two key changes were made to the GSM network to enable GPRS: the Channel Coding Unit at the BTS was upgraded, and a Packet Control Unit was added at the BSC or between the BSC and MSC.

Network Architecture of GPRS

GPRS is a packet-switched wireless data communication technology. It lets mobile devices access the internet and other data services through a cellular mobile network.

Key elements of a GPRS architecture include:

• Mobile Station (MS)
• Base Station Subsystem (BSS)
• Serving GPRS Support Node (SGSN)
• Gateway GPRS Support Node (GGSN)
• Home Location Register (HLR)
• Authentication Center (AuC)
• Operation and Maintenance Center (OMC)
• Charging Gateway (CG)

For simplicity, we can divide the entire GPRS network into these basic elements:

• Packet Control Unit (PCU): The PCU separates GSM and GPRS traffic. It distinguishes between circuit-switched and packet-switched traffic from the user, sending them to the GSM and GPRS networks, respectively.

In GPRS, the PCU has two paths:

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

• Serving GPRS Support Node (SGSN): Similar to the MSC in a GSM network. SGSN functions include:

  • Data compression to minimize the size of transmitted data units.
  • Authentication of GPRS subscribers.
  • Routing data to the correct GGSN when a connection to an external network is needed.
  • Mobility management as the subscriber moves between PLMN areas, possibly from one SGSN to another.
  • Collecting traffic statistics.

• Gateway GPRS Support Node (GGSN): The GGSN is the gateway to external networks like PDNs (packet data networks) or IP networks. Its main functions are similar to the GMSC in a GSM network:

  • Routing mobile-destined packets from external IP networks to the correct SGSN within the GPRS network.
  • Routing packets originating from a user to the appropriate external IP network.

• Border Gateway (BG): A router that interfaces different operators’ GPRS networks. The connection between two border gateways is called a GPRS tunnel. It’s more secure to transfer data between operators using their own PLMN networks through a direct connection, rather than the public Internet. This requires both operators to agree on providing such connectivity, along with terms and conditions including charging.

• Charging Gateway (CG): The CG handles charging GPRS users based on Quality of Service or their chosen plan (prepaid or postpaid). The charging data, called Charging Data Records (CDRs), is generated by all SGSNs and GGSNs. The CG collects and processes these CDRs, then sends them to the Billing System.

• DNS Server: Located at the ISP or IP network. It converts domain names to IP addresses needed to establish internet connections and deliver web pages to the user’s screen.

• Intra PLMN: An IP-based network connecting all GPRS network elements within a single PLMN area.

• Inter PLMN: A connection between two different PLMN areas.

GPRS Frame Structure

GPRS uses FDMA to divide 25 MHz into 124 channels, and each channel is further divided using TDMA into 8 time slots, like GSM. Each frequency and time slot creates one physical channel.

In GSM, a 26-Frame Multiframe (MF) and a 51-Frame MF are used for traffic/SACCH and signaling channels, respectively. Logical channels are time-multiplexed on physical channels. In GPRS, a 52-Frame MF structure is used for both traffic and signaling.

Each time slot follows the 52-frame MF structure. Resource allocation is called a radio block or RLC block. A radio block consists of 4 consecutive bursts in 4 consecutive TDMA frames within the same time slot.

In a GPRS frame structure, the 52-frame MF consists of 12 radio blocks for user data, 2 PTCCH frames for Timing Advance calculation, and 2 idle frames for neighbor cell measurements. Only for initial network entry logical frames like FCCH, SCH, and BCCH is the 51-Frame MF structure of GSM used in GPRS.

Figure 2 depicts the GPRS 52-frame Multiframe structure. PTCCH frames are marked with ‘T’ and idle frames with ‘X’. As shown, four consecutive time slots (carrying bursts) in four consecutive TDMA frames are needed to fill 456 data bits (each burst carries 114 bits).

Different logical frames go into these 12 blocks, both downlink and uplink, to carry signaling and traffic data.

• 1 TDMA Frame = 8 TNs (Time Slot Nos) (4.615 ms)
• One 52 Frame MF = 4.615 * 52 = 240 ms
• One superframe = 25.5 * (52 Frame MF) = 240 * 25.5 = 6.12 sec
• One hyperframe = 2048 * (6.12 sec) = 3 h 28 min 53 sec 760 ms

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

Mapping of Coding Schemes Based on Stealing Flags

Stealing flags tell the receiver which coding scheme is being used at the transmitter. There are 8 stealing flags to represent 4 coding schemes.

GPRS IP Addressing

GPRS uses the same IP version 4 (IPv4) and IP version 6 (IPv6) as other internet services/protocols.

GPRS Handset Classes

• Class A: These handsets have two transceivers, so they can send and receive data and voice at the same time. Both voice and data calls can work simultaneously.

