Chapter 8

The Gb interfaceContents:8.1 Structure of the Gb

1. Gb interface structure2. Evolved service model 3. Physical implementation of Gb 4. Layer 2: Network Service

8.2 Protocol structures 1. Frame Relay- Frame structure2. The address field of Frame Relay3. The Frame Relay network 4. Frame Relay procedures

8.3 NS Frame formats1. 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

Chapter 8

The Gb interface8.4 BSSGB procedures and messages

1. Paging2. Signalling Procedures between NM SAPs 3. Flow control messages 4. BSSGP UL unitdata

8.5 Gb appendix1. Estimation of Gb overhead

Chapter 8

The Gb interface8.1 Structure of the Gb

1. Gb interface structure2. Evolved service model 3. Physical implementation of Gb 4. Layer 2: Network Service

Gb interface structure

NS

BSSGP

Physical

NS

LLC GMM NM

BSSGP

Relay GMM NM

Physical

Gb

Physical Layer (08.14)NS: Network Service (08.16)NM: Network ManagementBSSGP: BSS GPRS Protocol (08.18)

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.

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

NS

NS Control part

NS Sub-network part

Creates NS virtual circuits together with identifieres and defines procedures to manage them

Defines the layer two protocol. In Rel 99 this is Frame Relay, starting with Rel 4 optional an IP network

PCU SGSN

Evolved service model up to Rel 6

BSSGP

LLC

BSSGP

Service model in an SGSN

BSSGP

RELAY

3GPP TS 44.064

GMM

LCS

LCS NM

Network ServiceNetwork Service

3GPP TS 48.016

Service model in an BSS

RL

RLC/MAC

GMM NM

PFM

PFM

GMM

RIM

LCS NM

GMM NM

PFM

PFMRIM

RIM

LCS

RIM

- "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 Flow

Contexts (PFCs);- "LCS" (location services) for functions associated with location services (LCS) procedures;- "RIM" (RAN Information Management) for functions associated with generic procedures to communicate

between two BSSs or with UTRAN via the core network. - “MBMS” (Multimedia Broadcast Multicast Service) for functions associated with Multimedia

Broadcast Multicast Service (MBMS) procedures.

Rel 5

Rel 5

Rel 99

3GPP TS 48.016

MBMS

MBMS

MBMS

MBMS

Rel 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.

SGSNGb

L1

BSSGP

L1

BSSGP

LLC

PCU

NS NSIn one PCU there is one NSE (Network Service Entity)In a SGSN many NSE are defined (one per PCU)identified by one NSEI (NSE Identifier)

Layer 2: Network Service

PCU 0

PCU 1

NSEI0

NSEI1

NSVCI 3

NSVCI 4

NSVCI 1

NSVCI 2

SGSN

link 3

link 4

link 1

link 2

NSVCI 3

NSVCI 4

NSVCI 1

NSVCI 2

NSEI0

NSEI1

1PCM

NS- Sub-networkservice

One Frame Relay linkor one IP endpoint

Chapter 8

The Gb interface8.2 Protocol structures

1. Frame Relay- Frame structure2. The address field of Frame Relay3. The Frame Relay network 4. Frame Relay procedures

Physical implementation of Gb

SGSNGb

NS

BSSGP

NS

BSSGP

LLC

PCU

PCU

BSC Frame RelaySub-network

SGSN

MSC

E1/T1-Line

E1/T1-Line

1

2

3

4

TRAU

(multiple) nailed-up connections, each 64 kbit/s

dedicated line

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- NUC through MSC and then via Frame Relay sub-network to the SGSN.

- As a dedicated line configuration (possibility 3 and 4) :- a direct line and then via Frame Relay sub-network to the SGSN- a direct connection to the SGSN.

For the last two configurations PCMs just carrying GPRStraffic have to be configured in the BSC.

A mixed configuration of links via A interface connection and dedicated line connection is possible.Please note that an IP Subnetwork may be based on other physical media!!!!!

