This chapter discusses the Gb interface, which connects a packet control unit (PCU) and a serving GPRS support node (SGSN). It describes the layered structure of the Gb interface and the protocols used at each layer. The physical layer can use different technologies. Above this is the network service layer, which provides virtual circuits using protocols like Frame Relay. The base station subsystem GPRS protocol (BSSGP) manages buffers and virtual circuits. This layered design allows evolution of the underlying network without changing higher layers. The chapter then examines specific aspects of the protocols used, including Frame Relay frame structure and procedures, network service addressing, and protocol data unit formats.

1. Chapter 8
The Gb interface
Contents:
8.1 Structure of the Gb
1. Gb interface structure
2. Evolved service model
3. Physical implementation of Gb
4. Layer 2: Network Service
8.2 Protocol structures
1. Frame Relay- Frame structure
2. The address field of Frame Relay
3. The Frame Relay network
4. Frame Relay procedures
8.3 NS Frame formats
1. NS addressing and load sharing
2. Connection of RAN Nodes to Multiple CN Nodes
3. General structure of a PDU
4. The network service Protocol Data Units (FR)
5. New NS-VC Start Up Procedure
6. The NS Service Data unit

2. Chapter 8
The Gb interface
8.4 BSSGB procedures and messages
1. Paging
2. Signalling Procedures between NM SAPs
3. Flow control messages
4. BSSGP UL unitdata
8.5 Gb appendix
1. Estimation of Gb overhead

3. Chapter 8
The Gb interface
8.1 Structure of the Gb
1. Gb interface structure
2. Evolved service model
3. Physical implementation of Gb
4. Layer 2: Network Service

4. Gb interface structure
PCU SGSN
Relay GMM NM LLC GMM NM
Physical Layer (08.14)
NS: Network Service (08.16)
NM: Network Management
BSSGP BSSGP BSSGP: BSS GPRS Protocol (08.18)
NS NS
Gb
Physical Physical
Gb identifies the interface between 1 PCU and 1 SGSN. Gb shows a layered structure which enables the use of
different technologies.
The physical layer defines the characteristics of the used medium. (PCM, STM-1,…)
Network Service (NS) layer is composed of two parts.
Creates NS virtual circuits together with identifieres and
NS Control part
defines procedures to manage them
NS
NS Sub-network part Defines the layer two protocol. In Rel 99 this is Frame
Relay, starting with Rel 4 optional an IP network
Base Subsystem GPRS Protocol (BSSGP) is used mainly for managing the buffers for flow control. Service
provided by this layer are :
•Network Management (NM): A local entity managing buffers and virtual circuits between the two nodes
•GPRS Mobility Management (GMM) deals with mobility messages between SGSN and PCU (for example
paging procedure)
•LLC in SGSN / Relay in PCU towards RLC/MAC are access points used for example for user data

5. Evolved service model up to Rel 6
Service model in an BSS Service model in an SGSN
RELAY LCS RIM PFM GMM NM MB LLC LCS RIM PFM GMM NM MB
MS MS
3GPP TS RL LCS RIM PFM GMM NM MBMS BSSGP LCS RIM PFM GMM NM MBMS
44.064
RLC/MAC BSSGP BSSGP
3GPP TS 48.016 3GPP TS 48.016
Network Service Network Service
- "RL" (relay) for functions controlling the transfer of LLC frames between the RLC/MAC function and BSSGP;
- "GMM" (GPRS mobility management) for functions associated with mobility management between an SGSN
and a BSS; and
- "NM" (network management) for functions associated with Gb-interface and BSS-SGSN node management;
- "PFM" (packet flow management) for functions associated with the management of BSS Packet
Rel 99
Flow Contexts (PFCs);
- "LCS" (location services) for functions associated with location services (LCS) procedures; Rel 5
- "RIM" (RAN Information Management) for functions associated with generic procedures to communicate
between two BSSs or with UTRAN via the core network. Rel 5
- “MBMS” (Multimedia Broadcast Multicast Service) for functions associated with Multimedia
Rel 6
Broadcast Multicast Service (MBMS) procedures.

6. Layer 2: Network Service
An SGSN and a BSS may be connected by different physical links. Each physical link is locally (i.e. at each side of
the Gb interface) identified by means of a physical link identifier. The exact structure of the physical link identifier is
implementation dependent.
Each physical link supports one or more Network Service Virtual Links (NS-VL). It defines a virtual communication
path between the BSS or the SGSN and the intermediate network, or between the BSS and the SGSN in case of
direct point-to-point configuration. Each NS-VL may be identified by means of a Network Service Virtual Link
Identifier (NS-VLI). The significance (i.e. local or end-to-end) and the exact structure of the NS-VLI depends on the
configuration of the Gb interface and on the intermediate network used. For example, in the case of a Frame Relay
network, the physical link is the FR bearer channel, the NS ‑VL is the local link (at UNI) of the FR permanent virtual
connection (PVC) and the NS-VLI is the association of the FR DLCI and bearer channel identifier.
Each NS-VL is supported by one physical link if the Frame Relay Sub-Network is employed. For an IP sub-network,
the NS-VL is mapped to an IP endpoint. The exact nature of the NS-VL depends on the intermediate network used
on the Gb interface. That means the SW in the SGSN and PCU handles NSVCI (Network Service virtual
connection Identifier), identifying different paths end to end. This design allows an evolution of the sub network
without the need to redesign higher layers.
LLC
BSSGP BSSGP
In one PCU there is one NSE (Network Service Entity)
NS NS In a SGSN many NSE are defined (one per PCU)identified by one
NSEI (NSE Identifier)
L1 L1
Gb SGSN
PCU