• Class B: These handsets can send/receive either data or voice, but not both at the same time.

• Class C: This type only allows one means of connectivity, either voice or data. For example, a GPRS card for a laptop/desktop will only provide GPRS data connectivity.

GPRS Coding Schemes

GPRS standards define four air interface coding schemes: CS1, CS2, CS3, and CS4. Each has different error correction capabilities and throughput:

• CS1: throughput <= 8kbps
• CS2: throughput <= 12kbps
• CS3: throughput <= 14.4kbps
• CS4: throughput <= 20kbps

GPRS Channels

Let’s look at GPRS logical channels. The logical channels used in GPRS networks are PBCCH, PPCH, PAGCH, PNCH, PRACH, PACCH, PTCCH, and PDTCH. These channels are divided based on their functions:

• Broadcast channel: Packet Broadcast Central Channel (PBCCH)
• Common control channels: Packet Paging Channel (PPCH), Packet Access Grant Channel (PAGCH), Packet Notification Channel (PNCH), Packet Random Access Channel (PRACH)
• Dedicated control channels: Packet Associated Control Channel (PACCH), Packet Timing Advance Common Control Channel (PTCCH)
• Dedicated traffic channel: Packet Data Traffic Channel (PDTCH)

Here’s what each GPRS Logical Channel does:

• PDTCH: Used for data traffic, bidirectional between the MS (Mobile Subscriber) and BSS (base station subsystem).
• PBCCH: Used for Broadcast signaling control, from the BSS to MSs.
• PRACH: Used for random access, from MSs to the BSS.
• PAGCH: Used for Access Grant indication, from the BSS to MSs.
• PPCH: Used for Paging, from the BSS to MSs.
• PNCH: Used for notification purposes, from the BSS to MSs.
• PACCH: Used for Associated control, bidirectional.
• PTCCH: Used for timing advance control, bidirectional.

GPRS QoS

Based on different GPRS applications, there are various GPRS QoS services. Applications include real-time multimedia, web browsing, and email. GPRS defines QoS profiles based on parameters like service precedence or priority, reliability, delay, and throughput.

GPRS Channel Allocation

In the Uplink, the MS sends a channel request to the network (BSS) on PRACH/RACH. The network responds on PAGCH/AGCH to the MS, indicating which PDCH the MS can use for data transfer. The network dictates which time slots the MS (UE) can use via a field called USF. The USF is carried in the MAC header within the downlink control/data block. USF is a 3-bit field.

In the downlink, the network (BSS) sends a paging request message to the MS on PPCH/PCH. The MS sends a packet channel request on PRACH/RACH to the network, and the network replies as mentioned above on PAGCH/AGCH. The MS sends a paging response to the network on PACCH.

GPRS MAC Modes

Let’s examine MAC modes in a GPRS network. There are two phases (phase-1 and phase-2) for TBF establishment in GPRS. A TBF (Temporary Block Flow) consists of a number of RLC/MAC blocks (of one or more LLC frames). Each TBF is assigned a unique TFI (Temporary Flow Identity) in both directions (uplink and downlink). This allows the same time slot to be allocated to more than one GPRS user.

The most common GPRS classes supported in both the network and MS (UE) are classes 10, 12, and 33. A TBF is temporary and maintained until data transfer is completed. It represents a connection between two RR entities at the L3 level.

• In 1-phase access, the network knows which allocation belongs to which MS when it receives the first TBF.

• In 2-phase access, the network identifies which resource allocation belongs to which MS. This is because the MS sends a packet resource request after receiving a channel assignment from the network. The network responds with a packet uplink assignment.

• Dynamic Allocation (DA): In DA, the MS decodes all the USF values on the downlink PDCHs associated with the uplink PDCHs.

• Extended Dynamic Allocation: Here, the MS does not monitor all USFs. Once the MS decodes one USF value, it transmits on multiple time slots in the uplink from the slot corresponding to the downlink side. Bitmaps are transmitted during the assignment message, indicating which time slots are needed in the downlink and uplink for transmission. A ‘1’ in the bitmap indicates transmission, and ‘0’ indicates no transmission. A field called Granularity indicates whether transmission should happen on consecutive 4 bursts or on 1 burst.

• Fixed Allocation: In fixed allocation, each MS is assigned a fixed allocation in the uplink to be used for transmission of radio blocks on PDCHs. This is also done using bitmaps communicated by the network to the MSs.

GPRS Standard References

For GPRS, read 3GPP 44.06 standard. ETSI GPRS standard documents are available from the following link: https://www.etsi.org/website/technologies/gprs.aspx

Conclusion

In this GPRS tutorial, we explored the GPRS network architecture, its frame structure, channel types, MAC modes, and security mechanisms that paved the way for more advanced technologies in mobile wireless communication.