Digital Enhanced Cordless Telecommunications

Digital Enhanced Cordless Telecommunications, usually known by the acronym DECT, is a digital communication standard, which is primarily used for creating cordless phone systems. It originated in Europe, where it is the universal standard, replacing earlier cordless phone standards, such as 900 MHz CT1 and CT2 DECT is European standard system. It is a CT-2 like system and the application are slightly different from the cellular system.

                  APPLICATIONS:

v     Use the public network to mobile communication within the home and the immediate vicinity.

v     Provide business communication locally. In this case, the PSTN has been removed the radio fixed point (RFP), also labeled or cluster controls.

v     Local loop using DECT to provide wireless local loops is a cost effective alternative to running copper wires to residential premises.

SYSTEM STRUCTURE:

                 

ü      Duplex method          – TDD

ü      Access method         – TDMA

ü      RF power of handset – 10 MW

ü      Channel bandwidth   - 1728 MHz/ channel

ü      Number of carriers    - 5 (a multiple carrier system)

ü      Frequency                  - 1800 MHz to 1900 MHz

CHARACTERISTICS:

                 

ü      Frame             – 10 MS

ü      Time slots      - 12

ü      Bit rates          - 388 kb/ slot

ü      Modulation     - GFSK ( Gaussian fsk)

ü      Hand off          - YES

            

mobile integrated radio systems. [MIRS]

MIRS is a cellular like system developer by Motorola to operate at the special mobile radio (SMR) band shown in the following figure.    

Band	Mobile transmission	Base transmission
A cellular band A	829-835, 845-8465 MHz	869-880, 890-891 MHz
B cellular band B	869,880,890-891.5 MHz	880-890,891.5-895 MHz
ESMR	806-821 MHz	851-866 MHz

v     MIRS system is called by the manufactures. The system providers call this system ESMR. The SMR band is in the part of FCC CFR in the private sector.

v     Public information on this system is limited however the operational function is the same as the cellular system. Since many existing SMR system are still in operation the MIRS frequency channels in the SMR band are restricted depending on the area in which they operate. Therefore a special frequency assignment will be imposed in the MIRS system.

v     Top level system specifications are as follows.

v      Full duplex communication system frequency

-          806 to 824 MHz mobile transmitter

-          851 to 869 MHz base transmitter

v     Channel band width – 25 MHz

v     Multiple access – TDMA

v     Number of time slots 0 6

v     Rate of speech coders – 4.2 kbps / slot

v     Modulation – 16 QAM

v     Speech coders – VSELP

v     No equalizer is implemented

v     Hand off

v     Transmission rate – 6.5 kbps/ slot

v     Forward error correction – 3 kbps

v     Inter leavens.

personal communication network. [PCN]

The PCN system was first initiated by LORD YOUND in 1988. The characteristics of PCN are as follow.

v     Operating frequency 1.7 to 1.88 GHz ( 1710-1785 MHz and 1805-1880 MHz)

v     Uses 30 GHz or up for microwave back bone system.

v     Covers both small cells and large cells.

v     Coverage inside and outside buildings

v     Hand over

v     Call delivery

v     Portable hand set

v     User intelligent network

PCN uses the DCS – 1800 system. Which is similar to GSM but up converts the frequency to 1.7 to 1.88 GHz therefore the network structure the signal structure and the transmission characterize are similar between PCN and GSM but operation frequencies are different.

the parts of basic cellular system

In a cellular communication system a geographical area as subdivided into small regions where each small region is called as a cell

      A basic cellular system consist of three parts as listed below

-          a mobile unit

-          a cell site

-          a mobile telephone switching office (MTSO)

      The mobile unit (Mobile Telephone) contains a control unit, a transceiver (combination of transmitter and receiver) and on antenna sister.

      Whenever a mobile unit wants to step a call it contents the cell site of the particular cell in which it exists at that time.

      The cell site consists of control unit, antennas power plant and data terminals.

      The cell site is infecting acting as an interface to the MTSO.

