GPRSIFA’2004
GPRS
GPRS (General Packet Radio Service) is an overlay on top of the GSM physical layer and network entities.
Advantages:
Short access time to the network for independent short packets (500-1000 bytes).
No hardware changes to the BTS/BSC
Easy to scale
High throughput (up to 21.4 kbps)
User friendly billing
GPRS
It uses exactly the same physical radio channels as GSM, only logical GPRS radio channels are defined.
Allocation of the channels is flexible: from one to eight radio interface timeslots can be allocated per TDMA frame.
The active users SHARE timeslots, and uplink and downlink are allocated separately.
The capacity allocation for GPRS is based on the actual need for packet transfer.
GPRS does not require permanently allocated physical channels.
GPRS offers permanent connections to the Internet with volume based charging.
IFA’2004
Class B Terminals
can monitor all services, but operate either GPRS or another service, such as GSM, one at a time.
Class C Terminals
Point-to-multipoint (PTM-G):
Multicast service to pre-determined group that may be dispersed over a geographic area.
Point-to-Point (PTP): Packet data transfer:
Connectionless based on IP and CLNS called PTP-CLNS.
Connection-oriented based on X.25 (PTP-CONS).
Also provides a bearer service for GSM’s SMS.
IFA’2004
GPRS Network Services
GPRS has parameters that specify a QoS based on precedence, a priority of a service in relation to another service (high, normal, and low), reliability and transmission characteristics required.
Three reliability cases are defined and four delay classes (end-to-end delay between the mobile terminals and the interface to the network external to GPRS).
IFA’2004
Serving GPRS support node = packet switch with mobility management capabilities
Gateway GSN = packet switch interworks with other networks
Internet or other networks
Reference Architecture in GPRS
There are a few new network entities called GPRS Support Nodes (GSN)
Responsible for delivery and routing of data packets between the mobile terminals and the external packet network.
Two types of GSN:
Router similar to the foreign agent in Mobile IP.
It controls access to the mobile terminals that may be attached to a group of BSCs. This is called a routing area or a service area of the SGSN.
Responsible for delivery of packets to the mobile terminal in the service area and from the mobile terminal to the Internet.
It also performs logical link management, authentication, and charging functions.
IFA’2004
Acts as a logical interface to the Internet.
It maintains routing information so that it can route the packets to the SGSN servicing the mobile terminal.
It analyzes the PDN address of the mobile terminal and converts it to the corresponding IMSI and is equivalent to the HA in Mobile IP.
IFA’2004
Reference Architecture in GPRS
New database: GPRS register (GR), colocated with the HLR. It stores routing information and maps the IMSI to PDN address (IP address, for example).
Um interface is the air-interface and connects the MS to the BSS.
The interface between the BSS and the SGSN is called Gb.
The interface between the SGSN and the GGSN is called the Gn interface.
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GPRS Interfaces
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Attachment Procedure:
Before accessing GPRS services, the MN must register with the GPRS network and become “known” to the PDN.
The MS performs an “attachment procedure” with an SGSN that includes authentication (checking with the GR).
The MS is allocated a temporary logical link identifier (TLLI) by the SGSN and a PDP (packet data protocol) context is created for the MS.
IFA’2004
Mobility Support in GPRS
This context is a set of parameters created for each session and contains the PDP type, such as IPv4, the PDP address assigned to the MS, the requested QoS parameters, and the GGSN address that serves the point of access to the PDN.
The PDN context is stored in the MS, the SGSN, and the GGSN.
A user may have several PDP contexts enabled at a time.
The PDP address may be statically or dynamically assigned (static address is most common).
The PDP context is used to route packets accordingly.
IFA’2004
Location and Handoff Management in GPRS
Based on keeping track of the MSs location and having the ability to route packets to it accordingly.
The SGSN and GGSN play the role of foreign and HA, respectively, as in Mobile IP.
There are three states in which the MS can be:
IDLE state – the MS is not reachable, and all PDP contexts are deleted
STANDBY state – movement across routing areas is updated to the SGSN but not across cells.