12

3

4

L1 L1

A

Frame Relay- Frame structure

FLAG0 1 1 1 1 1 1 0

Address field (byte 1)

Address field (byte 2)

Frame Check Sequence byte 1

Frame Check Sequence byte 2FLAG

0 1 1 1 1 1 1 0

User data with variable length

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.

Opening flags, unique sequence of bits defining the start of a frame.

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 used.

The data part length can be from 1 to 4096 bytes. For GPRS the 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 Check Sequence (or an unkknown DLCI) the frame will be immediately discarded. No retransmission mechanism is defined.

Clossing flag, the same sequence of bits as for the opening flag will close the frame.

Frame

Relay

frame

The address field of Frame Relay

Bit8 7 6 5 4 3 2 1

C/REA0

DLCI (MSB)

FECNBECN DEEA1

DLCI (LSB)

DLCI (LSB)

EA0

FECNBECN DE

EA1

Extension Address bit indicates whether another octett header follows or not (in GPRS only 2 bytes of header are used).

meaning

signaling

reserved

addresses of virtual connections

management function of layer 2

reserved

DLCI (10 bits)

0

1 - 15

16 - 991

992 – 1007

1008 – 1022

1023 reserved for layer 2 information

C/R Command/Response bit, not used in GPRS

The Data Link Connection Identifier (DLCI) identifies all FR packets that

belong to the same receiver. The DLCI is the Frame Relay address. There are DLCI with 10, 16, 17 and 23 bits possible. In GPRS networks mostly the 10 bit DLCI format is used.

The DLCI value is divided into two parts with a various number of bits: the More and Less Significant Bits (M/L-SB).

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.

One frame on its way through the network. The indicated DLCI values are examples

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.

UNI

USER A

USER B

USER C

FRAD Frame Relay Access Device

UNI User to Network Interface

FRAD X

X

X

XX

FRAD

FRAD

A Permanent Virtual Connection between two users defines a

dedicated path through the network

DLCI: 27

DLCI: 23

DLCI: 91DLCI: 56

DLCI: 23

Frame Relay switch

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 EnquiryThis 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.StatusThis 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.

Full Status enquiry

Full Status

Status enquiry

Status

DTE DCE

Full Status enquiry

Full Status

T391

.

.

.

N391 * T391

T391 defines the period between 2 Status messages

N391 defines the how often the Full status is requested

N392 is the error recovery counter which defines the number of unsuccessful polling cycles in a certain time frame before the FRL is put to the Disabled state. It is closely related to the value

N393 *which is a counter. The system will try N393 times before the links are put in Disabled state.

DTE Data Terminal Equipment, e.g. BSC or SGSNDCE 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!

Chapter 8

The Gb interface8.3 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

NS addressing and load sharing

PCU 0

PCU 1

NSEI0

NSEI1

NSVCI 3

NSVCI 4

NSVCI 1

NSVCI 2

SGSN

link 3

link 4

link 1

link 2

NSVCI 3

NSVCI 4

NSVCI 1

NSVCI 2

NSEI0

NSEI1

1PCM

NS- Sub-networkService (DLCI)

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 (applies only to NS SDUs)

Cell 1

Cell 2

Cell 3

Cell 4

Cell 5

Cell 6

Cells served by PCU 1

Traffic coming from cell 1 is sent through any NSVCI

NS-UDT messages that carry all payload and signaling also contain the BSSGP Virtual Connection Identifier (BVCI). The BVCI (BSSGP Virtual Connection Identifier) represents a single cell, BSSGP Signaling entity or Point-to-Multipoint (PTM) entity inside the BSS.

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.

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.

 

NSVC 2

NSVC 1

BVCI=4

SGSN 2

NSEI=2NSVC 4

NSVC 3

BVCI=5

BVCI=3

NSEI=2

SGSN 1

NSEI=1

BSS 1

BVCI=3NSEI=1

Rel 5

Radio Cell 1

Radio Cell 1

Traffic coming from cell 1 may be handled by different CN nodes!

