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links and connections required to be supported by the billing system, which would oth-
erwise require a separate connection to each of the individual GSNs. It can be used to
buffer and consolidate information before passing it on to the billing system.


4.3.4 Lawful interception gateway (LIG)
It is a requirement in many countries for the law enforcement agencies (LEA) to be able
to monitor traf¬c. The LIG is introduced into the network for this purpose. As user traf¬c
traverses the GPRS backbone it is possible to capture their data and forward it to the
LEA. However, this interception of user data does normally require a court order.


4.3.5 Domain name system (DNS)
In most cases, when a subscriber wishes to make a connection via GPRS to an external
network, they will select an access point name (APN) from a list in the mobile device. A
domain name system (DNS) is required so that the SGSN can make a query to resolve the
APN to an IP address of the correct GGSN. As with DNS in a standard IP network, this
APN is just text, such as ˜Internet™ or ˜home network™. A common scenario would be to
de¬ne two general access points: net and wap. Net would indicate a connection directly
to the Internet, and wap a connection to a wireless access protocol (WAP) gateway. An
operator can bill differently based on the access point, therefore many operators have
different tariffs for WAP access and Internet access. Further details on the DNS system
may be found in chapter 5.


4.3.6 Border gateway (BG)
A border gateway (BG) is used as the gateway to a backbone connecting different network
operators together. This backbone is referred to as an inter-PLMN backbone, or global
roaming exchange (GRX). The operation and con¬guration of this connection is according
to a roaming agreement between operators. The BG is essentially an IP router and is
generally implemented as the same hardware platform as the GGSN.
84 GENERAL PACKET RADIO SERVICE


4.4 NETWORK INTERFACES
GPRS introduces new interface de¬nitions in the network as well as over the air. The
interfaces are open standards as described by 3GPP and are shown in Figure 4.3. This
enables a multi-vendor network to be constructed with minimum amount of modi¬cation.
Since GPRS uses much of the GSM network, standardized interfaces are also required
between the GSM equipment and the GPRS equipment.

• Ga: this is used for transferring the charging records known as call detail records
(CDRs) from the SGSN and the GGSN to a CG. It uses an enhanced version of GTP
known as GTP .
• Gb: the Gb interface resides between the SGSN and the BSS. Its function is to transport
both signalling and data traf¬c. This interface is based on frame relay and is described
in more detail in Section 4.7.
• Gc: this interface is between the GGSN and the HLR, and provides the GGSN with
access to subscriber information. The protocol used here is MAP and the interface is
used for signalling purposes only. This interface can be used to activate the mobile
device for mobile terminated packet calls. It requires that the mobile device is given
a unique IP address, which is often not the case and thus it may not be implemented.
The GGSN is essentially an IP device and may not have MAP capabilities, so the
speci¬cation allows the GGSN to pass requests to the SGSN so that they can be
forwarded to the HLR on its behalf. The Gc is an optional interface.
• Gd: the Gd interface connects the SGSN to an SMS gateway, thus enabling the SGSN
to support SMS services.
• Gf: this interface connects the SGSN to the EIR and allows the SGSN to check the
status of a particular mobile device, such as whether it has been stolen or is not type
approved for connection to the network.