READY state – every movement of the MS is indicated to the SGSN.
IFA’2004
The reason for the three states approach:
If the MS updates its location too often, it consumes battery power and wastes the air-interface resources.
If the update is too rare, a system wide paging is needed: again waste of resources.
IFA’2004
Location Management in GPRS
During the STANDBY state there are two types of routing area updates:
Intra-SGSN RA update
The SGSN already has the user profile and PDP context.
A new temporary mobile subscriber identity is issued as part of routing area update “accept”.
The HLR need not be updated.
Inter-SGSN RA update
The new RA is serviced by a new SGSN.
The new SGSN requests the old SGSN to send the PDP contexts of the MS.
The new SGSN informs the home GGSN, the GR, and other GGSNs about the user’s new routing context.
IFA’2004
Mobility management in GPRS starts at handoff initiation.
The MS listens to the BCCH and decides which cell it has to select.
The MS measures the RSS of the current BCCH and compares it with the RSS of the BCCH of the adjacent cells and decides on which cell to attach it to.
There is an option for handoff similar to GSM (MAHO).
Handoff procedure is very similar to mobile IP.
IFA’2004
The location is updated with a routing update procedure:
1. When an MS changes a routing area (RA), it sends
an RA update request containing cell identity and the
identity of the previous routing area, to the new SGSN.
2.The new SGSN asks the old SGSN to provide the
routing context (GGSN address and tunneling
information) of the MS.
3. The new SGSN then updates the GGSN of the home
network with the new SGSN address and new tunneling
information. It also updates the HLR.
The HLR cancels the MS information context in the old SGSN and loads the subscriber data to the new SGSN.
The new SGSN acknowledges the MS.
The previous SGSN is requested to transmit undelivered data to the new SGSN.
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Short Messaging Service (SMS)
Very popular in Europe
Users of SMS can exchange alphanumeric messages of up to 160 characters.
Service is available wherever GSM exists making it a very attractive wide area data service.
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Short Messaging Service (SMS)
Uses the same network entities as GSM (with the addition of the SMS center – SMSC), the same physical layer, and intelligently reuses the logical channels of the GSM system to transmit messages.
It has an almost instant delivery if the destination MS is active.
It supports a store-and-forward delivery if the MS is inactive.
IFA’2004
Short Messaging Service (SMS)
Two types of services:
Cell broadcast service – message is transmitted to all MSs that are active in a cell and that are subscribed to the service (unconfirmed, one-way message).
Used to send weather forecast, stock quotes, game scores, and so on,
PTP service – MS sends a message to another MS using a handset keypad, a PDA or a laptop connected to the handset, or by calling a paging center.
IFA’2004
Short Messaging Service (SMS)
A short message (SM) can have a certain priority, future delivery time, expiration time, or it might be one of several predefined messages.
A sender may request an acknowledgement of message receipt.
A recipient can manually acknowledge message or have predefined messages for acknowledgement.
A SM will be delivered and acknowledged whether a call is in progress.
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The SMSC sorts and routes the messages appropriately.
The SM are transmitted through the GSM architecture using SS-7.
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A dedicated function called SMS-interworking MSC (SMS-IWMSC) allows the forwarding of the SM to the SMSC using a global SMSC ID.
Mobile Terminated Short Message:
It is forwarded by the SMSC to the SMS-gateway MSC (SMS-GMSC) function in a MSC.
It either queries the HLR or sends it to the SMS-GMSC function at the home MSC of the recipient.
Subsequently, the SM is forwarded to the appropriate MSC, and it delivers the message to the MS.
It queries the VLR for details about the location of the MS, the BSC controlling the BTS providing coverage to the MS, and so on.
IFA’2004
Short Messaging Service (SMS)
SMs are transmitted in time slots that are freed up in the control channels.
If the MS is in idle state, the short messages are sent over the SDCCH at 184 bits within approximately 240 ms.
If the MS is the active state (handling a call), the SDCCH is used for call setup and maintenance.
The SACCH is used for delivery at around 168 bits every 480 ms.