7. Layer 2: Network Service
One Frame Relay link
or one IP endpoint
PCU 1
N N
S
NSVCI 1 link 1 NSVCI 1 S
E E
I NSVCI 2 link 2 NSVCI 2 I
1 1
1PCM SGSN
PCU 0
N
NSVCI 3 link 3 NSVCI 3 S
N
E
S
I
E NSVCI 4 link 4 NSVCI 4 0
I
0
NS- Sub-network
service

8. Chapter 8
The Gb interface
8.2 Protocol structures
1. Frame Relay- Frame structure
2. The address field of Frame Relay
3. The Frame Relay network
4. Frame Relay procedures

9. Physical implementation of Gb
The physics (Layer 1, L1) of the Gb interface in most cases is based on multiples of PCM timeslots ( Frame Relay
link may have maximal the capacity of one PCM). Basically it can be realized in 4 different ways:
- As A interface connection (possibility 1 and 2):
- NUC through MSC to the SGSN 1
- NUC through MSC and then via Frame Relay sub-network to the SGSN. 2
- As a dedicated line configuration (possibility 3 and 4) :
- a direct line and then via Frame Relay sub-network to the SGSN 3
- a direct connection to the SGSN. 4
A
For the last two configurations PCMs just carrying GPRS
traffic have to be configured in the BSC. TRAU MSC
A mixed configuration of links via A interface connection
and dedicated line connection is possible. 1
Please note that an IP Subnetwork may 2
E1/T1-Line
be based on other physical media!!!!!
P
LLC SGSN
BSC C Frame Relay
3 Sub-network
BSSGP BSSGP U
NS NS
L1 L1
E1/T1-Line 4
dedicated line
Gb SGSN
PCU
(multiple) nailed-up connections, each 64 kbit/s

10. Frame Relay- Frame structure
Frame Relay is a layer 2 protocol, which is used in data networks, e.g. X.21, V.35, G.703/704,... . It is designed
to be used on PCM lines. One Frame Relay link consists of one or several PCM timeslots (max=all of one PCM).
It is effective and has only a small overhead but offers no mechanism for retransmissions. Therefore it is used
preferably on reliable connections. The DLCI (Data Link Connection Identifier) in the address field is used to
route the Frames in a Frame Relay network. It identifies a channel between two adjacent nodes (layer 2
identifier). Frame relay offers communication paths between 2 nodes. PVC Permanent Virtual Connections will
allow communication via several nodes.
FLAG
0 1 1 1 1 1 1 0 Opening flags, unique sequence of bits defining the start of a frame.
Address field (byte 1) The address field contained in the Frame Relay Header can have
different length, ranging from 2 to 4 bytes. For GPRS only 2 bytes are
Address field (byte 2) used.
Frame
Relay User data with
variable length The data part length can be from 1 to 4096 bytes. For GPRS the
frame maximum length is limited to 1600 bytes (enough for 1 LLC frame)
In case of detection of an error detected with the help of the Frame
Frame Check Sequence byte 1
Check Sequence (or an unkknown DLCI) the frame will be immediately
Frame Check Sequence byte 2 discarded. No retransmission mechanism is defined.
FLAG Clossing flag, the same sequence of bits as for the opening flag will
0 1 1 1 1 1 1 0 close the frame.

11. The address field of Frame Relay
Extension Address bit indicates whether Bit
EA EA
another octett header follows or not (in GPRS 8 7 6 5 4 3 2 1
0 1
EA
only 2 bytes of header are used). DLCI (MSB) C/R
0
EA
C/R Command/Response bit, not used in GPRS DLCI (LSB) FECNBECN DE
1
The Data Link Connection Identifier
DLCI (LSB) DLCI (10 meaning
(DLCI) identifies all FR packets that
belong to the same receiver. The DLCI is the Frame Relay bits)
0 signaling
address. There are DLCI with 10, 16, 17 and 23 bits 1 - 15 reserved
possible. In GPRS networks mostly the 10 bit DLCI format is
16 - 991 addresses of virtual connections
used.
992 – 1007 management function of layer 2
The DLCI value is divided into two parts with a various 1008 – 1022 reserved
number of bits: the More and Less Significant Bits (M/L-SB).
1023 reserved for layer 2 information
FECNBECN DE
Frame Relay allows a congestion control using two flags (only of interest if a Frame Relay
network is used):
The FECN (Forward Explicit Congestion Notification), which signals an overload in forward direction (the node
cannot send as much data packets as necessary since the line respectively the network element has a too small
capacity), and the BECN (Backward Explicit Congestion Notification), which signals that the network node itself
cannot handle the amount of data packets received and therefore the incoming data stream should be reduced.
With the DE (Discard Eligibility) bit the overloaded node is informed, whether a data packet may be discarded in
case that the reduction of the data stream by the DTE was not sufficient. If this still does not reduce the load
situation, data packets with the DE bit set to 0 are also discarded.

