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3.1     BSS Architecture for GPRS
        GPRS and circuit switched GSM will co-exist within the existing GSM infrastructure,
        which will lead to rapid implementation and wide coverage of GPRS. The new GPRS
        functions in the BSC are:
            •    Support of GPRS logical channels
            •    Frame relay and BSSGP to SGSN
            •    Allocation of PDTCH in cells
            •    Paging distribution
            •    Broadcast of GPRS information
            •    Support of MAC and RLC And in the BTS:
            •    Support of new channel coding schemes.
        GPRS requires new software in the BSS and new hardware to implement the PCU. The
        PCU is co-located with the BSC node. The BSC may be a combined BSC/TRC or a
        stand-alone BSC. The PCU can only serve one BSC and there will only be one PCU per
        BSC. The new PCU HW is available for both BYB 501 and BYB 202 (Ericsson specific
        Building Practice). A new open interface, the Gb interface, is introduced between the
        PCU (BSC) and SGSN. The existing Abis interface is reused for GPRS and will thus
        carry both circuit switched and GPRS traffic. The BSS structure is shown in Figure 5-1.



                                       Abis                     BSC      Gb   SGSN
                CCU
                                                               PCU
                CCU



          PCU         Packet Control Unit                     (Hardware and Software)
          CCU         Channel Control Unit                    (Software)

                                                    Fig. 3.1
                                       Structure of BSS and interfaces

3.2     Base Station Controller (BSC) and Packet Control Unit
        (PCU)
        The PCU is responsible for the GPRS packet data radio resource management in BSS.
        In particular the PCU is responsible for handling the MAC and RLC layers of the radio
        interface and the BSSGP and Network Service layers of the Gb interface. The Gb
        interface is terminated in the PCU. The PCU consists of both central software and
        hardware devices with regional software. It will have one or more Regional Processors
        (RP). An RPP (type of RP that the PCU is built on) can work with both the Gb and the
        Abis interfaces or with Abis alone. The function of the RPP is to distribute PCU frames
        between Gb and Abis. Where there is just one RPP in the PCU it will work with both the



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        Gb and the Abis interfaces. Where there is more than one RPP, each RPP may work with
        either Abis or with both Gb and Abis.
        Where more than one RPP is used (except for the two RPPs in an active/standby
        configuration) they will communicate with each other using Ethernet. A cell cannot be
        split between two RPPs. If an RPP does not handle the cell to which the message is
        destined, the message is forwarded via the Ethernet to the right RPP.
        A duplicated Ethernet connection is provided in the backplane of the PCU magazine. In
        addition some HUB boards are needed to connect the RPPs via the Ethernet. The HUB
        boards are doubled for redundancy reasons. The PCU connects to the Gb devices via
        the Group Switch (GS), and to the Abis devices via the GS and the SubRate Switch
        (SRS). The RPPs are connected to the group switch via DL2s and to the central
        processor CP via the RP bus. The GPRS traffic is multiplexed with the circuit switched
        traffic in the SRS. The PCU architecture is scalable to achieve cost effective solutions for
        both small and large PCUs. In order to enable capacity expansions several magazines
        containing RPPs and HUB boards may be connected.


3.3     A Packet Transfer from SGSN and a Telephone Call to Two
        Separate MSs
        Figure 5-2 illustrates when a data packet toward MS2 is using the same hardware (Gs,
        Abis, group switch, subrate switch and RBS) as the telephone call to MS1. The call to
        MS1 is transferred from MSC/VLR to the speech coder (TRAU) in TRC or BSC/TRC. In
        the A interface the speech rate is 64kbit/s. The speech coder transforms the speech rate
        into 16kbit/s. The packet that is to be sent from SGSN to MS2 is forwarded from RPP1 to
        RPP2 over the backplane using ethernet. In this example RPP2 handles cell 11, which is
        the cell that handles MS2. The speech frames from the CS call and the radio blocks for
        GPRS are multiplexed together in the subrate switch and transferred to the RBS for cell
        11.

