CMC ICT3207 Call Flow

Associate Professor

Dept. of Electrical and Electronic Engineering

University of Dhaka

Dr.

Mohammad

J

unaebur

R

ashid (

JR

)

1

ICT3207: Cellular and Mobile Communication (3.0 Cr)

Course Teacher

Bangladesh University of Professionals

Lecture 18

Multiple access schemes

• GSM time-slot (normal burst)

-Tail are all set to 0 and can be used to enhance the receiver performance.

-The training sequence is used to adapt the parameters and select the strongest signal. -A flag S indicates whether the data field contains user or network control data.

• GSM bursts

-A normal burst for data transmission

-A frequency correction burst allows the MS to correct the local oscillator to avoid

interference

-A synchronization burst with an extended training sequence synchronizes the MS with BTS

in time.

-An access burst is used for the initial connection setup.

3

Lecture 18

Lecture 18

5

Lecture 18

Channel coding

• Channel coding improves transmission quality when interference, multipath fading, and

Doppler shift are encountered.

• As a result, the bit error rate and frame error rate or word error rate are reduced, but

throughput is also reduced. Four kinds of channel codings are used in GSM:

• Convolutional codes (L, k) are used to correct random errors: k is the input block bits, and

L is the output block bits.

• Convolutional codes have three different rates in GSM: (1) the one-half rate (L/k = 2), (2)

the one-third rate (L/k = 3), and (3) the one-sixth rate (L/k = 6).

• Fire codes (L, k) are used as a block code to detect and correct a single burst of errors,

where k is the information bits and L is the coded bits.

• Parity check codes (L, k) are used for error detection. L is the bits of a block, k is the

Lecture 18

Channel coding

• Concatenation codes use Convolutional code as an inner code and fire code as an outer

code. Both the inner code and the outer code reduce the probability of error and correct most of the channel code. The advantage of using concatenation code is a reduction of the

implementation complexity as compared with a single coding operation.

• GSM's speech code is sent at a rate of 13 kbps, which represents 260 bits in each 20-ms

speech block. After channel coding, each block contains 456 bits and the transmission rate is

22.8 kbps, or 114 bits for time slots.

• Adding the overhead bits such

as tail bits (6), training bits (26), flag bits (2), and guard time bits

(8.25), the total bits of a traffic channel is 156 bits in one time

7

Lecture 18

Interleaving

• Interleaving scrambles and/or spreads a sequence of bits prior to transmitting them. The

sequence of bits is put back in order at the receiving end. Bursts of errors occur during transmission because of signal fading.

• After being received, these bursts of errors are then converted to random errors and put

back in the correct sequence. Interleaving’s major drawback is the corresponding delay at the

receiving end.

• Interleaving schemes are relatively simple in GSM. A code word of 456 bits could be

spread into the following format:

1. Four full bursts—divide 456 bits into 4 parts, each one filling up a whole burst. This interleaving format takes 4.615 ms × 4 = 18.46 ms.

2. Eight half bursts—divide 456 bits into 8 parts, each one filling up half a burst. This

Lecture 18

Radio resource (RR) management

• In a mobile network, radio channels must allocate for call setup, handover and release, on a

call bias.

• There are three management functions; location, handover, and roaming. The

implementation of the RR functions require some kind of protocol between the mobile station and the network.

Link protocol

• In addition to the user's information, the signaling transfer information exchanges must be

sent and understood by every piece of signaling transport equipment.

• Most information exchange functions are distributed to different kinds of equipment. There

are three link protocols to provide information exchanges.

-Radio link protocol (RLP), specified in GSM link access protocol over the radio link called

LAPDm.

9

Lecture 18

Radio resource (RR) management

Interface associated with link protocols

• Non-call-related signals correspond to protocols in the MSC that are different from those in

other MSCs or other HLRs and are grouped together in the Mobile Application Part (MAP).

We can distinguish them by MAP/X, where X can be B, C, D, and so forth. MAP/B Protocol between BSC and relay MSC

MAP/C Protocol between GMSC and an HLR

MAP/B Protocol between another MSC/VLR and HLR

Lecture 18

Radio resource (RR) management

11

Lecture 18

Mobility management (MM)

• The mobility of cellular system users requires mobility management for location updates,

handovers, and roaming.

• A handover occurs when a voice channel changes as the mobile station enters another cell

during a call.

• Roaming is the ability to initiate a call in one network system and deliver it to another

network system by using MM and location update management.

Location update management

• Cellular networks are spreading rapidly, leading to overloaded systems, unacceptable

delays, and increasing computational costs due to inefficient Location Management.

• Users can move one cell to another cell, updating their location with the network based

12

Lecture 18

Mobility management (MM)

• When a user receives a call, the network must page cells within the cells (also referred to as

polling) to find that user as quickly as possible. This creates the dynamics behind much of

Location Management.

• The network can require more frequent Location Updates (LUs), in order to reduce polling

costs, but only by incurring increased time and energy expenditures from all the updates.

• Conversely, the network could only require rare LUs, storing less information about users

to reduce computational overhead, but at a higher polling cost.

• Additionally, cell sites themselves can be optimized in order to create regions that require

less handoff and quicker locating of users.

13

Lecture 18

Mobility management (MM)

Cell selection

• Choosing the best cell by a MS depends on three factors:

(1) the level of the signal received by the MS,

(2) the maximum transmission power of the mobile station, and

(3) two parameters p1 and p2 specified by the cell. This is called the C1 criterion

C1 = A - max (B, O)

A = received level average – p1

B = p2 - maximum RF power of the MS p1 = a value between -110 and -48 dBm

p2 = a value between 13 and 43 dBm

• Both values of p1 and p2 are broadcast

from the cells.

MS maximum power = 29 to 43 dBm

• The cell selection algorithm is as follows:

- A SIM must be inserted.

- The strongest C1 is chosen by obtaining C1 from candidate cells; the C1 has to be higher

than 0.

Lecture 18

Mobility management (MM)

Authentication

Authentication protects the network against unauthorized access.

- _{First Phase: A PIN (personal identification number) code protects the SIM. The PIN is}

checked by the SIM locally, so the SIM is not sent out over the radio link. - Second phase: The GSM network

makes an inquiry by sending a random

number (RAND). The 128-bit RAND

is sent from the network to the MS, and mixes with the MS's secret

parameter, K_i, in an A3 processing

algorithm, which produces a

15

Lecture 18

Mobility management (MM)

-_{Encryption: Encryption protects against unauthorized listening. The MS uses the RAND}

received from the network and mixes K_i through a different algorithm, called A8, and

generates K_c (64 bits). The ciphering sequences are generated from the K_c (Fig. 15.12). The

frame number and K_c move to a ciphering algorithm, A5, and generate S2 (114 bits), which

Lecture 18

Mobility management (MM)

• SIM (MS side) and AUC (network side) are the repositories of the subscriber's key K_i.

• Key K_i never transmits over the air.

• Both sides perform A3 and A8 computations.

User identity protection - security management

Assignments

1. Link budget analysis in mobile communication system

2. Analysis of cell planning and link budgeting in WiMAX

3. Coverage area and power budget calculation in GSM 4. Indoor planning of a cellular network