Billing
Centre
HLR/AuC
MSC/VLR CGW
Gs Gr Ga Gc Ga
Gi
IP Network
Gn
Gb

BSS GGSN Gp
BSC SGSN
GRX
BG
Gf Gd




EIR SMS-GW

Figure 4.3 GPRS interfaces
4.4 NETWORK INTERFACES 85


• Gi: this is a reference point rather than an interface and refers to the connection between
the GGSN and some external network. Currently IPv4, IPv6 and PPP1 are supported by
GPRS and the Gi interface simply has to be able to support the required protocol for this
particular access point. For example, the access point may be required to transport IPv4
packets; the underlying network is not speci¬ed and may be Ethernet, asynchronous
transfer node (ATM), frame relay or any other transport protocol.
• Gn: the Gn interface resides between the GSNs. It consists of a protocol stack which
includes IP and GTP. GTP is explained in detail in Section 4.8. The GTP tunnel is also
used between two SGSNs and also between an SGSN via a BG to another operator™s
GGSN. It is not used between two GGSNs unless they have BG functionality. This
tunnel ensures that the operator™s IP network is completely separated from the IP used
for the mobile device to connect to the external network. The GTP tunnel actually
consists of two parts, the GTP-U which is used to carry user data and the GTP-C
which is used to carry control data.
• Gp: this has similar functionality to the Gn interface and also consists of a GTP pro-
tocol. It is required when the SGSN and GGSN are in different PLMNs. It introduces
further routing and security functions to the Gn interface. Connection is via BGs and
possibly an intermediate inter-PLMN network which may be owned by a third party,
hence the increased security functions.
• Gr: this interface is between the SGSN and the HLR, providing the SGSN with access
to subscriber information. The SGSN and HLR will be in different networks in the
case of roaming users. The protocol used here is MAP and the interface is used for
signalling purposes only.
• Gs: this is another optional interface. It is used for signalling between the SGSN and the
visitor location register (VLR), which is usually co-located with the mobile switching
centre (MSC) and an SGSN, it uses the BSS application part plus (BSSAP+) protocol.
This is a subset of the BSSAP protocol to support signalling between the SGSN and
MSC/VLR. To some extent, the SGSN appears to be a BSC when communicating
with the MSC/VLR. This interface enables a number of ef¬ciency saving features by
coordinating signalling to the mobile device such as combined updates of the location
area (LA) and routing area (RA) and IMSI attach/detach which reduces the amount of
signalling over the air interface.


In addition to the ˜G™ interfaces, two relevant interfaces are those across the air, for both
GPRS and UMTS:


• Um: this is the modi¬ed GSM air interface between the mobile device and the ¬xed
network which provides GPRS services.
• Uu: this is the UMTS air interface between the mobile device and the ¬xed network
which provides GPRS services.

1
Older networks may support X.25 rather than or in addition to PPP.
86 GENERAL PACKET RADIO SERVICE


4.4.1 Network operation mode
A network can be in three different modes of operation. These modes depend on whether
the Gs interface is present and how paging of the mobile device is executed.

• Network operation mode 1. A network which has the Gs interface implemented is
referred to as being in network operation mode 1. CS and PS paging is coordinated in
this mode of operation on either the GPRS or the GSM paging channel. If the mobile
device has been assigned a data traf¬c channel then CS paging will take place over
this data channel rather than the paging channel (CS or PS).
• Network operation mode 2. The Gs interface is not present and there is no GPRS paging
channel present. In this case, paging for CS and PS devices will be transferred over the
standard GSM common control channel (CCCH) paging channel. Even if the mobile
device has been assigned a packet data channel, CS paging will continue to take place
over the CCCH paging channel and thus monitoring of this channel is still required.
• Network operation mode 3. The Gs interface is not present. CS paging will be trans-
ferred over the CCCH paging channel. PS paging will be transferred over the packet
CCCH (PCCCH) paging channel, if it exists in the cell. In this case the mobile device
needs to monitor both the paging channels.

The network operation mode being used is broadcast as system information to the mobile
devices.



4.5 GPRS AIR INTERFACE
When a mobile network operator introduces GPRS, the service is run in conjunction with
GSM. The operator shares the bandwidth allocated to them from the telecommunications
regulator between the GSM and GPRS services. GSM and GPRS traf¬c can also share
the same TDM frame, although they cannot share a single burst concurrently. Introducing
GPRS may cause higher blocking for GSM calls and the operator may have to redimension
the network to counter this problem. GPRS uses the same modulation technique, Gaussian
minimum shift keying (GMSK), as GSM and since it uses the same time-slots and frame
format as GSM there are 114 bits available during a time slot for subscriber data. However,
the GSM structure of multiframes consisting of either 26 traf¬c channel (TCH) frames or
51 control frames has been replaced with a 52-frame format. As illustrated in Figure 4.4,
the new multiframe consists of 12 blocks of 4 consecutive frames, which are referred to
as radio blocks. Two idle (I) frames and two frames (T) which are used for the packet
timing advance control channel (PTCCH) are also components of this multiframe. The
time devoted to the idle frames and the PTCCH can be used by the mobile device for
signal measurements.
In a GSM system a time slot is dedicated for one user at a time (unless half-rate mode
is used). The GPRS system is different in this respect, since each of the radio blocks
consisting of 456 bits (57 — 2 — 4) can actually be used by separate users, where the
4.5 GPRS AIR INTERFACE 87