General structure of a PDU

 

octet 1

octets 2, 3, ...n

8 7 6 5 4 3 2 1

PDU type

other information elements

The first octett defines the type of PDU

SNS-ACKSNS-ADDSNS-CHANGEWEIGHTSNS-CONFIGSNS-CONFIG-ACKSNS-DELETESNS-SIZESNS-SIZE-ACK

NS-UNITDATANS-RESETNS-RESET-ACKNS-BLOCKNS-BLOCK-ACKNS-UNBLOCKNS-UNBLOCK-ACKNS-STATUSNS-ALIVENS-ALIVE-ACK

Information element

Presence Format Length

Only for IP sub network

For IP/FR sub networkPDU types defined:

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.

A set of messages which are only used in an IP subnetwork, the so called Sub-Network Service Control PDUs (SNS PDUs) are defined (starting with Rel 4).

The Network Service Protocol Data Units (FR)

Remarks

This PDU is used to test a NS-VC

This PDU acknowledges a received NS-ALIVE PDU and is sent on the NS-VC where the NS-ALIVE PDU was received

This PDU indicates that a NS-VC shall be blocked at the recipient entity

This PDU acknowledges that a NS-VC has been blocked for use

This PDU indicates that the NS peer entity is trying to reset one NS-VCs

This PDU acknowledges the reset of the indicated NS-VCs

This PDU is used to report error conditionsThis PDU indicates that a NS-VC shall be unblocked at the recipient entity

This PDU acknowledges that a NS-VC has been unblocked

NS PDU Type

NS-ALIVE

NS-ALIVE-ACK

NS-BLOCK

NS-BLOCK-ACK

NS-RESET

NS-RESET-ACK

NS-STATUS

NS-UNBLOCK

NS-UNBLOCK-ACK

NS-UNITDATA This PDU transfers one NS SDU between the BSS and SGSN

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.

Messages that handle procedures for establishing a new Network Service Virtual Connection (NS-VC) or close a connection.

For ‘data, traffic’

New NS-VC Start Up Procedure

RST (DLCI=103, NS-VCI=12, NSEI=520, cause)

RSTA (DLCI=103, NS-VCI=12, NSEI=520)

UBLO (DLCI=103)

PCU SGSN

Tns-test (e.g. 10s)

UBLA (DLCI=103)ALV (DLCI=103)

ALVA (DLCI=103)

If a new Network Service Virtual Connection (NS-VC) is taken into service the following startup procedure can be monitored (indicated values are examples)

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.

ALV (DLCI=103)

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 element

PDU type

NS SDU Control Bits

BVCI

NS SDU

Presence Format Length

M V 1

M V 1

M V 2

M V 1-?

Contains the BVCI as mandatory IE!

Length has to be derived by lower layers!

Allows to request or confirm a change flow

Chapter 8

The Gb interface8.3 The BSSGB protocol

1. The BSSGP protocol2. The BSSGP PDU types3. DL user data on Gb 3GPP 48.018 4. UL user data on Gb 3GPP 48.018

The BSSGP protocol

SGSNGb

L1 L1

LLC

PCU

NS NS

BSSGP BSSGP

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.

SGSN

Signalling entityPTM entity

Cell 1

Cell 2

BVCI = 0

BVCI = 1

BVCI = 2

BVCI = 3

BVCI = ? Cell ?

