Wireless communication networks have become much more pervasive than any one could have imagine when the cellular concept was first developed in the 1960s and 1970s.The wide spread success of cellular has led to the development of newer wireless systems and standards for many other types of telecommunication traffic.
Personal communication networks (PCN) refers to a wireless networking concept where any user can make or receive call,no matter where they are, using a light weight,personalized communicator.Personal communication services (PCS) refers to new wireless system that incoperate more network features and more personalized than existing cellular radio systems.
Second Generation(2G) Cellular networks
Unlike first generation cellular systems that relied on FDMA/FDD and analog FM,second generation standards use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques.The most popular 2G standard includes 3 TDMA standards and one CDMA standards:
1.Global System Mobile (GSM),which supports 8 time slotted users for each 200kHz radio channel.(in the PCS spectrum band only)
2.Interim Standard 136 (IS-136),wich support 3 time slotted users for each 30kHz radio channel.
3.Pacific Digital cellular(PDC), a japanese TDMA standards that is similar to IS-136.
4.Interim Standard 95 Code devision Multiple access(IS-95),also known as cdmaone, which supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 channel.
All 2G technologies offer at least a 3 times increase in spectrum efficiency as compared to the first generation analog technologies.
Evolution to 2.5 Mobile Radio Networks
The 2G technology use circuit switched data modems that limit data users to single circuit-switched voice channel.2G networks only support single user data rates on the order of 10 kilobits per second,which is too low for rapid email and internet browsing applications.This is because 2G standards were designed before the wide spread use of internet.
Even with relatively small user data rates, 2G standards are able to support limited internet browsing and sophisticated short messaging(SMS) capabilities using circuit-switched approach.The new technology 2.5G allow existing 2G equipment to be modified and supplemented with new base station add-ons and subscriber unit software upgrades to support higher data rate transmission for web browsing and e-mail traffic. 2.5G also support new web browsing format language,called wireless application protocol(WAP),that allow standard web pages to be viewed in compressed format specially designed for small hand held potable wireless devices.Japan introduced its own propriety wireless data service and internet micro browser technology called I-mode,on its PDC.I-mode supports games,color graphics and interactive web page browsing using 2G PDC data rate 0f 9.6 kilobits per second.
Evolution for 2.5G TDMA Standards
Three TDMA upgrade option include:
1.High Speed Circuit Switched Data (HSCSD)
2.General Packet Radio Service (GPRS)
3.Enhanced Data rates for GSM Evaluation (EDGE)
These option provide significant improvement in internet access speed over GSM and IS-136 and support the creation of new internet-ready cell phones.
HSCSD for 2.5G GSM
HSCSD allows single mobile subscriber to use consecutive user time slots in GSM standard,in order to obtain high speed data access.HSCSD relaxes the error control coding algorithm that specified in GSM for data transmission and increase data rate to 14.4 kbps.By running up to 4 consecutive time slots,HSCSD able to provide rate up to 57.6 kbps to individual users.Service provider has to implement a software change at existing GSM base station to activate HSCSD.HSCSD is ideal for streaming internet access and real-time interactive web sessions.
GPRS for 2.5G
GPRS is packet based data network,wich is well suited for non-real time internet usage,and applications where the user downloads much more data than its uploads(like retrieval of email,faxes).GPRS supports multi-user network sharing of individual radio channels and time slots.GPRS can support many users than HSCSD but in bursty manner.GPRS subscriber units are automatically instructed to tune to dedicated GPRS radio channels and particular time slot for "always on"access to the network.Implementation of GPRS merely requires the GSM operator to install new router and internet gateway at the base station,along with new software that redefines the base station air interface and time slots,no new RF hardware needed.As the case of packet network throughput experience by user may decreases as more users attempt to use the network or due to poor propagation conditions.GPRS allow dedicated peak 21.4kbps per channel data rate,if all 8 time slots of GSM used for GPRS,an individual user is able to achieve 171.2 kbps (8x21.4).GPRS originally designed to provide packet data access over GSM networks,but later extended to work with IS-136 as well.
EDGE for 2.5G
EDGE requires new hardware and software upgrades to the existing base station.EDGE introduce a new digital format,8-PSK(octal phase shift keying) in addition to existing GMSK modulation.EDGE allows for 9 different air interface formats,known as multiple modulation and coding schemes(MCS),with varying degree of error control protection.So each user connection may adaptively determine the best MCS setting(incremental redundancy) for the particular radio propagation condition and data access requirement of the user.EDGE is sometimes referred to as Enhansed GPRS,EGPRS.Incremental redundancy,is packets transmitted first with maximum error protection and data rate throughput,but subsequent packets are transmitted with less error protection and less throughput.If all 8 time slots are dedicated to EDGE,raw peak throughput data rate of 547.2kbps can be provided.
IS-95B for 2.5 CDMA
IS-95B provides high speed packet and circuit switched data access on a common CDMA radio channel by dedicating multiple orthogonal user channels(Walsh function).IS-95B supports medium data rate(MDR) service by allowing a dedicated user to command up to 8 different user Walsh code simultaneously and in parallel throughput of 115.2kbps per user(8 x 14.4kbps).IS-95B also specifies hard hand off procedure that allow subscriber unit to search different radio channels in the network with out instruction from the switch,and maintain link quality.
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