1. Master in progettista di servizi radiomobili Web Based Overview
Physical and Logical Channels
General Packet Radio System (GPRS)
Overview
© 2 dicembre Gianni Redaelli

2. Introduction General Packet Radio Service (GRPS) today
“Packet overlay” network on top of the existing GSM (Digital) circuit switched voice-based network
TCP/IP-based: allows data packets to be conveyed across the mobile network using packet switching
“Always on” / “always connected”
After initial “log-on”, user is permanently connected to IP services
Instant access, no further log-on
Flat rate (about $30/mo in the LA area)
User perceived performance: fluctuates (as GPRS users defer to voice users) to a max of 50Kbps
Network resources only used when information ready to be exchanged – bandwidth on demand …
More efficient utilization of air-time

3. GSM - GPRS Provides high speed packet data access
uses modified GSM hardware (different phones/cards)
Several time slots can be (dynamically) allocated to transmit a block of data

4. GSM/GPRS Architecture, simplified

5. Uplink/downlink
The uplink channel is shared by a number of mobiles, and its use is allocated by a BSC (Base Station Controller)
The MS (Mobile Station) requests use of the channel in a “packet random access message”.
The BSC allocates an unused channel to the mobile and sends a “packet access grant message” in reply
The downlink is fully controlled by the serving BSC and random access is not needed

6. GPRS Mobility Tunnelling

7. Supporting Nodes – main functions

8. GPRS/GSM Protocols - Overview

9. SNDCP: Sub Network Dependent Convergence Protocol
SNDCP: Sub Network Dependent Convergence Protocol. This is part of layer 3 of a GPRS protocol specification.
SNDCP interfaces to the Internet Protocol at the top, and to the GPRS-specific Logical Link Control (LLC) protocol at the bottom.
Interfaces A, Gs, Gr, GC, and D are used for signalling without involving user data transmission in GPRS.
Note that the A interface is used for both signalling and voice transmission in GSM.
Interfaces Um, Gb, Gn, Gp and Gi are used for both signalling and transmission in GPRS

10. Routing in GPRS

11. Mobile Address Allocation
GGSN Address Pools
Static, dynamic allocation at GPRS packet data activation
HLR
The HLR may keep a static IP address that is fetched by the SGSN at GPRS attach
RADIUS
The GGSN may interact with an external AAA server in order to perform AAA functions as well as dynamic IP address allocation at GPRS packet data activation
DHCP server
The GGSN may interact with an external DHCP server to perform dynamic IP address allocation at GPRS packet data activation
IPv6 auto configuration
AAA server - Authentication, Authorization and Accounting Remote Authentication Dial-In User Service (RADIUS) for user authentication.

12. GPRS initial State
Dialup users of data networks that use the point-point protocol (PPP) make use of a network address identifier (NAI) for user identification. NAI is also being used for roaming and tunneling service in data networks.

13. GPRS Attach
When a MS is turned on, the first function it performs is a GPRS attach
GSM access authentication (towards Home Network, HLR (Authentication Center))
User profile is downloaded from HLR to the serving SGSN
When the GPRS attach is complete, the MS is physically connected to the visited network

14. The Packet Data Protocol (PDP) Context
In order to be able to send and receive data, the mobile must set-up a packet data bearer(carrier/conveyor/bringer)
The PDP conveyer is associated with contexts in each nodes that data are traversing (passing thro’ , navigating)
The bearer is set-up via “PDP context activation” procedures
PDP context describes requirements of the connection to the packet networks: Type, network address, Access Point Name (APN), QoS, etc.
SGSN validates request against subscription information downloaded from HLR during GPRS Attach
Access point name sent to DNS, IP address(s) of suitable GGSNs returned
Logical connection using GPRS tunnels (GTP) between SGSN and GGSN
IP address allocated (GGSN pool, DHCP, RADIUS)

15. PDP Context Activation
GEA-GPRS Encryption Algorithms