12. The Frame Relay network
A Frame Relay network may be used between many SGSN and many PCU. Frame Relay allows two types of
connections:
Permanent Virtual Connections (PVC) are maintained all the time,
Switched Virtual Connections (SVC) are established and released on demand.
The Frame Relay Link on the Gb interface uses PVC only.
Frame Relay switches use routing tables which associate a port and DLCI incoming to another port and DLCI
outgoing. The DLCI value in the will be replaced.
A Permanent Virtual
DLCI: 23 Connection between
One frame on its way through two users defines a
the network. The indicated dedicated path
DLCI values are examples through the network
DLCI: 23
X USER B
UNI FRAD
USER A FRAD
DLCI: 27
X DLCI: 91 X DLCI: 56
X X FRAD USER C
FRAD Frame Relay Access Device
Frame Relay switch
UNI User to Network Interface

13. Frame Relay procedures
Status Message
• Status Enquiry - Request the status of a PVC/ verify link integrity
• Status- Mandatory response of Status Enquiry, indicates status of PVC and/or link integrity verification
Messages used for PVC status. A more detailed status reports is received if the type of report is set to
„Full Status“. In this case also the DLCI of the link is checked.
The messages use DLCI 0.
Status Enquiry
This message is sent to request the status of permanent virtual connections or to verify link integrity. Sending a
STATUS message in response to a STATUS ENQUIRY message is mandatory.
Status
This message is sent in response to a STATUS ENQUIRY message to indicate the status of permanent virtual
connections or for a link integrity verification. Optionally, it may be sent at any time to indicate the status of a single
PVC.
T391 defines the period between 2 Status messages
DTE DCE
Full Status enquiry N391 defines the how often the Full status is requested
T391 Full Status N392 is the error recovery counter which defines the
Status enquiry number of unsuccessful polling cycles in a certain time
frame before the FRL is put to the Disabled state. It is
N391 * T391 Status closely related to the value
. N393 *which is a counter. The system will try N393 times
before the links are put in Disabled state.
.
Full Status enquiry
DTE Data Terminal Equipment, e.g. BSC or SGSN
.
Full Status DCE Data Communication Equipment
The values of T391 and N391 need to be the same on PCU and SGSN side. If they are different it can happen that
BSSGP is not started, this means: no transfer of user data and GMM/SM signaling between SGSN and PCU!

14. Chapter 8
The Gb interface
8.3 NS Frame formats
1. NS addressing and load sharing
2. Connection of RAN Nodes to Multiple CN Nodes
3. General structure of a PDU
4. The network service Protocol Data Units (FR)
5. New NS-VC Start Up Procedure
6. The NS Service Data unit

15. NS addressing and load sharing
A NS link is identifies by a NS Virtual Connection Identifier (NS-VCI). There is a 1:1 relation between a NS-VCI and
an underlaying Frame Relay DLCI since Frame Relay is the Sub-Network Service.
Each Network Service Entity Identifier (NSEI) identifies one PCU that belongs to one defined Base Station
Subsystem (BSS). Hence, the NSEI can be seen as the „name“ of a PCU in the network. Different NS-VCIs that
lead to the same PCU (NSEI) belong to the same NS Virtual Connection Group. To ensure better reliability due to
redundancy each bearer should be located on a different E1 or T1 physical line, but this is not mandatory. For a
given MS, packets will always take the same NSVCI in order to guarantee the order of the packets.
NS-VC group with load sharing
Cells served by (applies only to NS SDUs)
PCU 1
Cell 1 PCU 1
Traffic coming from cell 1 is N N
Cell 2 sent through any NSVCI S NSVCI 1 link 1 NSVCI 1 S
E E
Cell 3
I NSVCI 2 link 2 NSVCI 2 I
NS-UDT messages that carry 1 1
all payload and signaling also 1PCM SGSN
contain the BSSGP Virtual PCU 0 N
NSVCI 3 S
Cell 4 Connection Identifier (BVCI). N link 3 NSVCI 3 E
S
The BVCI (BSSGP Virtual I
Connection Identifier)
E NSVCI 4 link 4 NSVCI 4
Cell 5 I 0
represents a single cell, 0 NS- Sub-network
Cell 6 BSSGP Signaling entity or Service (DLCI)
Point-to-Multipoint (PTM)
entity inside the BSS.

