Transcript GPRS - School of Electrical and Computer Engineering at Georgia

PERSONAL COMMUNICATION
SYSTEMS: GPRS
Ian F. Akyildiz
Broadband & Wireless Networking Laboratory
School of Electrical and Computer Engineering
Georgia Institute of Technology
Tel: 404-894-5141; Fax: 404-894-7883
Email: [email protected]
Web: http://www.ece.gatech.edu/research/labs/bwn
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
– Support for voice/data and data only terminals
– High throughput (up to 21.4 kbps)
– User friendly billing
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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.
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GPRS Mobile Terminal Types
– Class A Terminals
operate GPRS and other GSM services
simultaneously.
– Class B Terminals
can monitor all services, but operate either
GPRS or another service, such as GSM, one
at a time.
– Class C Terminals
operate only GPRS service.
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GPRS Network Services
 Point-to-Multipoint (PTM-M):
Multicast service to all subscribers in a given area.
 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.
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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).
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GPRS Reliability Classes
Reliability Classes
Probability for
Class
Lost Packet
Duplicated
Packet
Out-of-Sequence
Packet
Corrupted
Packet
1
10-9
10-9
10-9
10-9
2
10-4
10-5
10-5
10-6
3
10-2
10-5
10-5
10-2
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GPRS Delay Classes
Delay Classes
128 Byte Packet
1,024 Byte Packet
Class
Mean
Delay
95%
Delay
Mean
Delay
95%
Delay
1
< 0.5s
< 1.5s
< 2s
< 7s
2
< 5s
< 25s
< 15s
< 75s
3
< 50s
< 250s
< 75s
< 375s
4
Best
Effort
Best
Effort
Best
Effort
Best
Effort
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Reference Architecture in GPRS
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GPRS - Network Architecture
Internet or
other networks
HLR
SGSN
MSC/
VLR
GGSN
SGSN
BSC/PCU
Gateway GSN = packet
switch interworks with other
networks
Serving GPRS support
node = packet switch with
mobility management
capabilities
GPRS makes use of
existing GSM base stations
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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:
– Serving GPRS Support Node (SGSN):
 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.
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Reference Architecture in GPRS
– Gateway GPRS Support Node (GGSN):
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.
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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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Mobility Support in GPRS
 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.
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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.
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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.
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Location and Handoff
Management in GPRS
 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.
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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.
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Location and Handoff
Management in GPRS
 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.
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Location and Handoff
Management in GPRS
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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Location and Handoff
Management in GPRS
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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.
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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.
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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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Short Messaging Service (SMS)
Each message is maintained and
transmitted by the SMSC.
The SMSC sorts and routes the
messages appropriately.
The SM are transmitted through
the GSM architecture using SS-7.
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Short Messaging Service (SMS)
 Mobile Originated Short Message:
– SM is first delivered to a service center.
– Before that, it reaches an MSC for processing.
 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.
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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.
 Failures can occur is there is state change when the SM
is in transit. The SM will have to be transmitted later.
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