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entity adds on a CRC which is 40 bits for CS-1 and 16 bits for CS-2, CS-3, and CS-
4. For CS-2, CS-3 and CS-4 the USF (3 bits) is modi¬ed to give better protection.
This is not required with CS-1 due to its lower data rate. CS-1, CS-2 and CS-3 also
have four tail bits added. The resulting bits are passed to the convolution entity, which
performs 1/2 rate convolution coding on CS-1, CS-2 and CS-3. The output of the convo-
lution coder is required to be 456 bits, therefore puncturing as described above may be
necessary.



4.6.6 Layer 1
Layer 1 is divided into two separate distinct sub-layers, the physical radio frequency
(RF) layer and the physical link layer. The physical RF layer performs the modulation
based on the data it receives from the physical link layer and at the receiver demodulates
the signal. The physical link layer provides framing, data coding and the detection and



Information
Interleaving


Block Coding
Ciphering

Convolution
Coding
Burst building

Reordering &
Partitioning RF Modulation
& Tx/Rx


Figure 4.26 Physical layer procedures
4.7 Gb INTERFACE PROTOCOLS 119


possible correction of physical medium transmission errors. Figure 4.26 illustrates the
procedures that are executed at the physical layer. These functions include forward error
correction through the use of convolution coding and interleaving of RLC/MAC radio
blocks over four consecutive bursts.



4.7 Gb INTERFACE PROTOCOLS
The Gb interface connects the BSS to the SGSN and is used for both signalling and data.
It is designed to allow many users to be multiplexed over the same physical resources and
allows different data rates to be available depending on the user requirements. Resources
are allocated to a user when data is either transmitted or received. This is in contrast to
the A interface used for circuit switched connections where a user has the sole use of a
dedicated resource throughout the call, irrespective of the amount of activity.



4.7.1 Layer 1 bis
There are a number of physical layer con¬gurations possible, and the actual physical
connection of this interface is subject to negotiation between the equipment vendor and
operators. The speci¬cation allows for point-to-point connections between the SGSN and
the BSS, but also allows for an intermediate frame relay network to be used. In situations
where an intermediate network is used, a number of physical layer technologies may be
used over the different links between switches, and between switches and the SGSN and
the BSS. Since the standards allow for the intermediate network, this does increase the
complexity of this interface and unfortunately introduces a large number of identi¬ers. In
situations where the MSC and SGSN are co-located, it may be advantageous to multiplex
channels within the same E1 (2.048 Mbps) or T1 (1.544 Mbps) link for both circuit
switched (A interface) and packet switched (Gb interface) connections. When multiple
64 kbps channels are used for this interface it is recommended that they are aggregated into
a single n — 64 kbps channel since this will take advantage of the statistical multiplexing
at the upper layer.



4.7.2 Frame relay
The network link layer is based on frame relay as de¬ned in GSM08.16. Virtual circuits
(VC) are established between the BSS and SGSN and the transmissions from a number
of users can be multiplexed over these VCs. In many cases it is expected that there will
be a direct link between the BSS and the SGSN; however, frame relay will permit an
intermediate network in between the SGSN and BSS.
The frame relay connection will allow different frame sizes with a maximum of
1600 bytes. The frame relay header is 2 bytes long. A number of permanent virtual cir-
cuits (PVC) are expected to be used between the SGSN and the various BSS to transport
120 GENERAL PACKET RADIO SERVICE


the BSSGP data packets. These links are to be set up using administrative procedures.
The actual PVC between the SGSN and the BSS is referred to as the network services
virtual connection (NS-VC). Thus the NS-VC identi¬er (NS-VCI) has Gb end-to-end sig-
ni¬cance, and uniquely identi¬es this connection between an SGSN and a particular BSS.
If an intermediate network is used, each of the frame relay links is referred to as a net-
work services virtual link (NS-VL). It is often the case that there are a number of paths
between the SGSN and a particular BSS. This can be useful for redundancy and load
sharing. It is therefore necessary to combine all of the NS-VCs between an SGSN and a
speci¬c BSS into a group, which is referred to as the network services virtual connection
group (NS-VCG). This group of connections is identi¬ed by a network services entity
identi¬er (NSEI), which identi¬es the actual BSS to the SGSN for routing purposes. It is
important to note that frame relay guarantees in-order delivery of frames. By introducing
different paths between the SGSN and the particular BSS, there needs to be a mechanism
introduced to maintain this ordering of frames.
Frame relay is a packet switched technology that was developed to provide high-speed
connectivity. It takes advantage of the low error rates on modern networks by leaving
retransmission to the end stations. By stopping all point-to-point error correction and ¬‚ow
control within the network itself, the nodes do not have to wait for acknowledgements
or negative acknowledgement. This can increase the throughput tremendously since with
acknowledgements, end-to-end delays are ampli¬ed. Point-to-point error checking is still
performed; however, if a frame is found to contain errors it is simply discarded. The
end nodes will typically use higher-layer protocols, such as TCP, to perform their own
error control mechanism. Frame relay is generally regarded as a successor to X.25 and a
precursor to ATM.