      After the mobile unit contacts the cell site, the cell site in contacts the MTSO  which processes the call further depending on whether it is a mobile to mobile call or mobile to fixed station call.

      A little thought from the reader will indicate that a fixed subscriber to mobile call originates through the telephone exchange and passes through the MTSO and cell site towards the mobile unit.

Grade of Service (GOS)

      In loss systems, the traffic carried by the network is generally lower than the actual traffic offered to the network by the subscribers.

      The overload traffic is rejected and hence is not carried by the network.

      The amount of traffic rejected by the network is an index of the quality of the service offered by the network.

      This is termed Grade of Service (GOS) and is defined as the ration of lost traffic to offered traffic.

      Offered traffic is the product of the average number of calls generated by the users an average holding time per call.

      On the other hand, the actual traffic carried by the network is called the carried traffic and is the average occupancy of the servers in the network.

      GOS is given by

GOS= A – AO

               A

Where

            A         = Offered traffic

            AO      = Carried traffic

            A-AO  = Lost traffic

      The smaller the value of grade of service, the better is the service.

      The recommended value for GOS in India is 0.002 which means that two calls in every 1000 calls or one call in every 500 calls may be lost.

      Usually, every common subsystem in a network has an associated GOS value.

      The GOS of the full network is determined by the highest GOS value of the subsystems in a simplistic sense.

      A better estimate takes into account the connectivity of the subsystems, such as parallel units.

      Since the volume of traffic grows as the time passes by the GOS value of a network deteriorates with time

      In order to maintain the value within reasonable limits. Initially the network is sized to have a much smaller GOS value than the recommended one so that the GOS value continues to be within limits as the network traffic grows.

      The blocking probability PB is defined as the probability that all the servers in a system are busy.

      When all the servers are busy, no further traffic can be carried by the system and the arriving subscriber traffic is blocked.

      At the first instance, it may appear that the blocking probability is the same measure as the GOS.

      The probability that all the servers are busy may well represent the fraction of the calls lost, which is what the GOS is all about.

      However, this is generally not true.

Example:

                                    In a system with equal number of servers and subscribers. The GOS is zero as there is always a server available to subscriber. 

multiplexing

                        Multiplexing is the process where multiple channels are combined for transmission over a common transmission path.

                  Different types of multiplexing

                                    There are two types of multiplexing. 

                        (1)       Frequency division multiplexing. 

                        (2)       Time division multiplexing. 

                  (1) Frequency division multiplexing (FDM):         

                                   

                                    Frequency division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission in a single communications line or channel. Each signal is assigned a different frequency within the main channel.

(2)             Time division multiplexing (TDM):

                                   

                                    Time division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into main segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing. 

Tone

(A)             BUSY TONE:

                       

It is a burst 400 Hz signal with silence period in between. The burst and silence durations have the sum value of 0.755 or 0.3755. A busy tone is sent to the calling subscriber whenever the switching equipment or junction line is not available to put through the call or the called subscriber line is engaged no distinction is made between these conditions. It is not possible for a calling subscriber to conclude on the basis of the busy that the called party was actually engaged in a conversation.

                  (B)    DIAL TONE:

                       

This tone indicates that the exchange is ready to accept dialed digits from the subscribed. The subscriber should start dialing only after hearing the dial tone otherwise; initial dial pulses may be missed by the exchange. Which may result in the call landing on a wrong number Most often the dial tone is send out by the exchange even before the handset is brought near the ear. Sometimes however, a few seconds may elapse before the dial tone is heard this happens practically in common control exchange which is use shared resources for user interfaces. The dial tone is 33 Hz or 50 Hz or 400 Hz continuous tone.

                  (C)    ROUTING TONE:

                       

Routing tone or call-in-progress tone is a 400 Hz or 800 Hz intermittent pattern. It has 0.5 on/off period. In analog electronic exchanges it is a 400 Hz pattern with 0.5 on period and 2.5 off period. In digital exchanges it has 0.15 on/off periods at 400 Hz.Pride and Prejudice