First generation cellular systems that provide data communications using circuit switching have difficulty passing modem signals through the audio filters of receivers designed for analog, FM, common air-interfaces. Inevitably, voice filtering must be deactivated when data is transmitted over first genera-tion cellular networks. Wireless data networks are mainly divided into two categories: Individual mobile data Shared/overlay mobile data CDPD is a data service for first and second generation U.S. cellular systems and uses a full 30 kHz AMPS channel on a shared basis. CDPD provides mobile packet data connectivity to existing data networks and other cellular systems without any additional bandwidth requirements. CDPD directly overlays with existing cellular infrastructure and uses existing base station equipment, making it simple and inexpensive to install. CDPD does not use the MSC, but rather has its own traffic routing capabilities. CDPD occupies voice channels purely on a secondary, non interfering basis, and packet channels are dynamically assigned (hopped) to different cellular voice channels as they become vacant, so the CDPD radio channel varies with time. As with conventional, first generation AMPS, each CDPD channel is duplex in nature. The forward channel serves as a beacon and transmits data from the PSTN side of the network, while the reverse channel links all mobile users to the CDPD network and serves as the access channel for each subscriber Collisions may result when many mobile users attempt to access the network simultaneously. Each CDPD simplex link occupies a 30 kHz RF channel, and data is sent at 19,200 bps. Since CDPD is packet-switched, s large number of modems are able to access the same channel as needed, packet-by-packet basis. GMSK BT=0.5 modulation is used in CDPD CDPD Transmission CDPD transmissions are carried out using fixed- length blocks. User data is protected using a Reed Solomon (63,47) block code with 6-bit symbols. For each packet, 282 user bits are coded into 378 bit blocks, which provide correction for up to eight symbols. Protocols used in CDPD Two lower layer protocols are used in CDPD. The mobile data link protocol (MDLP) is used to convey information between data link layer entities (layer 2 devices) across the CDPD air interface. The MDLP provides logical data link connections on a radio channel by using an address contained in each packet frame. The MDLP also provides sequence control to maintain the sequential order of frames across a data link connection, as well as error detection and flow control. The radio resource management protocol (RRMTh) is a higher, layer 3 protocol used to manage the radio channel resources of the CDPD system and enables an M-ES (the mobile and system) to find and utilize a duplex radio channel without interfering with standard voice services. The RRMP also handles channel hopping commands, cell hand- offs, and M-ES change of power commands. Protocols MDLP, RRMP, X.25 Channel Data Rate 'bps; 19,200 Channel Bandwidth (kHz) 30 Spectrum Efficiency (b/Hz) 0.64 Random Error Strategy cover with burst protect Burst Error Strategy RS 63,47(6 bits per symbol) Fading Performance withstands 2.2 ms fade Channel Access slotted DSMA/CD . Advance Radio Data Information Systems (ARDIS) is a private network service provided by Motorola and IBM. It is based on MDC 4800 and RD-LAP (Radio Data Link Access Procedure) protocols developed at Motorola ARDIS provides 800 MHz two-way mobile data communications for short-length radio messages in urban and in-building environments, and for users traveling at low speeds. ARDIS has been deployed to provide excellent in- building penetration. When a mobile sends a packet, many base stations which are tuned to the transmission frequency attempt to detect and decode the transmission, in order to provide diversity reception for the case when multiple mobiles contend for the reverse link. Protocol MDC 4800 RD-LAP Speed (bps) 4800 19,200 Channel Bandwidth (kHz) 25 25 Spectrum Efficiency (b/Hz) 0.19 0.77 Random Error Strategy convolutional 1/2, k=7 trellis coded modulation, rate = 3/4 Burst Error Strategy interleave 16 hits interleave 32 bits Fading Performance withstands 3.3 ms fade withstands 1.7ms fade Channel Access CSMA slot CSMA General Packet Radio Service (GPRS) is a packet-based data network. GPRS is well suited for non-real time internet usage, including the retrieval of email, faxes, and asymmetrical web browsing. GPRS supports multi-user network sharing of individual radio channels and time slots. Similar to the CDPD the GPRS standard provides a packet network on dedicated GSM or IS-136 radio channels. GPRS retains the original modulation formats specified in the original 2G TDMA standards But it uses a completely redefined air interface in order to better handle packet data access. GPRS subscriber units are automatically instructed to tune to dedicated GPRS radio channels and particular time slots for always on access to the network. When all eight time slots of a GSM radio channel are dedicated to GPRS, an individual user is able to achieve as much as 171.2 kbps So per slot data rate is 21.4 kbps As is case of any packet network , the data throughput experience by as individual GPRS user decreases substantially as more users attempt to use the network Or as propagation conditions become poor for particular use No new base station RF hardware is required. Implementation of GPRS just requires the GSM operator to install new routers and internet gateways at the base station, along with new software that redefines the base station air interface standard for GPRS channels and time slots. EDGE is a more advanced upgrade to the GSM standard and requires the addition of new hardware and software at existing base stations. EDGE introduces a new digital modulation format , 8-PSK (octal phase shift keying), which is used in addition to GSM’s standard GMSK modulation. EDGE allows for nine different air interface formats , known as multiple modulation and coding schemes (MCS), with varying degrees of error control protection. Each MCS state may use either GMSK (low data rate) or 8-PSK (high data rate) modulation for network access, depending on the operating conditions. EDGE uses the higher order 8-PSK modulation and a family of MCSs for each GSM radio channel time slot so that each user connection may adaptively determines the best MCS setting for the particular radio propagation conditions and data access requirements of the user This adaptive capability to select the best air interface is called incremental redundancy. In Incremental redundancy packets are transmitted first with maximum error protection and maximum data rate throughput, and then the subsequent packets are transmitted with less error protection and less throughput, until the link has an unacceptable outage or delay. Rapid feedback between the base station and the subscriber unit then restore the previous acceptable air interface state. Incremental redundancy insures that the radio link for each user will quickly reach a condition which uses a minimum overhead as a result user capacity on network is maximized. When EDGE uses 8-PSK modulation without any error protection, and all 8 time slots of a GSM radio channel are dedicated to a single user, a raw peak throughput data rate of 547.2 kbps can be provided. In practice , the slotted schemes used in EDGE when combine with practical network contention issues and error control coding requirements, limits practical raw data rates to bout 384 kbps for a single dedicated user on a single GSM channel.
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