General packet radio service

Details General packet radio service General packet radio service

1998-04-20

2003-03-11

Nguyen, Steven (Department: 2665)

Multiplex communications

Communication over free space

Combining or distributing information via time channels

C370S322000, C370S329000, C370S432000, C370S342000, C455S509000, C455S450000

Reexamination Certificate

active

06532227

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to general packet radio service (GPRS) and more particularly to a method and apparatus for de-allocation of GPRS physical channels.
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. facsimile transmissions, internet access, etc, 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 know as the General Packet Radio Service (GPRS) is currently being standardised by the European Telecommunications Standards Institute (ETSI). 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 Multiplexed Access) frames. The length of 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. A series of four consecutive time slots on a physical channel is known as a radio block and represents the shortest transmission unit for packet switched data on a physical channel. Physical channels are similarly defined for conveying signalling information. With the introduction of GPRS, physical channels will be dynamically assigned 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.
There is illustrated in
FIG. 1
the basic ‘architecture’ of a GSM cellular network which supports GPRS. The terminology used in
FIG. 1
is defined, by convention, as follows:
MS
Mobile Station
PC/PDA
Personal Computer/Personal Data Assistant
BSS
Base Station Subsystem
BTS
Base Tranceiver Station
BSC
Base Station Controller
GPRS
General Packet Radio Service
HLR
Home Location Register
SGSN
Serving GPRS Support Node
GGSN
Gateway GPRS Support Node
MSC
Mobile Switching Centre
SS7
Signalling System number 7
PSTN
Public-Switched Telephone Network
The full benefits promised by packet switched transmission are only achieved if a data transmission can be set up very quickly, i.e. there is little point in providing for the transmission of short duration packets if the time required to set up each of those transmissions is relatively long. Rapid set-up is achieved in the proposed GPRS system by defining a ‘virtual’ channel or ‘context’ between a MS and the SGSN when the MS first becomes active in the cellular network. This virtual channel is not an actual physical channel but involves creating and storing parameters such as the MS identifier, encryption key, etc, at the SGSN and at the MS. When an actual data transmission is initiated between the MS and the SGSN using GPRS, there is no need to generate and/or transfer this information and the connection can be set up extremely quickly. The proposed GPRS air-interface protocols are set out in the GSM Technical Specification GSM 03.64 (ETSI).
FIG. 2
shows schematically the radio link protocol stacks which are described in the GSM 03.64 specification. The terminology used in
FIG. 2
is defined by convention as follows:
IP
Internet Protocol
SNDCP
Subnetwork Dependent Convergence Protocol
LLC
Logical Link Control
RLC
Radio Link Control
MAC
Medium Access Control
GSM RF
GSM Radio Frequency
Um
GPRS/MS interface
BSSGP
Base Station Subsystem GPRS Protocols
ATM
Asynchronous Transfer Mode
FR
Frame Relay
L1bis
Layer 1 bis
SGSN
Serving GPRS Support Node
BSC
Base Station Controller
Gb
SGSN/BSC interface
The assignment of physical channels at the radio interface for packet switched transmission is carried out at the RLC/MAC layer by RLC/MAC layer control messages. For example, a MS may initiate a packet transfer by making a ‘random access’ request using a RLC/MAC message. The number of time slots (or radio blocks) which the MS wishes may be conveyed using a further message. Similarly, the BSS may initiate a packet transfer to the MS using RLC/MAC messages. As already described, when a virtual channel is created between a MS and a SGSN, a MS identifier is assigned to the MS. This identifier is known as a Temporary Logical Link Identifier (TLLI). When a data transmission is initiated between the MS and the BSS and one or more physical channels are allocated, a new RLC/MAC identifier known as a Temporary Flow Identifier (TFI) is assigned to the MS. During the data transmission, RLC/MAC messages will include the TFI in one of their fields to identify the receiving (or transmitting ) MS.
It is envisaged that, in a mixed circuit switched/packet switched network, priority will be given to the circuit switched service. This means that if the amount of traffic in the network approaches the network capacity, and a request is made for a circuit switched call, it is necessary to re-allocate physical channels from the packet switched service to the circuit switched service. Re-allocation may be achieved by waiting for ongoing packet switched transmissions to be completed and, upon completion, assigning the released channels to the switched circuit service. However, it is considered more efficient to interrupt ongoing packet switched transmissions on these channels and to immediately allocate the released channels, completing the interrupted transmissions only when the demand for switched circuit transmission has fallen sufficiently (or other non-interrupted packet switched transmissions have been terminated). The existing GPRS proposals require that when capacity is required for switched circuit transmission, RLC/MAC control messages are transmitted to the MSs instructing them to cease transmission on certain specified physical channels and to request access to other physical channels from the BSS. More specifically, a new RLC/MAC control message is defined and which comprises a message identifier field defining the message as a resource reassignment message and a TFI field specifying the MS to receive the message. It is of course necessary to transmit a resource reassignment message separately for each mobile station currently occupying a physical channel which must be de-allocated and the de-allocation process is therefore relatively slow. It is noted that each packet switched transmission generally occupies an uplink and a downlink channel so that two separate RLC/MAC control

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