PTP functional entities

The BSSGP PDU types

 

octet 1

octets 2, 3, ...n

8 7 6 5 4 3 2 1

PDU type

other information elements

The first octet defines the type of PDU

PDUs between RL and BSSGP SAPsDL-UNITDATAUL-UNITDATARA-CAPABILITYPTM-UNITDATA

PDUs between GMM SAPsPAGING PSPAGING CSRA-CAPABILITY-UPDATERA-CAPABILITY-UPDATE-ACKRADIO-STATUSSUSPENDSUSPEND-ACKSUSPEND-NACKRESUMERESUME-ACKRESUME-NACK

PDU types defined (Rel 6)

PDUs between NM SAPsBVC-BLOCKBVC-BLOCK-ACKBVC-RESETBVC-RESET-ACKBVC-UNBLOCKBVC-UNBLOCK-ACKFLOW-CONTROL-BVCFLOW-CONTROL-BVC-ACK FLOW-CONTROL-MSFLOW-CONTROL-MS-ACKFLUSH-LLFLUSH-LL-ACKLLC-DISCARDEDSGSN-INVOKE-TRACESTATUSDOWNLOAD-BSS-PFCCREATE-BSS-PFCCREATE-BSS-PFC-ACKCREATE-BSS-PFC-NACKMODIFY-BSS-PFCMODIFY-BSS-PFC-ACKDELETE-BSS-PFCDELETE-BSS-PFC-ACK

BVCI = 1

BVCI = 0 or PTP

BVCI = 0

PTP

PTP

BVCI = 0 or 1 or PTP

PTP

BVCI = 0

BVCI = 0

PTP

PTP Mapping of theBSSGP PDU to A functional entity

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 Capability a)

MS Radio Access Capability/11.3.22

O TLV 7-?

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 CCOService UTRAN CCO/11.3.47.

O TLV 3

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.

DL user data on Gb 3GPP 48.018

SGSNGb

L1 L1

LLC

PCU

NS NS

BSSGP BSSGP

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.

UL user data on Gb 3GPP 48.018

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.

UL-UNITDATA

 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.

Chapter 8

The Gb interface8.5 BSSGB procedures and messages

1. Paging2. Signalling Procedures between NM SAPs 3. Flow control messages 4. BSSGP UL unitdata

Paging

·                   - 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 cellsTo 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

BSS SGSN

NS UDT (PAGING PS)

NS UDT (PAGING CS)

(PDU type, IMSI or P-TMSI, QoS Profile, Location Area or Routeing Area)

(PDU type, IMSI, DRX Parameters, Location Area or Routeing Area )

Signalling Procedures between NM SAPs

NS UDT (BSSGP-PDU: BVC-Reset)

NS UDT (BSSGP-PDU: BVC-RESET-ACK)

(PDU type, BVCI, Cause, Cell Id.)

(PDU type, BVCI, Cell Id.)

BVC Reset ProcedureThe 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 BSSan 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.  

Signaling Procedures between NM SAPsSGSN

NS UDT (BSSGP-PDU: BVC-BLOCK)

NS UDT (BSSGP-PDU: BVC-BLOCK-ACK)

NS UDT (BSSGP-PDU: BVC-UNBLOCK)

NS UDT (BSSGP-PDU: BVC-UNBLOCK-ACK)

(PDU type, BVCI, Cause)

(PDU type, BVCI)

(PDU type, BVCI)

(PDU type, BVCI)

BVC Blocking and Unblocking ProcedureThe 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.

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.

Flow control

PCU

MS flow control MS flow control MS flow control

BVC flow control

SGSNFlow control commands

Calculation of leak rate R and buffer size Bmax per MS and BVC

First level

Second level

Fig. 1 BSS Flow control: Cascaded Flow Control (MN1889EU10MN_0001 Point-to-point packet flow, 13)

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.

Flow control messages

FLOW-CONTROL PDU

(Tag, Bucket Size, Leak Rate)BSS SGSN

C defines the periodicity of the message

FLOW-CONTROL-ACK PDU

(Tag)

FLOW-CONTROL PDU

(Tag, Bucket Size, Leak Rate)

The Packet Flow

PFC Flow Control(optional)Requires support of MS(Rel 99)and network (Rel 5)

PDPcontext 1

LLC PDUs

PDPcontext 2

LLC PDUs

PDPcontext 3

LLC PDUs

PDPcontext n

LLC PDUs SGSN

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.