4.7.3 Base station system GPRS protocol (BSSGP)
The base station system GPRS protocol (BSSGP) resides above the frame relay network
and is used to transport both control and user data over the Gb interface. The primary
function of this layer is to introduce and provide the required QoS for the user as well as
routing information between the BSS and the SGSN. On the uplink, the BSC will take
RLC/MAC frames from the mobile device and reassemble a complete LLC packet from
these to be passed within a single BSSGP packet to the SGSN. On the downlink the
BSC will extract the LLC frame from the BSSGP packet and segment it into the required
number of RLC/MAC frames to be transported to the mobile device. To complete this
task, the BSC makes use of the TLLI which is provided by the SGSN and is carried
within the BSSGP packet header. It uses this to identify the correct resources provided
to the RLC/MAC that this particular packet corresponds to. Each RLC/MAC“BSSGP
association is linked via the TLLI. As well as the TLLI, the SGSN provides further
information to the BSSGP protocol for the speci¬c mobile device. This information
includes:

• The radio capability of the mobile device indicating the simultaneous number of time
slots the device is capable of handling.
4.7 Gb INTERFACE PROTOCOLS 121


• The QoS pro¬le which de¬nes the peak bit rate, whether the BSSGP packet is Layer
3 signalling or data (signalling may be transferred with higher protection), whether
the LLC frame being carried is ACK/SACK or not (it may be transferred with higher
priority if it is ACK/SACK), the precedence class as well as the transmission mode
(RLC/MAC acknowledged mode using ARQ or unacknowledged transfer) to be used
when transmitting the LLC frame between the BSC and the mobile device.
• A time period for which the packet is valid within the BSS. Any packets held up for
longer than this period are to be discarded locally.

The precedence class, lifetime and peak bit rate may be incorporated into the BSC radio
resource scheduling algorithm for ef¬cient transfer of LLC frames. In periods of conges-
tion the BSS may initiate a network controlled cell reselection for a particular mobile
device to ensure ef¬ciency and maintain the maximum number of service requests. If
such an event occurs, the BSS will update any internal references to the location of the
mobile device and inform the SGSN. It is, however, the responsibility of the SGSN to
cope with any LLC packets that have been discarded.
Figure 4.27, shows the format of the BSSGP frames; the various ¬elds are explained
below, and follow the format shown in the bottom of the ¬gure.


32 Down Link 1 32 Up Link 1
TLLI
PDU type TLLI PDU type

TLLI QoS Profile
TLLI QoS Profile

PDU Lifetime
PDU Lifetime

Cell ID
MS Radio Access Capability

Cell ID
MS Radio Access Capability Options

Options Cell ID Options
Alignment

Options Alignment
LLC-PDU

LLC-PDU




8 Information Element Coding 1
byte 1
T Information element ID (IEI)

byte 2
L Length indicator


byte 3-n
V Information element value



Figure 4.27 BSSGP data frames
122 GENERAL PACKET RADIO SERVICE


Table 4.10 Examples of BSSGP
PDU types
Value PDU type
(hex)
x00 DL unit data
x01 UL unit data
x02 RA capability
x03 DPTM unit data
x05 Paging packet switched
x06 Paging circuit switched
x0b Suspend
x0c Suspend ack
x28 Flow control MS
x29 Flow control MS ack


• PDU type: identi¬es the type of PDU and thus the frame format to follow. Table 4.10
is a sample list of assigned PDU types.
• QoS pro¬le: de¬nes the peak bit rate, whether the SDU is signalling or data, the type
of LLC frame (ACK/SACK or not), the precedence class and the transmission mode
to use over the air.
• MS radio access capability: de¬nes the radio capability of the mobile device. This ¬eld
is optional and only present if the SGSN is aware of the mobile device capability.
• PDU lifetime: de¬nes the time period that the PDU is considered valid within the BSS.
This period is set by upper layers in the SGSN.
• Cell identi¬er: to support location-based services, the uplink PDU includes the cell
identity where the LLC was received.
• Localized service area (LSA): this is an optional ¬eld as it is an operator-de¬ned group
of cells for which speci¬c access conditions apply. This may, for example, be used for
negotiating cell reselection.

Unlike the RLC/MAC between the mobile station and the BSC, the BSSGP does not
provide error correction, and if a retransmission is required this is performed between
the mobile station and the SGSN at the LLC layer. This is because, like frame relay, it
assumes that the link is reliable.
There is a one-to-one mapping of the BSSGP protocol between an SGSN and a partic-
ular BSS. If the SGSN controls more than one BSS, then there will be additional separate
mappings to each of these BSSs from the SGSN. The BSSGP virtual connection (BVC) is
carried over a single NS-VC group, which is a collection of frame relay links between an
SGSN and a particular BSS; the NS-VC group can carry more than one BVC. Each cell
supporting GPRS is allocated and identi¬ed by a BVCI. The BVCI and the NSEI are used
within the SGSN to identify the cell where a mobile device in ready mode resides. The
SGSN does not need to know the BVCI of a mobile device in standby mode, it simply
needs to know which NSEIs relate to the routing area that the mobile device is in for
paging purposes. If a mobile device requires to send data or is paged for downlink data
then the SGSN will be informed of the cell (BVCI) where the mobile device is located.
4.7 Gb INTERFACE PROTOCOLS 123




Gb
BTS BVC
I-1
BVCI-2
NSEI-1
BTS
BSC 1
SGSN 1
BSC 2
BVCI-3
NSEI-2
BTS I-4
BVC

BTS


Figure 4.28 Example use of BVCI and NSEI identi¬ers



Figure 4.28 shows how the BVCI and NSEI can be used to successfully transport data

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