Major cellular system types include those operating according to the Global Services for Mobile(GSM)Standard, the TIA/EIA/IS-95 Mobile Station-Base Station compatibility Standard for Dual Mode Wide Band Spread Spectrum Cellular System, the TIA/EIA/IS-136 Mobile Station-Base Station Compatibility Standard, and the TIA/EIA 553 Analog Standard(AMPS/TACS). Other major cellular systems include those operating in the personal communications system (PCS) band according to the IS-95 based ANSI-J-STD-008 1.8–2.0 Ghz standard or, those operating according to the GSM based PCS1900 (1900 Mhz frequency range) standard.
Currently, each of the major cellular systems standards bodies are implementing data services into their digital cellular specifications. A packet data service specification has been finalized for GSM and, packet data service specifications compatible with the IS-95 and IS-136 standards are being prepared. Another example of a data service is the TIA/EIA IS-99 Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System (IS-99). IS-99 defines a connection based packet service for IS-95-A based networks. The IS-99 system provides a standard for asynchronous data service (Service Option 4) and digital Group-3 facsimile service (Service Option 5).
In an IS-99 based system, a radio link protocol (RLP) is utilized to provide an octet stream service over IS-95-A forward and reverse traffic channels. Each octet comprises 8 bits of digital data. The octet stream service carries the variable length data packets of the point to point protocol layer. The RLP divides the point to point protocol packets into IS-95-A traffic channel frames for transmission. There is no direct relationship between point to point protocol packets and IS-95-A frames. A large packet may span several IS-95-A traffic channel frames, or a single traffic channel frame may include all or part of several point to point packets. The RLP does not take the higher level traffic channel framing into account, but operates on a featureless octet stream, delivering the octets in the order received from the point to point layer. The data may be transmitted on the traffic channel as primary traffic, or, for example, along with speech, as secondary traffic. Data may also be transmitted in a signaling subchannel. IS-95 multiplex option 1 may be used at full rate, half rate and eighth rate for primary traffic and at rate 1, rate ⅞, rate ¾, and rate ½, for secondary traffic.
The RLP utilizes RLP control frames to control the transmission of data and RLP data frames for the transmission of data at the RLP level.
The format of RLP control and data frames is defined so that each RLP frame includes an 8-bit sequence number field (SEQ). Each RLP data frame SEQ field contains the sequence number of that particular data frame. The sequence numbers are used to identify each received data frame and allow determination of data frames that have not been received. The RLP control frame SEQ field is not used to indicate the sequence number of the control frame, but contains the next data frame sequence number, to allow quick detection of erased data frames.
In addition to the SEQ field, each RLP data frame includes a number of data bits, with up to a maximum number of data bits allowed for each frame. The maximum number of data bits allowed in a data frame depends upon the IS-95 multiplex subchannel used. For example, for primary traffic on the traffic channel, using multiplex option 1 at IS-95 full rate, the maximum number of data bits allowed is 152, and, for primary traffic on the traffic channel, using multiplex option 2 at IS-95 half rate, the maximum number of data bits allowed is 64. When less than the maximum number of bits are transmitted in a frame, padding is used to fill out the data field to 152 bits. Each RLP data frame also includes a RLP frame type (CTL) field, and a data length (LEN) field. The LEN field indicates the length of the data in the frame in octets. For unsegmented data frames, the CTL frame is one bit and is set to 0. For segmented data frames, the CTL frame contains 4 bits and can be set to indicate whether the data in the frame includes the first LEN octets, the next LEN octets, or, the last LEN octets of the unsegmented data frame.
The RLP control frame may function as a negative acknowledgement (NAK) RLP control frame. A (NAK) RLP control frame includes a 4 bit frame type (CTL) field, a four bit length (LEN) field, an 8 bit FIRST field, an 8 bit LAST field, a reserved field (RSVD), a frame check sequence field (FCS) and padding. An RLP control frame having the frame type field set to indicate negative acknowledgement (NAK) may then be used to request retransmission of a particular data frame, or, a particular sequence of data frames. For example, a mobile station expecting a data frame having a particular sequence number, would transmit a NAK control frame to the base station if the mobile determined that the data frame was missed. The FIRST and LAST fields of the RLP NAK control frame are used to indicate the particular data frame, or, sequence (indicated as a range beginning at the sequence number indicated by the FIRST field and ending at the sequence number indicated by the LAST field) of data frames that are requested to be retransmitted. In IS-99, the number of requests for retransmission of a data frame is a set number and the initiation of the requests for retransmission is controlled by a NAK retransmission timer. When RLP frames are carried as primary or secondary traffic, the retransmission timer is implemented as a frame counter. When RLP frames are carried in the signaling subchannel, the retransmission timer is implemented as a timer having a duration equal to a predetermined value, T1m, that is defined in Appendix D of IS-95-A. The NAK retransmission counter for a data frame is started upon the first transmission of a NAK RLP control frame requesting retransmission of that data frame.
If the data frame has not arrived at the receiver when its NAK retransmission timer expires, the receiver sends a second NAK control frame requesting retransmission of that data frame. This NAK control frame is transmitted twice. The NAK retransmission timer for this data frame is then restarted. If the data frame has not arrived at the receiver when its NAK retransmission timer has expired twice, the receiver sends a third NAK control frame requesting retransmission of that data frame. Each NAK control frame transmitted as the result of a retransmission timer expiring a second time is transmitted three times.
A NAK abort timer is then started in the receiver upon transmission of the third NAK control frame. The NAK abort timer is implemented, and expires, identically to the NAK retransmission timer. If the data frame has not arrived at the receiver when its NAK abort timer has expired, the NAK is aborted and no further NAK control frames are transmitted for that data frame.
The IS-99 NAK scheme results in a maximum number of three retransmission requests, that include a maximum number of six NAK RLP control frames, being transmitted for a particular unreceived data frame.
As cellular telecommunications systems evolve, various high speed data (HSD) service options will be implemented into the different cellular system standards. For example, several HSD options are being considered for implementation into the IS-95-A standard. These HSD options may include IS-95-A based systems having the capability to transmit data at rates of up to 78.8 kbps. Use of any of these options in IS-95-A will increase the range of services and applications that can be supported. For an IS-99 based system, an increase in the number of services and applications that the system may support will require that the system support data services having different bandwidth, delay sensitivity and quality of service requirements (QoS).
Different bandwidth, delay sensitivity and quality of service requirements may require different bit error rate (BER), and, delay requirements. A fixed frame header and fixed NAK retransmission procedure such as that of IS-99 may not be optimally configured for certain data services that must be supported. For example, it may be that a service with low QoS requirements (high BER allowed) may experience large delays from a NAK retransmission procedure in a system having a predetermined number of retransmissions, when it is not really necessary to retransmit missing data frames the predetermined number of times in order to provide acceptable service. Another example of non-optimization in a data packet service using a fixed frame header, such as that of IS-99, could occur if a service required high bandwidth and included large numbers of sequenced data frames to be transmitted as high speed data. This service may use long data sequences having a number of data frames greater than X, which is the maximum number indicated by the full SEQ field of the fixed frame header. In this case, the count in the SEQ field would have to be restarted before a long data sequence was finished. Restarting the count in the sequence field may require more complicated processing of the transmitted and received data than having each frame in the data sequence numbered sequentially. Additionally, if a data service uses a shorter data sequence having a number of data frames less than the maximum number indicated by the SEQ field, this may be nonoptimal because bits reserved for the SEQ field go unused in each data frame, when these bits could be used to carry data.