wide-area wireless ip connectivity with gprs

8/22/2019 Wide-Area Wireless IP Connectivity With GPRS

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Wide-Area Wireless IP

Connectivity with the General

Packet Radio Service

Muhammad Ali Raza Anjum


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GPRS Overview

In general, a GPRS network can be viewed as a specialIP network

It offers IP connectivity to IP terminals on the go

To provide such a mobile connectivity service, the GPRS

network must feature additional functionality comparedwith standard IP networks

From a high level point of view, however, the GPRSnetwork resembles a typical IP network

In the sense that it provides typical IP routing andinterfaces to the external world through one or more IProuters


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GPRS Overview

Figure on next slide captures schematically this highlevel conceptual view of a GPRS network

By using shared radio resources, the mobile users gainaccess to remotepacket data networks (PDN) through aremote access router

In GPRS terminology this is designated as GGSN

The access to a remote PDN can be envisioned as beingsimilar to a typical dial-up connection.

Indeed a user establishes a virtual connection to the

remote PDN. With GPRS, however, a user may dial-up to manyremote PDNs simultaneously

So it can be charged by the volume of the transferreddata, not by the duration of a connection.


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GPRS Overview

High-level conceptual view of a GPRS network


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GPRS Overview

GPRS can offer both transparentand nontransparentaccessto a PDN

With transparent access the user is not authenticated by theremote PDN

And is assigned an IP address from the address space of the

GPRS network. With nontransparent access, the user credentials are sent to

the remote PDN

And the user is permitted to access this PDN only if he or sheis successfully authenticated

In this case, the user is assigned an IP address from theaddress space of the remote PDN

Note that, irrespectively of the access type to a PDN, a user isalways authenticated by the GPRS network before he or sheis permitted access to GPRS services


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GPRS Overview

The nontransparent access is particularly useful foraccessing secure intranets (e.g., corporate networks) orISPs

Whereas the transparent access is most appropriate forusers who do not maintain subscriptions to third-partyISPs or intranets

The GPRS network forms an individual subnet, whichcontains all users who use transparent access to remotePDNs

External PDNs perceive this subnet as being a typical IPnetwork

Figure on next slide illustrates some more detailedaspects of a GPRS network.


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GPRS Overview

The GPRS bearer service


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GPRS Overview

A mobile station (MS) is shown on the left, and thegateway GPRS serving node (GGSN) is shown on theright

Among other things, the GGSN offers IP routingfunctionality and it is used for interfacing with external IPnetworks.

From the MS point of view, the GGSN can be thought ofas a remote access router

It must be noted that, in general, the GGSN mayinterface not only with IP networks but also with severalother types of PDNs (e.g., with X.25 networks)

In this lecture we mainly focus on IP and, unlessotherwise indicated, it is assumed that GPRS interfaceswith IP PDNs only.


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GPRS Bearers

The GPRS network effectively provides a GPRS bearer

It provides a communication channel with specificattributes between the MS (the terminal) and the GGSN(the router)

Over the GPRS bearer, the MS may send IP packets tothe GGSN and it may receive IP packets from the GGSN

The GPRS bearer is dynamically set up at the beginningof an IP session

When the user dials to a specific PDN and then it can

be tailored to match the specific requirements of anapplication

For example, it can be set up with specific QoSattributes, such as delay, throughput, precedence, andreliability


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GPRS Bearers

Figure also illustrates the internal structure of a GPRSbearer

This includes the protocols and the GPRS nodesinvolved in the provisioning of this bearer

The MS communicates through the radio interface (theso-called Um reference point) with a base transceiverstation (BTS)

BTS provides mainly physical-layer functionality

In GPRS, the BTS handles the transmission and the

reception of packet data on the GPRS physical channels Data received by the BTS is processed (e.g., decodedand de-interleaved) and then relayed to the nexthierarchical node in the GPRS architecture, that is, to thepacket control unit(PCU).


