Authentication and authorization?

1. What is the difference between authentication and authorization?

Authentication verifies who you are.
Authentication establishes identity.

Authorization is what you are authorized/allowed to do or access.
Authorization decides what privileges a given person or program has.

2. Why is authorization insignificant if we don’t also have authentication?

Without authentication we don’t know who the user is, so we can’t possibly know what privileges that user should have.

Mapping of logical Channels

Generally two configurations are mainly used

Separate SDCCH: FCCH + SCH + BCCH + CCCH
–Addresses a channel configuration in which no SDCCH are available on TS 0.
–In this case SDCCH sub channels are defined on TS 1
–Rest of the TS are used by Traffic channels

Combined SDCCH: FCCH + SCH + BCCH + CCCH + SDCCH/4
–Addresses a channel configuration in which all control channels are assigned to TS 0
–In this case TS1 is also available for Traffic channels

Logical channels can time-share TS 0 in different TDMA frames of 51 frame Multi frame.

Broadcast control channel (BCCH)

Broadcast control channel (BCCH)

1. Downlink -> Uplink
2. Normal Burst
3. BCCH Occur in timeslot 0 of some specific carriers known as BCCH carriers
4. After locking on to the frequency and frame structure in the cell, MS needs some more general information broadcast on the BCCH for call setup purposes

• Cell Identity (CI)
• Network Identity (LAI)
• Control Channel structure
• BCCH Frequencies of neighboring cells
  
• GPRS Supported or not.

Normal Burst for GMSK

The time interval of a TS corresponds to 156.25 bits.

Normal Burst for GMSK: 114 bits of useful information

• Used to carry Traffic or control information
• Tail bits are sequence of zero bits to indicate the start and the end of burst
• Data bits carries Traffic or control information
• Training bits carries a training sequence used by adaptive equalizer to estimate the channel
• Flag bit indicates type of information being transmitted (User info/signaling info (TCH or FACCH)
  
• Guard bits are used to avoid overlapping between adjacent timeslots

Hyper, Super & Multiframes

Frame number is generated by BTS. BTS broadcast current frame number 5 times in 51 frame multiframe.

Why uplink uses lower freq band and downlink uses higher freq bad?

- lower band gives better radio propagation which is more critical for power-limited mobiles

- higher band means more power, so BTS uses this. Also, if mobile uses higher band, then there will be more interference.

Why cells are hexagonal while practically, they are circles?

A honeycomb of hexagons completely tiles an area, indicating complete coverage at the expense of distorting the shape of the cells. Circular tilings either overlap or leave gaps. A hexagon is to be taken as indicating a circular cell where the circle has the same center as the hexagon and a circumference passing through its vertices. The call of a mobile user passing through a grid of circular cells needs to be handed off each time the user leaves one cell and enters another. This has to be done by passing through the area where the cells overlap. At this point, the mobile phone is in the coverage area of two cells, the one it is about to leave and the one it is entering. In this way, the overlapping areas can indicate that the need for a hand-off is imminent, and the cell that is the target for the hand-off.