                                                 GS
                        Gb                                            Gb Dev
                               ETC
                             (RTG-0)
                                                                     GSL Dev



                                                                     GSL Dev
                        A
                               ETC
                             (RALT-0)                                GSL Dev




                              TRAU

                A-bis                                                          A-bis
                    ETC                                                  ETC
                   RBLT-0                                               RBLT-0
         BTS                                                                           BTS



                                                SRS
                                                                               GPRS Call
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                                             Fig. 3.2
                                  Structure of BSS and interfaces

3.4     Capacity of BSC
        One BSC can handle one PCU. One PCU can handle
           • 16 RPPs (in reality, a BYB 501 handles 14 and a BYB 202 15 RPPs)
           • 4096 (theoretically) PDCHs (1750, in reality)
           • 512 cells.
        One RPP can handle
           • 512 cells
           • 256 PDCHs (180, in reality)
           • One Gb interface
           • Two DL2 connections of 2Mbit each.


3.5     Base Transceiver Station (BTS)
        GPRS will be implemented in the BTS software and no new BTS hardware is required.
        The fact that a software-only upgrade is needed allows for rapid introduction with full
        coverage. Existing sites can be reused, since GPRS is supported both on RBS (Radio
        Base System) 2000 and the RBS 200 platforms SPU++ (Signal Processing Extension)
        and ‘SPU+(Signal Processing Unit) with SPE’. The RBS 200 platform SPP does not
        support GPRS. Channel Coding Schemes CS-1 and CS-2 are both supported. The only
        exception is RBS 2301 without a DSP cluster, which only supports CS-1. Note that if the
        operator has set the preferred channel-coding scheme to CS-2, the BSC will switch to
        CS-1 in that cell when the BTS is not capable of CS-2.


3.6     Operation Support System (OSS)
        OSS provides support for GPRS related parameter setting. It also provides alarm
        surveillance of the new GSN nodes. Configuration management of the new GSN nodes is
        supported by means of a web interface (Netscape) in the OSS.


3.7     Transmission


3.7.1 Abis Interface
        The existing transmission and signaling links over the Abis interface are reused for
        GPRS, thus providing an efficient and cost effective introduction. Modified TRAU
        (Transcoder Radio Adapter Unit) frames are used for the support of GPRS Coding
        Schemes 1 and 2. No additional transmission links are needed (unless of course the
        number of TRXs (Transceivers) per site is increased).

3.7.2 Gb Interface
        General



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        The Gb interface is a new open interface between the PCU and SGSN. The PCU can be
        connected to an SGSN over the Gb interface either:
            1. Directly from a standalone BSC or a combined BSC/TRC.
            2. Via a TRC from a standalone BSC.
            3. Via an MSC from a standalone BSC or combined BSC/TRC.
        A BSC can use one or more physical links to connect to a SGSN. When using an E1
        interface the size of the physical links is between 1 and 31 64 kbits/s time slots, i.e.
        between 64 kbits/s and 1984 kbits/s. When using a T1 interface the size of these physical
        links is between 1 and 24 64 kbits/s time slots, i.e. between 64 kbits/s and 1536 kbits/s.
        Note: If more than one 64kbits/s circuit are used on the same physical link the time slots
        must be contiguous to each other.
        Gb protocols
        The protocol used to provide layer 3 is BSSGP. BSSGP is a GPRS specific protocol. It
        conveys the necessary routing information to be able to transfer an LLC PDU
        transparently across the radio network to the MS. Layer 2 is called the NS layer. This
        layer is further divided into two separate layers. The upper layer is called the network
        service control. The lower layer is called the sub-network service. The protocol used to
        provide the network service control layer is the network service control protocol. The
        network service control protocol provides a generic way of encapsulating BSSGP PDU
        and transferring them via the sub-network service. The protocol used to provide the sub-
        network service layer is frame relay. Frame relay is a frame mode interface specification
        providing a signaling and data transfer mechanism between end-points and the network.
        The end-points of the Gb interface are the BSC and the SGSN. Frame relay transparently
        transfers NS PDUs between an SGSN and a BSC.