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GPRS Bearers

The PCU offers radio resource management

It is responsible for allocating uplink and downlinkresources to the various MSs on a demand basis

The radio resource allocation is implemented with a

packet scheduling function that takes into account theQoS committed to each active MS

The PCU communicates with the serving GPRS supportnode (SGSN) over a Frame Relay interface (Gb)

The SGSN provides mobility management functionality,session management, packet scheduling on thedownlink, and packet routing/tunneling


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GPRS Bearers

The interface between the SGSN and the GGSN (Gn) isentirely based on IP, typically on IPv4

The GGSN provides mainly routing and optionallyscreening functionality

It can be considered as a remote access routerinterfacing with the external PDNs

It is a fact that we have two IP layers within the GGSN

This implies that some sort of IP-to-IP tunneling is

applied across the Gn interface


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GPRS Protocols

The Subnetwork Dependent Convergence Protocol(SNDCP) runs between the MS and the SGSN

It is the first layer that receives the user IP datagrams fortransmission

SNDCP basically provides

acknowledged and unacknowledged transport services,

compression of TCP/IP headers (conformant to RFC 1144 [8])

compression of user data (conformant to either V.42bis or V.44) datagram segmentation/ reassembly, and

PDP context multiplexing


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GPRS Protocols

The segmentation/reassembly function ensures that thelength of data units sent to LLC layer does not exceed amaximum pre-negotiated value

For example, when this maximum value is 500 octets,

then IP datagrams of 1,500 octets will be segmented intothree SNDCP data units

Each one will be transmitted separately andreassembled by the receiving SNDCP layer

As discussed, a Packet Data Protocol(PDP) Contextessentially represents a virtual connection between anMS and an external PDN.


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GPRS Protocols

The PDP Context multiplexing is a function that: routes each data unit received on a particular PDP Context to

the appropriate upper layer and

routes each data unit arrived from an upper layer to theappropriate PDP Context

For example, assume a situation where the MS has setup two PDP Contexts, both with type IP but with differentIP addresses

One PDP Context could be linked to a remote ISP, andthe other could be linked to a remote corporate network

In this case, there are two different logical interfaces atthe bottom of IP layer, one for each PDP Context

The SNDCP layer is the entity that multiplexes data toand from those two logical interfaces


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GPRS Protocols

The LLC protocol also runs between the MS and theSGSN

LLC basically provides data link services

In particular, LLC provides one or more separate logical

links (LLs) between the MS and the SGSN, which aredistinguished into user-LLs (used to carry user data) and

control-LLs (used to carry signaling)

There can be up to four user-LLs

While there are basically three control-LLs: one forexchanging GPRS mobility management and sessionmanagement signaling, another to support SMS , anda third to support locat ion services(LCS)


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GPRS Protocols

The user-LLs are established dynamically

In the context of the PDP Context Activation procedure

Their properties are negotiated between the MS and theSGSN during the establishment phase

Negotiated properties typically include the data transfer mode (acknowledged versus unacknowledged) the maximum length of transmission units, (3) timer values, and

flow control parameters.

On the other hand, the control-LLs have predefined

properties that are automatically set up right after the MSregisters to the GPRS network

It should be noted that each user-LL carries data for oneor more PDP Contexts, all sharing the same QoS.


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GPRS Protocols

Control-LLs operate only in unacknowledged mode

Which basically provides an unreliable transport service

User-LLs operate either in unacknowledged mode or inacknowledged mode, depending on the reliability

requirements The latter mode provides reliable data transport by Detecting and retransmitting erroneous data units

maintaining the sequential order of data units, and

providing flow control.

