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GPRS Mobility and Session Management


GMM/SM GMM/SM


LLC LLC
Relay
BSSGP BSSGP
RLC/MAC RLC/MAC
Frame Relay
Frame Relay

GSM RF GSM RF Layer 1 bis Layer 1 bis

Um Gb
Mobile Device BSS SGSN

Figure 4.7 Control plane for GPRS/GSM

To other end system (includes TCP or UDP header)
Application

IP IP
Relay

SNDCP
SNDCP GTP GTP

LLC UDP UDP
LLC
Relay
BSSGP BSSGP IP IP
RLC/MAC
RLC/MAC
Frame Relay Frame Relay Layer 2 Layer 2

GSM RF Layer 1 bis Layer 1 bis Layer 1 Layer 1
GSM RF
Um Gb Gn Gi
Mobile Device BSS SGSN GGSN

Figure 4.8 User plane for GPRS/GSM

control and user plane protocols use GTP between the SGSN and GGSN. The control
plane uses GTP-C and the user plane uses GTP-U.
Figure 4.9 shows a 1500-byte payload passing through the mobile device protocol stack
to be transported in the 456 bits available within a radio block over the air interface. This
radio block consists of four TDMA frames, each making available 114 bits. The number
of actual bits used for user data transfer and for error detection and correction depends
on the speci¬c coding scheme being used.
Figure 4.10 shows a generalized diagram of the layered process for GPRS from the
perspective of the user equipment (UE). The SGSN stack is similar, but will have a
number of differences including the use of BSS GPRS protocol (BSSGP) rather than
RLC/MAC to transport the LLC. The left-hand side of this diagram has been presented
in Chapter 3. The right-hand side blocks SNDCP, LLC and RLC/MAC are described in
the following sections.
A packet received at the UE indicates which upper layer entity the packet is to be
routed towards using the 4-bit protocol discriminator ¬eld in the Layer 3 header. The
protocol discriminator (PD) may for example identify that the received packet mobility
management, SMS messages, or a user IP packet.
4.6 GPRS PROTOCOLS 95


max 1500 bytes Payload

TCP/IP Payload

SNDCP TCP/IP Payload

LLC information frame (max 1600 bytes)
LLC Payload BCS
SNDCP TCP/IP



RLC/MAC Info FCS RLC/MAC Info RLC/MAC Info FCS
FCS

456 bits 456 bits 456 bits
114 114 114 114 114 114 114 114
114 114 114 114


3 1 57 26 1 57 8.25


Figure 4.9 Packet fragmentation for transport

Signalling SNDCP
CM SM CC SS
& SMS
Logical Link
Mobility Management (MM) Control (LLC)


Radio Resource
Radio Resource
Sublayer
Management
RR PDU's
Protocol Discriminator PBCCH PDTCH
PCCCH
PACCH
RLC/MAC

PDCH
SDCCH


SDCCH
SACCH


SACCH
FACCH
AGCH
BCCH
RACH

PCH




SAPI 0 SAPI 3

Data Link Layer


Physical Layer


Figure 4.10 Mobile device layered architecture

The RLC/MAC layer supports four radio priority levels as well as an additional level
for signalling messages. This information is used by the BSS to determine the access
priority and the transfer priority under heavy load.


4.6.1 Physical and logical channels
Information about a cell™s ability to deal with GPRS subscribers is broadcast on the GSM
broadcast channel (BCCH). GPRS introduces a number of additional control channels
96 GENERAL PACKET RADIO SERVICE


to the air interface; some of these are mandatory and some are optional. When the new
channels were introduced, the naming scheme was to simply put a P in front of the old
channel name, if such a channel existed; for example, the BCCH becomes the PBCCH.
Each of these channels is transferred via a packet data channel (PDCH). This PDCH
equates to a physical channel taken from the total pool of GSM and GPRS resources.


4.6.1.1 Broadcast and control channel (BCCH)
The broadcast and control channel (BCCH) transmits general information from the base
station to all mobile devices in the cell. One small part of this information is to indicate
whether or not GPRS is supported in this particular cell. If GPRS is supported and the
optional packet broadcast and control channel (PBCCH) is con¬gured, the position of this
channel is also indicated on the BCCH. The PBCCH is then used to broadcast information
to mobile devices which is required for packet transmission operations. The information
transmitted on the BCCH is also reproduced so that mobile devices connected in packet
switched mode can listen to a single channel (PBCCH) for all general cell information.
As mentioned, the PBCCH is optional; if GPRS is supported but the PBCCH is not, then
information for GPRS devices is broadcast on the BCCH.