3.8     Frame Relay
3.8.1 General description of the Frame Relay protocol
        Frame Relay is a high-performance WAN protocol that operates at the physical and data
        link layers of the OSI reference model. Frame Relay originally was designed for use
        across Integrated Services Digital Network (ISDN) interfaces. It is used over a variety of
        other network interfaces as well.
        Frame Relay is an example of a packet-switched technology, which enables end stations
        to dynamically share the network medium and the available bandwidth.
        Frame Relay typically operates over WAN facilities that offer more reliable connection
        services and a higher degree of reliability than the facilities available.
        Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer
        3 (the network layer) as well. This enables Frame Relay to offer higher performance and
        greater transmission efficiency than X.25, and makes Frame Relay suitable for current
        WAN applications, such as LAN interconnection. Devices attached to a Frame Relay
        WAN fall into the following two general categories:
        A-Data terminal equipment (DTE): which generally are considered to be terminating
        equipment for a specific network and typically are located on the premises of a customer.
        Examples of DTE devices are terminals, personal computers, routers, and bridges.
        B-Data circuit-terminating equipment (DCE): which are carrier-owned internetworking
        devices. The purpose of DCE equipment is to provide clocking and switching services in


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        a network, which are the devices that actually transmit data through the WAN. In most
        cases, these are packet switches.
        The connection between a DTE device and a DCE device consists of both a physical
        layer component and a link layer component, in which physical component defines the
        mechanical, electrical, functional, for the connection between the devices. One of the
        most commonly used physical layer interface specifications is the recommended
        standard (RS)-232 specification. The link layer component defines the protocol that
        establishes the connection between the DTE device, such as a router, and the DCE
        device, such as a switch.

3.8.2 How Frame Relay works
            •   Check the integrity of the frame using the Frame Checksum (FCS). If it indicates
                an error, discard the frame
            •   Look up the DLCI in the distribution table of the node. If the DLCI is not defined
                for this link, discard the frame.

            •   Relay the frame towards its destination by sending it on the outgoing port or trunk
                specified in the distribution table.




                                              Fig 3-3
                                FR Frame Structure & Header Format




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3.8.3 Data-Link Connection Identifier
        Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs).
        DLCI values typically were assigned by the Frame Relay service provider (for example,
        the telephone company). Frame Relay DLCIs have local significance, which means that
        their values are unique in the LAN, but not necessarily in the Frame Relay WAN.

3.8.3 Frame Relay Virtual Circuits
        Frame Relay provides connection-oriented data link layer communication, This service is
        implemented by using a Frame Relay virtual circuit, which is a logical connection created
        between two data terminal equipment (DTE) devices across a Frame Relay packet-
        switched network (PSN). Virtual circuits provide a bi-directional communication path from
        one DTE device to another and are uniquely identified by a data-link connection identifier
        (DLCI).
        A number of virtual circuits can be multiplexed into a single physical circuit for
        transmission across the network. This capability often can reduce the equipment and
        network complexity required to connect multiple DTE devices. A virtual circuit can pass
        through any number of intermediate DCE devices (switches) located within the Frame
        Relay PSN.
        Frame Relay virtual circuits fall into two categories:
        Switched Virtual Circuits: Switched virtual circuits (SVCs) are temporary connections
        used in situations requiring only data transfer between DTE devices across the Frame
        Relay network. A communication session across an SVC consists of the following four
        operational states:
        Call setup: The virtual circuit between two Frame Relay DTE devices is established.
        Data transfer: Data is transmitted between the DTE devices over the virtual circuit.
        Idle: The connection between DTE devices is still active, but no data is transferred. If an
        SVC remains in an idle state for a defined period of time, the call can be terminated.
        Call termination: The virtual circuit between DTE devices is terminated.
        After the virtual circuit is terminated, the DTE devices must establish a new SVC if there
        is additional data to be exchanged. It is expected that SVCs will be established,
        maintained, and terminated using the same signaling protocols used in ISDN.
        Permanent Virtual Circuits: Permanent virtual circuits (PVCs) are permanently
        established connections that are used for temporary and permanent data transfers
        between DTE devices across the Frame Relay network. PVC does not require the call
        setup and termination states that are used with SVCs. PVCs always operate in one of the
        following two operational states:
        Data transfer: Data is transmitted between the DTE devices over the virtual circuit.
        Idle: The connection between DTE devices is active, but no data is transferred.
        Unlike SVCs, PVCs will not be terminated under any circumstances when in an idle state.
        DTE devices can begin transferring data whenever they are ready because the circuit is
        permanently established.