16. Connection of RAN Nodes to Multiple CN Nodes
Use of Concepts on the Gb Interface when Intra Domain Connection of RAN Nodes to Multiple CN Nodes
applies in the BSS.
RAN sharing respectively CN redundancy may be reasons for that.
Rel 5
For a pool area the BSS sets up several NSEs, and each of these NSEs goes towards different SGSNs. In this way
the BSS have one NSE towards each of the connected SGSNs. Alternatively, several NSEs in the BSS are
connected towards each of the SGSNs supporting the pool areaOne or more NS-VCs are set up between each of the
NSEs in the BSS and the corresponding peer NSEs in the SGSNs.
In an IP network, an NS-VC is identified by a pair of IP addresses and UDP ports at both the BSS and the SGSN. In
a FR network, the identity of an NS-VC is unique within an NSEI.
BSS 1 SGSN 1
BVCI=3 NSVC 1
NSEI=1
NSEI=1
NSVC 2
BVCI=4
Radio Cell 1 Traffic coming from cell 1 may
be handled by different CN
SGSN 2 nodes!
Radio Cell 1 BVCI=3
NSVC 3
NSEI=2 NSEI=2
BVCI=5 NSVC 4

17. General structure of a PDU
8 7 6 5 4 3 2 1
octet 1 PDU type The first octett defines the type of PDU
octets 2, 3, ...n other information elements
For IP/FR
PDU types defined: Only for IP sub network
sub network NS-UNITDATA
SNS-ACK NS-RESET
A set of messages which are only SNS-ADD NS-RESET-ACK
used in an IP subnetwork, the so SNS-CHANGEWEIGHT NS-BLOCK
called Sub-Network Service SNS-CONFIG NS-BLOCK-ACK
Control PDUs (SNS PDUs) are SNS-CONFIG-ACK NS-UNBLOCK
defined (starting with Rel 4). SNS-DELETE NS-UNBLOCK-ACK
SNS-SIZE NS-STATUS
SNS-SIZE-ACK NS-ALIVE
NS-ALIVE-ACK
GSM rec 8.16 defines for each type of PDU a list of Information Elements which are present (mandatory M or –
conditional C) of a certain format (V, TLV or TV) and a certain length in octetts.
Information Presence Format Length
element

18. The Network Service Protocol Data Units (FR)
NS PDU Type Remarks Depending on value of timer Tns-test a Test
NS-ALIVE This PDU is used to test a NS-VC Procedure using control messages Alive (ALV)
and Alive Acknowledge (ALVA) supervises the
NS-ALIVE-ACK This PDU acknowledges a received NS- availability of the NS link. Due to the periodical
ALIVE PDU and is sent on the NS-VC appearance of ALV/ALVA messages this status
where the NS-ALIVE PDU was received check procedure is also called „heartbeat
check“. After the first ALV is answered with a
NS-BLOCK This PDU indicates that a NS-VC shall
ALVA from the other side, every 10 (?) seconds
be blocked at the recipient entity
another ALV is sent into the same direction.
NS-BLOCK-ACK This PDU acknowledges that a NS-VC
has been blocked for use
NS-RESET This PDU indicates that the NS peer
entity is trying to reset one NS-VCs
Messages that handle procedures for establishing
NS-RESET-ACK This PDU acknowledges the reset of the a new Network Service Virtual Connection (NS-VC)
indicated NS-VCs
or close a connection.
NS-STATUS This PDU is used to report error
conditions
NS-UNBLOCK This PDU indicates that a NS-VC shall
be unblocked at the recipient entity
NS-UNBLOCK- This PDU acknowledges that a NS-VC
ACK has been unblocked
NS-UNITDATA This PDU transfers one NS SDU
between the BSS and SGSN For ‘data, traffic’

19. New NS-VC Start Up Procedure
If a new Network Service Virtual Connection (NS-VC) is taken into service the following startup procedure can be
monitored (indicated values are examples)
PCU SGSN
RST (DLCI=103, NS-VCI=12, NSEI=520, cause)
RSTA (DLCI=103, NS-VCI=12, NSEI=520)
UBLO (DLCI=103)
UBLA (DLCI=103)
ALV (DLCI=103)
Tns-test (e.g. 10s) ALVA (DLCI=103)
ALV (DLCI=103)
1. New NS-VC is reset using NS control messages Reset (RST) and Reset Acknowledge (RSTA). Both
messages contain DLCI of the appropriate Frame Relay PVC, NS-VCI as identity of the NS-VC and NSEI as
identifier of the BSS to which the NS-VC leads to.
2. After the NS-VC was reset it is unblocked to enable data transport using Unblock (UBLO) and
Unblock Acknowledge (UBLA) messages. Since relation between DLCI and NS-VCI was already defined within the
Reset procedure all following NS control procedures use only DLCI value to identify the link.
3. Depending on value of timer Tns-test a Test procedure using control messages Alive (ALV) and Alive
Acknowledge (ALVA) supervises the availability of the NS link. Due to the periodical appearance of ALV/ALVA
messages this status check procedure is also called „heartbeat check“. After the first ALV is answered with a ALVA
from the other side, every 10 seconds another ALV is sent into the same direction.