Another service provided by the LLC layer is ciphering This service can be provided in both acknowledged andunacknowledged mode of operation

Therefore, all LLs can be secured and protected fromeavesdropping


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GPRS Protocols

The RLC and MAC protocols run between the MS andthe PCU

The RLC provides the procedures for unacknowledgedor acknowledged operation over the radio interface

It also provides segmentation and reassembly of LLCdata units into fixed-size RLC/MAC blocks

In RLC acknowledged mode of operation, RLC alsoprovides the error correction procedures

That enables the selective retransmission of

unsuccessfully delivered RLC/MAC blocks Additionally, in this mode of operation, the RLC layerpreserves the order of higher layer data units provided toit


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GPRS Protocols

Note that, while LLC provides transport servicesbetween the MS and the SGSN

The RLC provides similar transport services between theMS and the PCU

The MAC layer implements the procedures that enablemultiple mobile stations to share a common radioresource

In particular, the MAC layer provides the procedures forthe arbitration between multiple mobile stations whichsimultaneously attempt to access the sharedtransmission medium

In the downlink direction (network to MS), the MAC layerprovides the procedures for queuing and scheduling ofaccess attempts


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GPRS Protocols

The MAC function in the network maintains a list of activeMSs, which are mobile stations with pending uplinktransmissions

These MSs have previously requested permission to contentfor uplink resources and the network has responded positively

to their requests Each active MS is associated with a set of committed QoS

attributes, such as delay and throughput

These QoS attributes were negotiated when the MSrequested uplink resources.

The main function of the MAC layer in the network is toimplement a scheduling function (in the uplink direction)

Which successively assigns the common uplink resource toactive MSs in a way that guarantees that each MS receives itscommitted QoS


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GPRS Protocols

A similar scheduling function is also implemented inthe downlink direction

The Base Station Subsystem GPRS Protocol(BSSGP)runs across the Gb interface, between PCU and SGSN

BSSGP basically provides Unreliable transport of LLC data units between the PCU and the

SGSN and

Flow control in the downlink direction

The flow control aims to prevent the flooding of buffers inthe PCU and to match the transmission rate on Gb (fromSGSN to PCU) to the transmission rate on the radiointerface (from PCU to MS).


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GPRS Protocols

Flow control in the uplink direction is not provided

It is assumed that uplink resources on Gb are suitablydimensioned and are significantly greater than thecorresponding uplink resources on the radio interface

BSSGP provides unreliable transport Because the reliability of the underlying frame relay

network is considered sufficient enough to meet therequired reliability level on Gb

BSSGP also provides addressing services These are used to identify a given MS in uplink and

downlink directions, and a particular cell


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GPRS Protocols

In the downlink direction, each BSSGP data unit typicallycarries an LLC data unit, the identity of the target MS, aset of radio-related parameters (identifying the radiocapabilities of the target MS), and a set of QoS attributes

needed by the MAC downlink scheduling function The identity of the target cell is specified by means of a

BSSGP virtual channel identifier(BVCI)

This eventually maps to a frame relay virtual channel

In the uplink direction, each BSSGP data unit typicallycarries an LLC data unit, the identity of the source MS,the identity of the source cell, and a corresponding set ofQoS attributes


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GPRS Protocols

The mobility management function in the SGSN uses thesource cell identity to identify the cell wherein the sourceMS is located

The GTP runs between the SGSN and the GGSN

In general, however, GTP also runs between two SGSNs GTP provides an unreliable transport function (usually

runs on top of UDP) and a set of signaling functions

These are primarily used for tunnel management and

mobility management The transport service of GTP is used to carry user

originated IP datagrams (or any other supported packetunit) into GTP tunnels.


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GPRS Protocols

GTP tunnels are necessary between the SGSN and theGGSN for routing purposes

They are also necessary for correlating user-originatedIP datagrams to PDP Contexts

By means of this correlation, a GGSN knows how totreat an IP datagram received from an SGSN (e.g., towhich external PDN to forward this datagram)

And an SGSN knows how to treat an IP datagramreceived from a GGSN (or another SGSN) (e.g., whatQoS mechanisms to apply to this datagram and to whichcell to forward this datagram)

wide-area wireless ip connectivity with gprs

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