Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-10-28
2002-08-13
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S349000, C370S392000, C370S401000, C370S466000, C370S469000, C370S474000
Reexamination Certificate
active
06434133
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mobile radio subnetwork dependent convergence protocols. The invention is applicable in particular, though not necessarily, to the Subnetwork Dependent Convergence Protocol (SNDCP) to be specified for the General Packet Radio Service (GPRS).
BACKGROUND OF THE INVENTION
Current digital cellular telephone systems such as GSM (Global System for Mobile communications) were designed with an emphasis on voice communications. Data is normally transmitted between a mobile station (MS) and a base station subsystem (BSS) over the air interface using the so called ‘circuit switched’ transmission mode where a physical channel, i.e. a series of regularly spaced time slots on one or more frequencies, is reserved for the duration of the call. For voice communications, where the stream of information to be transmitted is relatively continuous, the circuit switched transmission mode is reasonably efficient. However, during data calls, e.g, internet access, the data stream is ‘bursty’ and the long term reservation of a physical channel in the circuit switched mode represents an uneconomic use of the air interface.
Given that the demand for data services with digital cellular telephone systems is increasing rapidly, a new GSM based service known as the General Packet Radio Service (GPRS) is currently being standardised by the European Telecommunications Standards Institute (ETSI) and is defined in overall terms in recommendation GSM 03.60. GPRS provides for the dynamic allocation of physical channels for data transmission. That is to say that a physical channel is allocated to a particular MS to BSS link only when there is data to be transmitted. The unnecessary reservation of physical channels when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS will therefore use the basic channel structure defined for GSM. In GSM, a given frequency band is divided in the time domain into a succession of frames, known as TDMA (Time Division Multiple Access) frames. The length of a TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into eight consecutive slots of equal duration. In the conventional circuit switched transmission mode, when a call is initiated, a physical channel is defined for that call by reserving a given time slot (1 to 8) in each of a succession of TDMA frames. Physical channels are similarly defined for conveying signalling information.
With the introduction of GPRS, a “traffic channel” for transmitting data is created by dynamically assigning physical channels for either switched circuit transmission mode or for packet switched transmission mode. When the network requirement for switched circuit transmission mode is high, a large number of physical channels may be reserved for that mode. On the other hand, when demand for GPRS transmission is high, a large number of physical channels may be reserved for that mode. In addition, a high speed packet switched transmission channel may be provided by assigning two or more slots in each of a succession of TDMA frames to a single MS.
The GPRS radio interface for GSM Phase 2+ (GSM 03.64) can be modelled as a hierarchy of logical layers with specific functions as shown in
FIG. 1
, where the mobile station (MS) and the network have identical layers which communicate via the MS
etwork interface Um. It will be understood that the model of
FIG. 1
does not necessarily represent the hardware contained in the MS and the network, but rather illustrates the flow and processing of data through the system. Each layer formats data received from the neighbouring layer, with received data passing from the bottom to the top layer and data for transmission passing from the top to the bottom layer.
At the top layer in the MS are a number of packet data protocol (PDP) entities. Certain of these PDP entities use point-to-point protocols (PTPs) adapted for sending packet data from one MS to another MS, or from one MS to a fixed terminal. Examples of PTP protocols are IP (Internet Protocol) and X.25 which are capable of interfacing with user applications (not shown in FIG.
1
). It is noted that two or more of the PDP entities may use the same PDP. Also on the top layer are other GPRS end point protocols entities such as SMS and signalling (L
3
M). A similar arrangement exists within the network and in particular at the Serving GPRS Support Node (SGSN).
Certain of the top layer entities use a common Subnetwork Dependent Convergence Protocol (SNDCP)—GSM 04.65—which, as its name suggests, translates (or ‘converges’) the different SNDCP user data into a common form (SNDCP protocol data units) suitable for further processing in a transparent way. SNDCP units are up to 1600 octets and comprise an address field which contains a network service access point identifier (NSAPI) which identifies the endpoint connection, i.e. the SNDCP user. Each MS may be assigned a set of NSAPIs independently of the other MSs. This architecture means that new PDPs and relays may be developed in the future which can be readily incorporated into the existing GPRS architecture.
Each SNDCP (or other GPRS end point protocol) unit is carried by one logical link control (LLC) frame over the radio interface. The LLC frames are formulated in the LLC layer (GSM 04.64) and include a header frame with numbering and temporary addressing fields, a variable length information field, and a frame check sequence. More particularly, the addressing fields include a service access point identifier (SAPI) which is used to identify a specific connection endpoint (and its relative priority and Quality of Service (QoS)) on the network side and the user side of the LLC interface. One connection endpoint is the SNDCP. Other endpoints include the short message service (SMS) and management layer (L
3
M). The LLC layer formats data received from these different endpoint protocols. SAPIs are allocated permanently and are common to all MSs.
The Radio Link Control (RLC) layer defines amongst other things the procedures for segmenting and re-assembling Logical Link Control layer PDUs (LLC-PDU) into RLC Data Blocks, and for retransmission of unsuccessfully delivered RLC blocks. The Medium Access Control (MAC) layer operates above the Phys. Link layer (see below) and defines the procedures that enable multiple MSs to share a common transmission medium. The MAC function arbitrates between multiple MSs attempting to transmit simultaneously and provides collision avoidance, detection and recovery procedures.
The physical link layer (Phys. Link) provides a physical channel between the MS and the network). The physical RF layer (Phys. RF) specifies amongst other things the carrier frequencies and GSM radio channel structures, modulation of the GSM channels, and transmitter/receiver characteristics.
When a MS becomes active in a network, it is necessary to define exactly how data is to be processed at each of the layers described above. This process also involves conducting preliminary negotiations between the MS and the network. In particular, control parameters known as SNDCP exchange identity (XID) parameters are exchanged between the two peer SNDCP layers, via the respective LLC layers, in an XID parameter negotiation stage. Initialisation of XID negotiation may occur at either the MS or the network. Upon receipt of an XID parameter, the peer entity either configures itself according to that parameter or carries out a further negotiation with the user entity. The field format for the SNDCP XID parameters is as follows:
bit
8
7
6
5
4
3
2
1
octet 1
Parameter Type
octet 2
Length = n − 1
octet 3
High-order octet
....
....
octet n
Low-order octet
FIG. 2
considers in more detail the SNDCP layer and its interfaces to the SNDCP users and the LLC layer, and is applicable to both the MS and the SGSN architectures. In particular,
FIG. 2
illustrates the compression of protocol and/or
Hsu Alpus H.
Nokia Mobile Phones Limited
Perman & Green LLP
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