20. The NS Service Data Unit
This PDU transfers one NS SDU (user data, BSSGP control messages, ..) between the BSS and SGSN.
It is used in both directions. BSS to SGSN, SGSN to BSS
Information Presence Format Length
element
PDU type M V 1
NS SDU M V 1
Control Bits Allows to request or confirm a change
flow
BVCI M V 2 Contains the BVCI as mandatory IE!
NS SDU M V 1-?
Length has to be derived by lower layers!

21. Chapter 8
The Gb interface
8.3 The BSSGB protocol
1. The BSSGP protocol
2. The BSSGP PDU types
3. DL user data on Gb 3GPP 48.018
4. UL user data on Gb 3GPP 48.018

22. The BSSGP protocol
Base Station Subsystem GPRS Protocol (BSSGP) is the layer 3 protocol between SGSN and PCU.
The main tasks of the BSSGP are:
• Provision of radio-related, QoS and routing information between the RLC/MAC layer of PCU and the
SGSN
• Provision of connectionless link between SGSN and BSS
• Handling of paging requests from the SGSN to the BSS
• Provision of flow control between SGSN and BSS
Uplink and downlink messages are handled on separated BSSGP channels. In downlink direction the radio related
information used by the RLC/MAC function of the BSS is provisioned by the SGSN. In the uplink direction this radio
related information is derived from the RLC/MAC and sent to the SGSN.
Furthermore the BSSGP allows the SGSN and BSS to operate node management control functions. Each BSSGP
Virtual Connection (BVC) is identified by means of a BSSGP Virtual Connection Identifier (BVCI) which has end-to-
end significance across the Gb interface. Each BVCI is unique within on Network Service Entity, that means: within one
BSS. The BVCI value 0000 hex shall be used for the signalling functional entities.
The BVCI value 0001 hex shall be used for the PTM functional entities.
All other values may be used freely by the BSS and shall be accepted by the SGSN.
LLC
BSSGP BSSGP BVCI = 0 Signalling entity
BVCI = 1 PTM entity
NS NS
SGSN BVCI = 2
Cell 1 PTP
L1 L1
BVCI = 3 Cell 2 functional
Gb SGSN
PCU BVCI = ? Cell ? entities

23. The BSSGP PDU types
8 7 6 5 4 3 2 1
octet 1 PDU type The first octet defines the type of PDU
octets 2, 3, ...n other information elements PDUs between NM SAPs
BVC-BLOCK
BVC-BLOCK-ACK
PDU types defined (Rel 6)
BVC-RESET
BVCI = 0
BVC-RESET-ACK
PDUs between RL and BSSGP BVC-UNBLOCK
SAPs BVC-UNBLOCK-ACK
DL-UNITDATA FLOW-CONTROL-BVC
UL-UNITDATA PTP Mapping of the
FLOW-CONTROL-BVC-ACK
RA-CAPABILITY BSSGP PDU to PTP FLOW-CONTROL-MS
PTM-UNITDATA BVCI = 1 A functional entity
FLOW-CONTROL-MS-ACK
FLUSH-LL
PDUs between GMM SAPs
FLUSH-LL-ACK
PAGING PS
BVCI = 0 or PTP BVCI = 0 LLC-DISCARDED
PAGING CS
SGSN-INVOKE-TRACE
RA-CAPABILITY-UPDATE
PTP BVCI = 0 or 1 or PTP STATUS
RA-CAPABILITY-UPDATE-ACK
DOWNLOAD-BSS-PFC
RADIO-STATUS PTP
CREATE-BSS-PFC
SUSPEND
CREATE-BSS-PFC-ACK
SUSPEND-ACK
CREATE-BSS-PFC-NACK
SUSPEND-NACK
BVCI = 0 MODIFY-BSS-PFC
RESUME
PTP MODIFY-BSS-PFC-ACK
RESUME-ACK
DELETE-BSS-PFC
RESUME-NACK
DELETE-BSS-PFC-ACK

24. DL user data on Gb 3GPP 48.018
DL-UNITDATA
Information element Type / Reference Presence Format Length
PDU type PDU type/11.3.26 M V 1
TLLI (current) TLLI/11.3.35 M V 4
QoS Profile QoS Profile/11.3.28 M V 3
PDU Lifetime PDU Lifetime/11.3.25 M TLV 4
MS Radio Access MS Radio Access
O TLV 7-?
Capability a) Capability/11.3.22
Priority Priority/11.3.27 O TLV 3
DRX Parameters DRX Parameters/11.3.11 O TLV 4
IMSI IMSI/11.3.14 O TLV 5 –10
TLLI (old) TLLI/11.3.35 O TLV 6
PFI PFI/11.3.42 O TLV 3
LSA Information LSA Information/11.3.19 O TLV 7-?
Service UTRAN CCO
Service UTRAN CCO O TLV 3
/11.3.47.
Alignment octets Alignment octets/11.3.1 O TLV 2-5
LLC-PDU b) LLC-PDU/11.3.15 M TLV 2-?
a) The field shall be present if there is valid MS Radio Access Capability information known by the SGSN; the field shall not be
present otherwise.
b) The LLC-PDU Length Indicator may be zero.