52 TDMA Multi-frame

Block Block Block Block Block Block Block Block Block Block Block Block
T I T I
0 1 2 3 4 5 6 7 8 9 10 11

TDM frame
1250 bits in 4.615msec
012 34567 01234567 01234567 01234 567


3 1 57 26 1 57 8.25 3 1 57 26 1 57 8.25 3 1 57 26 1 57 8.25
3 1 57 26 1 57 8.25


Figure 4.4 52-frame format


users would essentially share the resources of the time slot. This is referred to as a radio
block and is a type of TDM within the TDM frame itself. A mobile device is assigned
these blocks when it is required to transfer data. This assignment is referred to as a
temporary block ¬‚ow (TBF).



4.5.1 Resource sharing
The air interface is shared between the GSM and GPRS users. It can be considered that
GPRS users are utilizing bandwidth that is left over by GSM voice users. Consider a
transceiver (TRX) that has ¬ve GSM users. There are three time slots remaining that
GPRS users can share. When a voice user makes a call, they are dedicated a time slot for
the duration of that call. Unless a time slot is exclusively reserved for GPRS users, GSM
users generally have priority of resource allocation. However, for GPRS, the remaining
time slots should be viewed as a pool of available resources that the data users can share.
Therefore the performance experienced by a GPRS user is based on three factors:

1. The number of GSM users in the current cell, which indicates the available time slots
for GPRS data traf¬c.
2. The number of GPRS data users that must share these time slots.
3. The number of time slots that the mobile device can work with.

From the perspective of the mobile operator, it allows them to introduce more services
which will, in theory, utilize space that otherwise would be wasted since it is not being
used by voice calls. Devoting time slots speci¬cally to GPRS, however, will increase call
blocking problems for GSM users since there are only a maximum of eight time slots
available per frequency. GPRS users can share a single time slot with each other, each
using a single or a number of radio blocks. Since they share the same time slot they
also share the time slots bit rate. Currently, in many cases the subscriber demand for
data services is rather low and as such dedication of time slots exclusively for GPRS
traf¬c is minimal. However, as the ratio of voice and non-voice traf¬c becomes more
even, operators will re-evaluate this position and redimension resource allocation in the
network appropriately. For example, there may be a lot of justi¬cation for reserving a
88 GENERAL PACKET RADIO SERVICE


TDM Frame


BCCH GSM 1 GSM 2 GSM 3 GSM 4 GSM 5 GPRS 2
GPRS 1
(a)


GPRS1
BCCH GSM 2 GSM 3 GSM 4 GSM 5
(b) GSM 1 GSM 6
GPRS 2


BCCH GSM 1 GSM 2 GSM 3 GPRS1 GPRS1 GPRS 2
GPRS2
(c)


Figure 4.5 TDM frames


larger number of time slots for the exclusive use of GPRS customers in central business
districts or city centre areas.

Example
Figure 4.5(a) shows a standard TDM frame with time slot 0 allocated to the broadcast
channel (BCCH) and other control channels. The seven time slots remaining are available
to GSM and GPRS users. Suppose that there are ¬ve GSM users and two GPRS users,
which will ¬ll the TDM frame. If an additional GSM user wishes to make a call, the two
GPRS users will be moved into a single time slot as in Figure 4.5(b). If three of the GSM
users now ¬nish their calls, the two GPRS users will be able to use the vacant time slots,
increasing their overall bit rate, as shown in Figure 4.5(c).

The vast majority of packet data transfer of GPRS is expected to be TCP/IP and

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