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Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Mobile Unit

Base Station

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Classification of Basic Cellular Systems: 1. Circuit-switched system,

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2. Packet-switched system

Circuit-switched System: In a circuit-switched system, each traffic channel is dedicated to a user until its call is terminated. Classification of Circuit Switched Systems: 1. Analog Cellular System

2. Digital Cellular System

Analog Cellular System: It consists of three subsystems 1. Mobile Unit

2. Cell Site (Radio Base Station)

3. Mobile Telephone Switching Centre/Office (MTSC/MTSO) 

Mobile unit: A mobile telephone unit contains a control unit, a transceiver, and an antenna system.

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Cell site (Base Station) : It provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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MTSO: The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and cellular administration.

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It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities.

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The cellular switch (analog or digital): It switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network.

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Data links: Provide supervision links between the processor and the switch and between the cell sites and the processor. (wire lines).

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Microwave radio links or T-carriers carry both voice and data between cell site and the MTSO. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Two types of channels are available between the mobile unit and the base station (BS). 1. Control channels: Used to exchange information having to do with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest BS. 2. Traffic channels: Carry a voice or data connection between users.

Call Setup between Mobile Users 

Mobile unit initialization:

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Cells with different frequency bands repetitively broadcast on different setup channels. When the mobile unit is turned on, it scans and selects the strongest setup channel and monitors that channel.

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The effect of this procedure is that the mobile unit has automatically selected the BS antenna of the cell within which it will operate. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Then a handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell. The handshake is used to identify the user and register its location.

As long as the mobile unit is on, this scanning procedure is repeated periodically to account for the motion of the unit.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Mobile-originated call:  A mobile unit originates a call by sending the number of the called unit on the preselected setup channel.  The receiver at the mobile unit first checks that the setup channel is idle by examining information in the forward (from the BS) channel. When an idle is detected, the mobile unit may transmit on the corresponding reverse (to BS) channel. The BS sends the request to the MTSO. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Paging: The MTSO then attempts to complete the connection to the called unit. The MTSO sends a paging message to certain BSs depending on the called mobile unit number. Each BS transmits the paging signal on its own assigned setup channel.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Call accepted: The called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO. The MTSO sets up a circuit between the calling and called BSs. At the same time, the MTSO selects an available traffic channel within each BS's cell and notifies each BS, which in turn notifies its mobile unit. The two mobile units tune to their respective assigned channels. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Ongoing call: While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Handoff: If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell.  The system makes this change without either interrupting the call or alerting the user.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Call blocking: During the mobile-initiated call stage, if all the traffic channels assigned to the nearest BS are busy, then the mobile unit makes a preconfigured number of repeated attempts. After a certain number of failed tries, a busy tone is returned to the user. Can termination: When one of the two users hangs up, the MTSO is informed and the traffic channels at the two BSs are released. Call drop: During a connection, because of interference or weak signal spots in certain areas, if the BS cannot maintain the minimum required signal strength for a certain period of time, the traffic channel to the user is dropped and the MTSO is informed. Calls to/from fixed and remote mobile subscriber: The MTSO can set up a connection between a mobile user in its area and a fixed subscriber via the telephone network. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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A basic digital system consists of four elements: 1. Mobile station (MS)

3. Base station controller (BSC)

2. Base transceiver station (BTS)

4. Switching subsystems

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MS: It consists of two parts, mobile equipment (ME) and subscriber identify module (SIM). SIM contains all subscriber-specific data stored on the MS side.

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BTS: Besides having the same function as the analog BTS, it has the Transcoder/Rate Adapter Unit(TRAU), which carries out coding and decoding as well as rate adaptation in case data rate varies.

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BSC: A new element in digital systems that performs the Radio Resource (RR) management for the cells under its control. BSC also handles handovers, power management time and frequency synchronization, and frequency reallocation among BTSs.Prof. K. Pushpa,SVECW, Bhimavaram 6/17/2012

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Switching subsystems:

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MSC: The main function of MSC is to coordinate the setup of calls between MS and PSTN users. VLR (Visitor Location Register):A database of all mobiles roaming in the MSC’s area of control. HLR(Home Location Register): A centralized database of all subscribers registered in a Public Land Mobile Network (PLMN). AUC (Authentication Center): Provides HLR with authentication parameters and ciphering keys that are used for security purposes. EIR (Equipment Identity Register): A database for storing all registered mobile equipment numbers. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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IWF: Provides the subscriber with data services that can acess data rate and protocol conversion facilities and interfaces with public and private data networks.