25. DL user data on Gb 3GPP 48.018
On the downlink, a DL-UNITDATA PDU contains information elements to be used by the RLC/MAC function and a
LLC-PDU. There is only one LLC-PDU per DL-UNITDATA PDU possible.
The SGSN provides the BSSGP with a current TLLI, identifying the MS. If a SGSN provides a second TLLI,
indicating that a MS has recently changed its TLLI, this is considered as the 'old' TLLI. A BSS uses the 'old' TLLI
to locate a MS's existing context. Subsequent uplink data transfers for this MS reference the current TLLI and not
the old TLLI.
The Local TLLI is derived from the P-TMSI (Packet Temporary Mobile Subscriber Identity). It is used if the MS
wants access to the network and has not changed its Routing Area (RA) since the P-TMSI was allocated.
Foreign TLLI is also derived from P-TMSI. Used in case of a Routing Area Update procedure.
Random TLLI is created by MS. Used if no P-TMSI is stored in the MS, e.g. for first Attach to a network. Also
used for Anonymous PDP Context Activation Request.
Auxiliary TLLI is created by SGSN. Only used in case of Anonymous PDP Context Activation as defined in GPRS
Release 97 and 98.
LLC
BSSGP BSSGP
NS NS
L1 L1
Gb SGSN
PCU

26. UL user data on Gb 3GPP 48.018
UL-UNITDATA
Information element Type / Reference Presence Format Length
PDU type PDU type/11.3.26 M V 1
TLLI TLLI/11.3.35 M V 4
QoS Profile QoS Profile/11.3.28 M V 3
Cell Identifier Cell Identifier/11.3.9 M TLV 10
PFI PFI/12.3.42 O TLV 3
LSA Identifier List LSA Identifier List/11.3.18 O TLV 3-?
Alignment octets Alignment octets/11.3.1 O TLV 2-5
LLC-PDU a) LLC-PDU/11.3.15 M TLV 2-?
a) The LLC-PDU Length Indicator may be zero.
On the uplink, an UL-UNITDATA PDU contain information elements derived from the RLC/MAC function, meaningful to
higher-layer protocols in a SGSN, and a LLC-PDU.
The BSS provides the TLLI, received from the MS, to the SGSN. Beside the TLLI the BSS provides a BVCI and a NSEI
indicating the point-to-point functional entity, upon which the LLC-PDU was received.

27. Chapter 8
The Gb interface
8.5 BSSGB procedures and messages
1. Paging
2. Signalling Procedures between NM SAPs
3. Flow control messages
4. BSSGP UL unitdata

28. Paging
NS UDT (PAGING PS)
(PDU type, IMSI or P-TMSI, QoS Profile, Location Area or Routeing Area)
BSS NS UDT (PAGING CS) SGSN
(PDU type, IMSI, DRX Parameters, Location Area or Routeing Area )
· - For packet-switched transmission - PAGING PS PDU
· - for circuit switched transmission - PAGING CS PDU (in case of Gs interface available)
· - PDU contains information to initiate paging for a MS within a group of cells
To enable data transmission from the SGSN to the MS, the SGSN sends a PAGING PS (Packet Switched) PDU. To
initiate a voice call from a MSC/VLR to a MS, the SGSN is also able to send a PAGING CS (Circuit Switched) PDU.
In both cases the PDU contains information to find a MS within a group of cells and to set up the call. The SGSN
provides the BSSGP with MS specific information. This includes:
QoS profile with bit rate parameter set to "best effort" and transmission mode set to "unacknowledged" an indication of
cells (so-called DRX Parameters) within the BSS shall page the MS. Here it is possible that the MS is paged in all cells
of a BSS, cells on a BSS within one Location Area (LA) or cells on a BSS within one Routing Area (RA).
Each PAGING PDU relates to only one MS, but on behalf of a special radio interface paging PDU it is also possible for
the BSS to page different MS at the same time.
The paging can be started with different MS identifications.
•IMSI and DRX Parameters for circuit-switched services
•IMSI for packet-switched services
•P-TMSI if SGSN provides the information
•TMSI and TLLI if SGSN provides the information

29. Signalling Procedures between NM SAPs
BVC Reset Procedure
The purpose of the BVC RESET procedure is to synchronize the initialization of GPRS BVC related contexts at a
BSS and SGSN. This enables the BSS and SGSN to begin communication in known states.
The reason to initiate a RESET procedure can be:
a system failure in the SGSN or BSS
an underlying network service system failure
a change in the transmission capability of the underlying network service
The BVC-RSET PDU includes the BVCI of the reset BVC, a cause element indicator and if necessary the cell
identifier, when the reset is for a PTP BVC and BSS is initiator of the reset.
The partner side sends an acknowledgement with BVC-RESET-ACK, which includes the same parameters with
the exception of cause indicator.
NS UDT (BSSGP-PDU: BVC-Reset)
(PDU type, BVCI, Cause, Cell Id.)
NS UDT (BSSGP-PDU: BVC-RESET-ACK)
(PDU type, BVCI, Cell Id.)

