general packet radio service gprs · gtp gprs tunneling protocol tcp transmission control protocol...
TRANSCRIPT
Packet data technologies
• cellular digital packet data (CDPD) for AMPS, IS-95 and IS-136
• general packet radio service (GPRS) for GSM
Two cellular packet data technologies
have been developed:
We will focus on GPRS
GPRS, GENERAL PACKET
RADIO SERVICE (1) General Packet Radio Service (GPRS) is a service designed for digital
cellular networks (GSM, DCS, PCS).
GPRS uses a packet-mode technique to transfer high-speed and low-speed data and signalling in an efficient manner over GSM radio networks.
GPRS can be used for carrying end user’s packet data protocol such as IP and X.25
GPRS is standardised in ETSI (European Telecommunications Standards Institute).
New GPRS radio channels are defined, and the allocation of these channels is flexible: from 1 to 8 radio interface timeslots can be allocated per TDMA frame, timeslots are shared by the active users, and up and downlink are allocated separately.
Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150 kbps per user.
GPRS
• packets can be directly routed from GPRS mobile stations to packed switched networks
• more user friendly billing
• billing may be based on the amount of transmitted data
• higher data rates
• the user can be on-line over a long period
GPRS, GENERAL PACKET
RADIO SERVICE (2)
GPRS introduces two new network nodes in the GSM PLMN:
The Serving GPRS Support Node (SGSN),
which is at the same hierarchical level as the MSC, keeps track of the individual MSs’
location and performs security functions and access control.
The Gateway GPRS Support Node (GGSN)
provides inter working with external packet-switched networks, and is connected with
SGSNs via an IP-based GPRS backbone network.
The HLR is enhanced with GPRS subscriber information.
HLR/
AUC
MSC/
VLR
BSS
GMSC IWF
IWFGGSNSGSN
Other
networks
SGSN: Serving GPRS Support Node
GGSN: Gateway GPRS Support Node
GPRS System Architecture
124
123
2
1
...
...
...
960 MHz
959.8 MHz
21 3 4 5 6 7 8
21 3 4 5 6 7 8
200 kHz
935.2 MHz
935 MHz
915 MHz
914.8 MHz
200 kHz
890.2 MHz
890 MHz
45 MHzseparation
Time slot
Downlink
Data burst, 156.25 bit periods = 15/26 ms = 576.9 s
Uplink
TDMA frame
TDMA frame
Delay
124
123
2
1
...
...
...
The GSM air interface
A GSM mobile station uses the same time
slots in the Uplink as in the Downlink
GPRS traffic
• A cell supporting GPRS may allocate physical channels for GPRS traffic
• Such physical channel is denoted as packet data channel PDCH
• The PDCHs are taken from the common pool of all channels in the cell
Radio resources of a cell are shared by all
GPRS and GSM users
• Mapping of physical channels to either GPRS or GSM is done dynamically
depending on traffic load, priorities, etc.
• A load supervision procedure monitors the load of the PDCHs in a cell
• PDCHs can be deallocated
Logical channels in GPRS
• on top of the physical channels there is a series of logical channels
• these are for signalling, paging, synchronisation, payload traffic, etc
PDTCH: packet data traffic channel - to transmitt user data
- one or many users per PDTCH
- one user may use several PDTCHs
Mapping of logical channelsMapping of logical channels onto physical channels has two components:
mapping in frequency and mapping in time
Frequency: TDMA frame number
Time: A multiframe of 52 TDMA frames
• 4 consecutive TDMA frames form one block (12 blocks per multiframe)
• 2time frames for the PTCCH (synchronization)
• 2 time blocks are idle
The multiframe carries all the logical GPRS channels
B0 B1 B2 B5B3 B4 B6 B7 B8 B9 B10 B11T X T X
Multiframe with 52 TDMA frames (240 ms)
T Frame for PTCCH
X Idle frame
GPRS System Architecture
In order to integrate GPRS into existing GSM architecture, a new
class of networks nodes, called “GPRS support nodes” (GSN) has
been introduced
GSNs are responsible for the delivery and routing of data packets
between the mobile station and the external packet data network
(PDN)
GSNs: SGSN and GGSN
SGSNA Serving GPRS Support Node (SGSN) is responsible
for the delivery of data packets from and to the mobile
station within its service area
Tasks: packet routing and transfer
mobility management (attach/detach and
location management)
logical link management
authentication
charging functions
The location register of the SGSN stores:
* location information (e.g. current cell, current VLR)
* user profiles (e.g. IMSIs and addresses used in the PDN of all GPRS users)
MS
BTS
BSC
SGSN
Intra-PLMN
GPRS backbone
SGSN GGSN
Gn
Gn Gn
GGSN
A Gateway GPRS Support Node (GGSN) acts as an interface
between the GPRS backbone network and the external PDN
GGSN: converts the GPRS packets coming from the