EC (Echo Canceller): Used on the PSTN side of the MSC for all voice circuits. XC (Transcoder): Usually installs in each BTS. But for the cost reason, it can be installed in BSC or MSC. OMC(Operational and Maintenance Center): This function resided in analog MSC but became a separated entity in digital systems.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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MS: Provides the voice and packet data services. It is also called UE (User Equipment).

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Node B: The name for base station in GSM.

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RNC (Radio Network Controller): Controls the radio resources of the Node Bs that are connected to it. Its function is similar to BSC. A device PCU (Packet Control Unit) converts the data stream into packet format.

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SGSN (Service GPRS Support Node): Analogous to MSC/VLR in the circuit-switched system. This includes mobility management, security, and access control functions. It interfaces to HLR.

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GGSN (Gateway GPRS Support Node): The point of interface with external packet data networks such as the Internet.

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CGF (Changing Gateway Function): Mainly for billing.

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RNS (Radio Network Subsystem): It consists of RNC and Node B. UTRAN consists of two or more RNS. Prof. K. Pushpa,SVECW, Bhimavaram

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1. Voice Quality

CM: For any given commercial communications system, the voice quality will be based on the following criterion: a set value x at which y percent of customers rate the system voice quality (from transmitter to receiver) as good or excellent; the top two circuit merits (CM) of the five listed below.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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1. Voice Quality MOS: As the percentage of customers choosing CM4 and CM5 increases, the cost of building the system rises.

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The average of the CM scores obtained from all the listeners is called mean opinion score (MOS). Usually, the toll-quality voice is around MOS ≥4.

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DRT (Diagnostic Rhyme Test): An ANSI standardized method used for evaluation of intelligibility. It is a subjective test method. Listeners are required to choose which word of a rhyming pair they perceived. The words differ only in their leading consonant.

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The word pairs have been chosen such that six binary attributes of speech intelligibility are measured in their present and absent states. This attribute profile provides a diagnostic capability to the test. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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2. Data Quality 

There are several ways to measure the data quality such as bit error rate, chip error rate, symbol error rate, and frame error rate. The chip error rate and symbol error rate are measuring the quality of data along the transmission path. The frame error rate and the bit error rate are measuring the quality of data at the throughput. 3. Picture/Vision Quality

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There are color acuity, depth perception, flicker perception, motion perception, noise perception, and visual acuity. The percentage of pixel (picture element) loss rate can be characterized in vertical resolution loss and horizontal resolution loss of a pixel.

Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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4. Service Quality 

Three items are required for service quality.

1. Coverage: The system should serve an area as large as possible. With radio coverage, however, because of irregular terrain configurations, it is usually not practical to cover 100 percent of the area for two reasons: a. The transmitted power would have to be very high to illuminate weak spots with sufficient reception, a significant added cost factor. b. The higher the transmitted power, the harder it becomes to control interference. 

Therefore, systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in hilly terrain.

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The voice quality and coverage criteria would be adjusted as per decided various terrain conditions. In hilly terrain, 90 percent of users must rate voice quality good or excellent in 75 percent of the served area. Prof. K. Pushpa,SVECW, Bhimavaram

6/17/2012

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4. Service Quality 2. Required grade of service: For a normal start-up system, the grade of service is specified for a blocking probability of .02 for initiating calls at the busy hour. This is an average value. 

However, the blocking probability at each cell site will be different. At the busy hour, near freeways, automobile traffic is usually heavy, so the blocking probability at certain cell sites may be higher than 2 percent, especially when car accidents occur.

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To decrease the blocking probability requires a good system plan and a sufficient number of radio channels. 3. Number of dropped calls: During Q calls in an hour, if a call is dropped and Q−1 calls are completed, then the call drop rate is 1/Q.

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This drop rate must be kept low. A high drop rate could be caused by either coverage problems or handoff problems related to inadequate channel availability or weak reception. Prof. K. Pushpa,SVECW, Bhimavaram 6/17/2012