IMSI

TLLI

Trace ref., type, id

OMC id

BSS PacketFlow ContextPFIAggregate BSSQoS Profile NegotiatedBSS PacketFlow Timer

BSS PacketFlow ContextPFIAggregate BSSQoS Profile NegotiatedBSS PacketFlow Timer

BSS PacketFlow ContextPFIAggregate BSSQoS Profile NegotiatedBSS PacketFlow Timer

BSS PacketFlow ContextPFIAggregate BSSQoS Profile NegotiatedBSS PacketFlow Timer

TLLI 1 TLLI 2 TLLI 3 MS Flow control

BVCI 1 BVC Flow control

BSS

PFI 1 PFI 2 PFI X PFC Flow control

The Packet Flow

BSSContext

PFC1

PFC2

TBF

Buffer 1

Buffer 2

BSS

Um

Gb

SGSN

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.

The Packet Flow

3.DL_UNITDATA(PFI1)

4.PFC FC(PFI1)

5.DL_UNITDATA(PFI2)

6.PFC FC(PF1, PFI2)

MS SGSNBSC

2.MS FC

C timer

1. TBF establishment

3.TBF reconfiguration

5. TBF reconfiguration

3.a PFC Download Procedure

1.DL_UNITDATA(PFI predefined)1) TBF is opened due to a DL UNITDATA having PFI signalling coming.

2) MS FLOW CONTROL is sent when the first C timer expiration occurs. 

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. 

3a) If BSC does not have valid PFC parameters, PFC Download procedure starts.  

4) At next C timer expiration a PFC Flow Control message including parameter for PFI1 is sent. 

5) Then during packet transfer mode, a DL UNITDATA having PFI2 not pre-defined could cause a reconfiguration of TBF in order to manage new services or in any case an internal scheduler reconfiguration can occur. 

6) At next C timer expiration a PFC-FC is sent including PF1 and PF2 parameters.

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

|1111---- |Filler 15

|----0000 |MNC digit 1 |X

|0010---- |MNC digit 2 |X

|***B2*** |LAC |10

|00001100 |RAC |12

|***B2*** |CI |21

|Alignment Octets

|00000000 |IE Name |Alignment Octets

|10000000 |IE Length |0

|LLC PDU

|00001110 |IE Name |LLC-PDU

Chapter 8

The Gb interface8.5 Gb appendix

1. Estimation of Gb overhead2. Configuration Example

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

(compressed) IPLLCNS BSSGBFRorIP

SNDCP

Example of Configuration

PCU2

PCU1

PCU3

BTS_3

BTS_6

RA 1

BTS_8

BTS_22

RA 2

LA

PCU3

BTS_22RALA

Bearer Channel_5

Bearer Channel_6

Bearer Channel_2

Bearer Channel_1

Bearer Channel_3

Bearer Channel_4

DLCI_16DLCI 17

DLCI_16DLCI 17

DLCI 16

DLCI 16DLCI_17

DLCI_17DLCI_18

DLCI 16

BVCI_3

BVCI_0NS-VCI_7

NS-VCI_2

NSEI_1

NS-VCI_5

NS-VCI_8

NS-VCI_3

BVCI_0

BVCI_6

NSEI_2

NS-VCI_4

NS-VCI_1

NS-VCI_11

BVCI_8

BVCI_0

NSEI_3

BVCI_22

BVCI_22

BVCI_0NS-VCI_6

NS-VCI_9

NSEI_7

BVCI_3

BVCI_0 NS-VCI_7

NS-VCI_2

NS-VCI_5

NS-VCI_8

NS-VCI_3

BVCI_0

BVCI_6

NSEI_1

NSEI_2

NS-VCI_4

NS-VCI_1

NS-VCI_11

BVCI_8

BVCI_0

NSEI_3

BVCI_22

BVCI_22

BVCI_0 NS-VCI_6

NS-VCI_9

NSEI_7

PAPU1

PAPU2

PAPU3

SGSN

BSSGPNSFR

DataSignal

Data & Signal

BSS2

BSS1