4.6.1.2 Common control channel (CCCH)
This is a GSM channel, but can be used for GPRS if PCCCH does not exist.
The CCCH is actual constructed from a number of channels, including the following:

• paging channel (PCH): a downlink channel used to page mobile devices
• random access channel (RACH): an uplink channel used to request a SDCCH channel
• access grant channel (AGCH): a downlink channel used to allocate the requested
SDCCH. It can also allocate a traf¬c channel (TCH) directly
• noti¬cation channel (NCH): this is used to notify mobile devices of voice group or
voice broadcast calls.

4.6.1.3 Packet common control channel (PCCCH)
The PCCCH is an optional channel that is transported on a PDCH; if it is not allocated
then information required for packet switch operation is transmitted on the CCCH. The
PCCCH may be implemented if the demand for packet data transfer warrants this or if
there is enough spare capacity within the cell since this will increase the QoS for packet
data access. It consists of the following:

• packet paging channel (PPCH): a downlink channel used to page mobile devices
prior to packet transfer. This paging can be used for both circuit switched and packet
switched paging
• packet random access channel (PRACH): an uplink channel used to request one or
more packet data traf¬c channels (PDTCH).
4.6 GPRS PROTOCOLS 97


• packet access grant channel (PAGCH): a downlink channel used to assign the requested
PDTCH channels
• packet noti¬cation channel (PNCH): this downlink channel is used to notify a group
of mobile devices of a point-to-multipoint packet transfer.

The actual GPRS traf¬c is transferred over the packet data traf¬c channel (PDTCH). This
corresponds to the actual resources that have been made available for this transfer. It may
be a single time slot, part of a time slot or a number of time slots up to the maximum of
eight, all of which must be on a single frequency.
The packet associated control channel (PACCH) is a signalling channel which is ded-
icated to a particular mobile device. It is required in both the uplink and downlink.
The information on this channel may consist of resource assignment information, power
control information or acknowledgements.
Figure 4.11 illustrates the downlink (a) and uplink (b) channels.
As discussed, it is not necessary for a cell with GPRS to implement the PCCCH
channels since the mobile device will be able to use the existing GSM control channels.
However, if there are any PDCHs that contain PCCCH the network will broadcast this
information on the BCCH. On the downlink the ¬rst radio block (B0) will be used as a
PBCCH, if a PBCCH exists. If required, up to three more blocks on the same PDCH can
be used for additional PBCCH information.
Since GSM and GPRS may dynamically share the same radio resources in the cell,
it is important that GPRS releases resources as soon as possible without introducing too
much signalling. The following situations for resource release are possible:

• wait for the assignment on the PDCH to terminate
• notify all users that have an assignment on the PDCH individually
• broadcast a message to deallocate the PDCH.

In practice a combination of the above may be used.


PDCH PDCH




PACCH PCCCH PDTCH PBCCH PACCH PCCCH PDTCH




PNCH PPCH PAGCH PRACH

(a) Down Link (b) Up Link

Figure 4.11 Downlink and uplink channels
98 GENERAL PACKET RADIO SERVICE


4.6.2 Subnetwork-dependent convergence protocol
(SNDCP)
The SNDCP is only used in the user plane to indicate a speci¬c PDP context and not
used by the mobility and session management. A subscriber may have a number of PDP
contexts open and each one of these is associated at this layer to a network services access
point identi¬er (NSAPI). The main functions of the SNDCP layer are to provide:

• multiplexing of PDPs;
• compression of user data (including IP header compression);
• segmentation of data packets to be passed to the LLC layer.

The LLC stipulates the maximum size of protocol data unit (PDU) it can carry within a
single segment. If the Layer 3 packet (e.g. IP packet), referred to as a network layer PDU
(N-PDU), cannot ¬t into this size then it is a requirement of the SNDCP layer to break
the Layer 3 packet into smaller segments (SN-PDU) that can be carried within one LLC
frame and then at the receiving end reassemble the N-PDU packet. The SNDCP layer
compression capability for the IP header conforms to the IETF header compression RFCs:
RFC 1144 and RFC 2507. It is also possible to compress the data in compliance with
ITU-T V.42bis. Although compressing the user data reduces the number of data bytes
transmitted over the air, it does have the negative effect of increasing the processing
power required by the MS and the SGSN.
Figure 4.12 illustrates how two different data connections can be active on a single
mobile device, both of these active connections sharing the same logical link between the
mobile device and the SGSN. The PDP connection types do not have to be the same;
for example, one session may be an IPv4 connection to the Internet while the other may
be a PPP or IPv6 connection to the home network. They are identi¬ed by their different
NSAPI identi¬ers. If a number of PDP contexts require the same QoS, they will have


Mobility and User data for pdp User data for pdp

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