SGSN into appropriate packet data
protocol (PDP) (e.g. IP or X.25) and sends them
out on the corresponding PDN
PDP addresses of incoming data packets are
converted to the GSM address of the user
The readdressed packets are sent to the
resposible SGSN. For this purpose, the GGSN
stores the current SGSN address of the user and
his or her profile in its location register
The GGSN also performs authentication and
charging functions
A GGSN is the interface to external PDN for several SGSNs
An SGSN may route its packets over different GGSNs to reach different PDNs
MS
BTS
BSC
SGSN
Intra-PLMN
GPRS backbone
SGSN GGSN
Gn
Gn Gn
GPRS backbone networks
All GSNs are connected via an IP-based backbone network
Within this backbone, the GSNs encapsulates the PDN packets and
transmit (tunnel) them using the GPRS Tunnel Protocol, GTP
Two kinds of GPRS backbones:
* Intra-PLMN backbone network connects GSNs of the same PLMN
=> a private IP-based network of the GPRS network provider
* Inter-PLMN backbone networks connect GSNs of different PLMNs
=> a roaming agreement between two GPRS network providers is
necessary to install such a backbone
MS
BTS
BSC
SGSN
Intra-PLMN
GPRS backbone
SGSN GGSN
Gn
Gn Gn
Gi
Gp
Inter-PLMN
GPRS backbone
Intra-PLMN
GPRS backbone
GGSN
Border
gateway
Border
gateway
BSC
BTS
SGSN
Packet data network (PDN)
(e.g., Internet, intranet
RouterHost
LAN
PLMN 1
Quality of Service
QoS requirements do differ: real time multi media, Web browsing, e-mail
In GPRS there is support for different QoS classes, which can be specified for
each individual session. This is an important feature!!!
GPRS allows defining QoS profiles using the parameters:
• service precedence
• reliability
• delay
• throughput
QoS parameters•service precedence: the priority of a service in relation to another service. There exist three
levels of priority: high, normal and low
• reliability: indicates the transmission characteristics required by an application. Three
reliability classes are defined, which guarantee certain maximum values for the probability
of loss, duplication, missequencing, and corruption (an undetected error) of packets
Class Lost packet Duplicate
packets
Out of
sequence
Corrupted
packets
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
QoS parameters
• delay: defines maximum values for the mean delay and the 95-percentile delay. The delay
is defined as the end-to-end transfer time between two communicating mobile stations or
between a mobile station and the Gi interface to an external network
• throughput: maximum/peak rate and the mean bit rate
Mean delay 95-percent delay
Class 128 bytes 1024 bytes
1 < 0.5 s < 2 s
2 < 5 s < 15 s
3 < 50 s < 75 s
4 Best effort Best effort
Class 128 bytes 1024 bytes
1 < 1.5 s < 7 s
2 < 25 s < 75 s
3 < 250 s < 375 s
4 Best effort Best effort
Simultaneous usage of PS- and CS-services
In a GSM/GPRS network one finds in parallell:
• conventional circuit switched services (speech, data, SMS)
• packet switched services, GPRS
Three classes of mobile stations:
• class A: supports simultaneous operation of GPRS and conventional GSM service
• class B: may register for GPRS and GSM simultaneously, but can just use one of the at a time
• class C: may attach for either GPRS or GSM
Further classes when it comes to the ability of multi-slot operation
Attachment and Detachment Procedure
First an MS must register with an SGSN of the GPRS network
• The network checks if the user is authorized
• The network copies the user profile from the
HLR to the SGSN
• The network assigns a Packet Temporary
Mobile Subscriber Identity (P-TMSI)
to the user
This procedure is called GPRS attach
For mobile stations using both CS and PS services, it is possible to perform a
combined GPRS/IMSI attach procedure
The disconnect from the GPRS network is called GPRS detach. It can be initiated
by the mobile station or by the network (SGSN or HLR)
Session Management, PDP Context
after a successful GPRS attach, the mobile station needs addresses used in the PDN
• an IP address in case the PDN is an IP network
• this address is called PDP address (Packet Data Protocol address)
• the Mobile station applies for such an address
• for each session a so called PDP Context is created
• the PDP Context describes the characteristics of the session
The PDP Context contains:
• the PDP type (e.g. IPv4)
• the PDP address assigned to the mobile station (e.g. 130.145.111.210)
• the requested QoS
• the address of a GGSN that serves as access point to the PDN
PDP Context
The context is stored in the MS, the SGSN and in the GGSN
With an active PDP context, the mobile station is visible for the PDN and is
able to send and receive data packets.