30. SGSN
Signaling Procedures between NM SAPs
BVC Blocking and Unblocking Procedure
The BVC blocking and unblocking procedure is initiated by the BSS to block one BVC because of Operation and
Maintenance intervention for a cell, equipment failure at the BSS or cell equipment failure at the BSS.
When a BSS blocks a BVC, the BSS marks that BVC as blocked and discards any traffic sent to the BVC in the
uplink direction. The cells associated with the BVC doesn't accept any data in the downlink direction.
NS UDT (BSSGP-PDU: BVC-BLOCK)
(PDU type, BVCI, Cause)
NS UDT (BSSGP-PDU: BVC-BLOCK-ACK)
(PDU type, BVCI)
To reset the block status the BVC-UNBLOCK PDU is used. This PDU is transmitted in the direction from BSS to
SGSN and includes as parameter the BVCI of the BVC, which is unblocked.
NS UDT (BSSGP-PDU: BVC-UNBLOCK)
(PDU type, BVCI)
NS UDT (BSSGP-PDU: BVC-UNBLOCK-ACK)
(PDU type, BVCI)

31. Flow control
The principle of the BSSGP flow control procedures is that the BSS sends to the SGSN flow control parameters
which allow the SGSN to locally control its transmission output in the SGSN to BSS direction (Flow Control is only
performed in DL!). The SGSN shall perform flow control on each BVC and on each MS. The flow control is
performed on each LLC-PDU first by the MS flow control mechanism and then by the BVC flow control mechanism.
If the LLC-PDU is passed by the individual MS flow control, the SGSN then applies the BVC flow control to the
LLC-PDU.
First level
MS flow control MS flow control MS flow control
Second level BVC flow control
Calculation of leak rate R
and buffer size Bmax per MS PCU SGSN
and BVC Flow control
commands
Fig. 1 BSS Flow control: Cascaded Flow Control (MN1889EU10MN_0001 Point-to-point packet flow, 13)

32. Flow control messages
FLOW-CONTROL PDU
BSS (Tag, Bucket Size, Leak Rate) SGSN
FLOW-CONTROL-ACK PDU
(Tag)
C defines the periodicity of the message
FLOW-CONTROL PDU
(Tag, Bucket Size, Leak Rate)

33. The Packet Flow
IMSI PDP PDP PDP PDP
TLLI context 1 context 2 context 3 context n
LLC PDUs LLC PDUs LLC PDUs LLC PDUs SGSN
Trace ref., type, id
OMC id
BSS Packet PFC Flow Control
BSS Packet
Flow Context (optional)
BSS Packet
Flow Context PFI 1 PFI 2 PFI X PFC Flow control
BSS Packet
PFIFlow Context Requires support of MS
PFI (Rel 99)
PFI Context
Flow
Aggregate BSS
Aggregate BSS
QoS Profile
QoSPFI
Aggregate BSS
Profile
and network (Rel 5)
Negotiated
QoS Profile BSS
Aggregate
Negotiated TLLI 1 TLLI 2 TLLI 3
BSS Negotiated
PacketProfile
QoS MS Flow control
BSS Packet
Flow Timer
Negotiated
BSS Packet
Flow Timer
BSS Packet
Flow Timer
Flow Timer
BVCI 1 BVC Flow control
BSS
A packet flow context defines the flow control in terms of buffer capacity, maximum throughput rate, etc.
for a single user. The management of these packet flow contexts is done with the Packet Flow
Management (PFM), which uses the BSSGP as means of transportation.
BVC Flow Control: The BSS informs the SGSN about the maximum size of the buffer for each BSSGP
Virtual Connection and a data transmission rate. Please note, that there is one BVC for each cell
supporting GPRS. The data transmission rate can be modified. Its rate simply represents the amount of
data, which can be currently transmitted in the cell. In other words, the BSS controls the flow of data from
the SGSN to it.