3.8.4 Frame Relay Network for GPRS Gb Interface.
        The protocol stack for the protocols related to Gb interface in charge of the connectivity
        between BSS and SGSN is depicted in the following picture:

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                                             BSSGP

                             Network Service Control (NS Control)
                                                                             Network Service
                                          Frame Relay
                                                                             L1 bis
                                    E1                        T1


                                             Fig. 3.3
                                     BSS and SGSN Protocols



        Main focus of this document will be on the Network Service Layer, as this layer is defined
        to make use of Frame Relay as link layer. Therefore, the different Gb design alternatives
        are based on how this layer is implemented.
        Network Service Layer: On the BSS side, the Gb interface is terminated in the Packet
        Control Unit (PCU). Every PCU will be configured with one NSE (Network Service Entity),
        representing the NS protocol instance in the BSS side, while the SGSN will be configured
        with as many NSEs as BSCs/PCUs are connected to it. The communicating Network
        Service Entities on both sides of a Gb interface are associated with each other by means
        of the same Network Service Entity Identifier (NSEI). A NSE communicates with only one
        peer NSE. The NSE is composed of an entity dependent on the intermediate
        transmission network used on the Gb interface, the Sub-Network Service, and of a
        control entity independent from that network, the Network Service Control.
        Sub-Network Service – Frame Relay Service: The sub-network service is implemented
        in GPRS Phase 1 using Frame Relay protocol The Gb interface may consist of:
            •   Direct point-to-point connections between the BSS and the SGSN; the BSS shall
                be considered as the user side of the user-to-network interface (UNI) and the
                SGSN shall be considered as the network side.
            •   Intermediate Frame Relay network may be placed between both ends of the Gb
                interface; both BSS and SGSN shall be treated as the user side of the user-to-
                network interface (UNI).
        Network Service Control: The Network Service Control protocol instances communicate
        via Network Service Virtual Connections (NS-VCs), which are Frame Relay Permanent
        Virtual Circuits (PVCs), with NS specific additional functionality for management of NS-
        VCs. An NS-VC is identified end-to-end by means of the NS-VC Identifier (NS-VCI). The
        NS control layer defines one NS-VC for each Frame Relay PVC and one NSE for each
        group of NS-VCs leading to the same peer NS user. The Network Service Control is
        responsible for the following functions:
            •   The BSSGP PDU are transmitted on the NS-VCs
            •   Load sharing function that distributes the BSSGP traffic on the available and
                unblocked NS-VCs of a group. The load sharing function assures in-order
                delivery of all BSSGP PDUs of the same MS, by sending them via the same NS-
                VC.




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3.8.5 Relationship Between GPRS Gb Interface and Frame Relay
        In order to provide the reader with a better understanding of the Gb network the following
        points highlight the main concepts:
            •   The Gb interface link layer is based on Frame Relay. One or many Frame Relay
                permanent virtual connections (Frame Relay PVCs) are established between the
                SGSN and the BSS for the transfer of signaling and user data.
            •   Frame Relay is also denoted Sub Network Service (Sub NS). Sub NS and NS
                Control together make up the Network Service (NS) layer.
            •   The NS Control protocol adds GPRS specific node management functionality to
                Frame Relay. An NS-VC can be considered an enhanced Frame Relay PVC and
                is identified end-to-end by the NS-VCI.
            •   The Network Service Control (NS Control) protocol provides means to support
                end-to-end communication paths called BSSGP Virtual Connections (BVCs) to
                the NS user protocol (BSSGP). These BVCs are multiplexed on the available
                NS-VCs (Frame Relay PVCs).
            •   Each cell will get a Point-to-Point BSSGP Virtual Connection (PtP BVC) between
                the PCU and the SGSN.
            •   Each PCU will have a Signalling BVC towards the SGSN. The Signalling BVC
                will control the establishment of the PtP-BVCs.
            •   NS Load sharing function will distribute the traffic from the BVCs into the
                available NS-VCs. This means that there is no direct relationship between the
                number of cells and the number of PVCs and that we will dimension the FR
                PVCs taking into account the traffic coming from the whole BSC.
            •   Load distribution function behaves differently at the PCU and SGSN. At the PCU,
                each BVC will be associated to a specific NS-VC when the BVC is created. For
                the downlink, note that traffic for a BVC will come from different subscribers
                located in that cell. The load distribution at the SGSN will use the identity of the
                user to use a specific NS-VC. Therefore, traffic for the same BVC will use
                different NS-VCs depending on the mobile identity.
            •   If an NS-VC/PVC fails, the BSSGP traffic will be routed using a different NS-VC
                (if any is available), providing with a seamless service in case of failure.




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