The mapping between the two addresses, PDP and IMSI lets the GGSN to
transfer data packets between PDN and MS.
A user may have several simultaneous PDP context active at the same time
The allocation of the PDP address can be static or dynamic
PDP addresses
Either static or danamic
static: the network operator of the user’s home-PLMN permanently assigns
a PDP address to the user
dynamic: a PDP address is assigned to the user upon activation of a PDP context
• by the operator of the user’s home-PLMN, dynamic home-PLMN PDP address
• by the operator of the visited network, dynamic visited-PLMN PDP address
• The home network operator decides which of the alternatives that may be used
• In case of dynamic PDP address assignment:
•The GGSN is responsible for the allocation and the activation/deactivation of the PDP addresses
Location ManagementMain task: keep track of the user’s current location so that
incomming packets can be routed to the MS
The MS frequently sends location update messages
to its current SGSN
•sending seldom:
its location (its current cell) is not
known exactely =>
paging for each packet => delays
•sending often:
known location, no paging delay =>
heavy use of uplink radio capacity
battery power In GPRS: A compromise
idle
ready
standby
READY timer
expired or force
to standby
GPRS detachGPRS attach
Transmission
of a packet
Application
Network layer
(IP, X.25)
SNDCP
LLC
RLC
MAC
PLL
RFL
Data link layer
Physical layer
Relay
RLC
MAC
PLL
RFL
BSSGP
Network
service
Phy. layer
RelayGTP
Data link
layer
LLC
IPBSSGP
Network
service
Phy. layer
SNDCP GTP
TCP/UDP
Phy. layer
Network layer
(IP, X.25)
TCP/UDP
IP
Data link
layer
Phy. layer
MS BSS SGSN GGSN
Um Gb Gn Gi
BSSGP BSS GPRS application protocol
GTP GPRS tunneling protocol
TCP Transmission control protocol
UDP User datagram protocolIP Internet protocol
SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer
The Data Link Layer
• LLC Layer: between the MS and the SGSN
• RCL/MAC Layer:between the MS and the BSS
LLC: provides a highly reliable logical link
between the MS and the SGSN
It is mainly an adaptive version of the
LAPDm in GSM and based on:
• sequence control
• in-order delivery
• flow control
• detection of transmission errors
• retransmission (automatic repeat request, ARQ)
Application
Network layer
(IP, X.25)
SNDCP
LLC
RLC
MAC
PLL
RFL
Data link layer
Physical layer
Relay
RLC
MAC
PLL
RFL
BSSGP
Network
service
Phy. layer
MS BSS
Um
SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer
RLC/MAC Layer
RLC: reliable link between the MS and the BSS
-segmentation and reassembly of LLC frames
into RLC data blocks
- ARQ of uncorrelated codewords
MAC: controls the access attempts of an MS on
the radio channel shared by many users
- contention resolution
- multiuser multiplexing on the PDTCH
- prioriting based on the negotiated QoS.
-The MAC protocol is based on
Slotted Aloha
Application
Network layer
(IP, X.25)
SNDCP
LLC
RLC
MAC
PLL
RFL
Data link layer
Physical layer
Relay
RLC
MAC
PLL
RFL
BSSGP
Network
service
Phy. layer
MS BSS
Um
SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer
GPRS applications
This device has the ability to maintain a
constant web connection through WiFi or
cellular GPRS networks. Thanks to which, the
Dash does not illustrate POI (points of
interest) information from a database that is
stocked up on the device itself, but retrieves
data about people, places, products and
services from Yahoo’s local search service.
This facilitates the Dash to offer more
intrinsic details.
GPRS applications
Parking meters from CALE ltd are equipped
with solar power meeters. They communicate
with a web service office located in central
Stockholm. This communication takes place
over either a WiFi connection or over GPRS.
Black list handling of up to 800,000 credit
cards.
GPRS applications
In Europe, there is a requirement on all
power suppliers that the current usage of
electrical power should be meetered online.
The actual value is in many cases sent over
GPRS!