34. The Packet Flow
The following figure shows a system model when PFC Management is enabled without Multiple TBF (Rel 6). On
the SGSN side, there is for each BVC, MS and PFC (if supported) a buffer .
Enhanced Flow Control (eFC) has been introduced in R5 in order to inform SGSN about rate can be used for a
specific PFC (=flow) especially in case of congestion. BSC then can favor some lows instead of other flow. Using
only MS Flow Control this mechanism was not possible. With eFC(=PFC Flow Control) it is possible to reduce the
traffic for background PFCs while allowing the RT traffic for the same user.
eFC introduces new messages on Gb interface but these changes are subordinate to an agreement between
SGSN and BSC. Each NE knows the capabilities of the other during the BVC RESET procedure reading the
Feature Bitmap Field. In this way there aren’t problem of SW misalignment between SGSN and BSC.
Gb
BSS
Um
SGSN
Buffer 1
PFC
1
TBF BSS
Context
Buffer 2
PFC
2

35. The Packet Flow
MS BSC SGSN
C timer
1.DL_UNITDATA(PFI predefined)
1) TBF is opened due to a DL UNITDATA having PFI signalling coming.
1. TBF establishment 2) MS FLOW CONTROL is sent when the first C timer expiration
occurs.
2.MS FC
3) Then during packet transfer mode, a DL UNITDATA having PFI not
pre-defined causes a reconfiguration of TBF in order to manage new
services or in any case internal scheduler reconfiguration. It is not
strictly necessary to have a TS reconfiguration, maybe only a
scheduler reconfiguration occurs.
3.DL_UNITDATA(PFI1)
3a) If BSC does not have valid PFC parameters, PFC Download
3.a PFC Download Procedure
procedure starts.
3.TBF reconfiguration
4) At next C timer expiration a PFC Flow Control message including
4.PFC FC(PFI1) parameter for PFI1 is sent.
5.DL_UNITDATA(PFI2) 5) Then during packet transfer mode, a DL UNITDATA having PFI2 not
pre-defined could cause a reconfiguration of TBF in order to manage
5. TBF reconfiguration new services or in any case an internal scheduler reconfiguration can
occur.
6.PFC FC(PF1, PFI2)
6) At next C timer expiration a PFC-FC is sent including PF1 and PF2
parameters.

36. BSSGP UL unitdata
|GPRS BSSGP, SMG#31, 08.18 V6.7.0 (TS 101 343) (BSSGP670) UUDT (= UL-UNITDATA)
|UL-UNITDATA
|00000001 |Message Type |1
|TLLI
|***B4*** |TLLI (current) |c000006b
|QOS Profile
|***B2*** |R Value |0
|-----000 |Precedence |High priority
|----0--- |A bit |RLC/MAC ARQ functionality
|---0---- |T bit |PDU contains Signalling
|--0----- |C/R bit |PDU contains ACK or SACK
|00------ |Reserved |0
|Cell identifier
|00001000 |IE Name |Cell identifier
|10001000 |IE Length |8
|**b12*** |MCC number |XXX |***B2*** |CI |21
|1111---- |Filler 15
|Alignment Octets
|----0000 |MNC digit 1 |X
|0010---- |MNC digit 2 |X
|00000000 |IE Name |Alignment Octets
|***B2*** |LAC |10
|10000000 |IE Length |0
|00001100 |RAC |12
|LLC PDU

37. Chapter 8
The Gb interface
8.5 Gb appendix
1. Estimation of Gb overhead
2. Configuration Example

38. Estimation of Gb overhead
Protocol overhead in octetts for one packet on Gb:
Protocol Min Header Max Header Specification
FR 6 6 GSM 3.60
NS 4 4 GSM 8.16 NS-UNITDATA
BSSGB 12 54 GSM 8.18 DL-UNITDATA or UL-UNITDATA
LLC 5 40 GSM 4.64 I or U-frames
SNDCP 3 4 GSM 4.65 SN-UNITDATA or SN-DATA PDU
Total 30 min 108 max
FR
or NS BSSGB LLC SNDCP (compressed) IP
IP

39. Example of Configuration
SGSN NSEI_1 PCU1
Bearer
PAPU1 Channel_1 NS-VCI_7 BVCI_0
NSEI_1
BVCI_0 NS-VCI_7 DLCI_16
NS-VCI_2 BVCI_3
DLCI 17 LA
BVCI_3 NS-VCI_2 Bearer RA 1
Channel_2 NSEI_2 PCU2
BTS_3
NSEI_2 DLCI 16 NS-VCI_5
BVCI_0
DLCI_17 BTS_6
NS-VCI_5 NS-VCI_8
BVCI_0 DLCI_18 BVCI_6
NS-VCI_8 Bearer RA 2
BVCI_6 Channel_3 NS-VCI_3
BSS1 BTS_8
NS-VCI_3 DLCI_16 NSEI_3 PCU3
DLCI 17 NS-VCI_4 BVCI_8 BTS_22
PAPU2 NSEI_3
Bearer NS-VCI_1
Channel_4 BVCI_0
BVCI_8 NS-VCI_4
DLCI 16 NS- BVCI_22
BVCI_0 NS-VCI_1 VCI_11
NS- Bearer
BVCI_22
VCI_11 Channel_5
DLCI 16
DLCI_17
PAPU3
NSEI_7 Bearer NSEI_7 PCU3 LA
BVCI_0 NS-VCI_6 Channel_6 NS-VCI_6 BVCI_0 RA
BSS2 BTS_22
BVCI_22 NS-VCI_9 NS-VCI_9 BVCI_22
BSSGP Data
NS Signal
FR Data & Signal