This document summarizes cellular systems and medium access control (MAC) for mobile computing. It defines cellular systems as using space division multiplexing with base stations transmitting within cell radii of varying distances. The advantages of cellular systems are higher capacity through frequency reuse, lower transmission power, and localized interference. Disadvantages include needing infrastructure and performing handovers between cells. The document then defines MAC as regulating user access to shared mediums and discusses motivations for specialized MAC protocols for mobile networks, including avoiding collisions.

1. MOBILE COMPUTING
Cellular System and Medium Access Control
by,
S.Subhalakshmi
M.Sc(Computer Science)
Nadar Saraswathi college of arts and science,Theni.

2. CONTENT:
Cellular System
Advantages
Disadvantages
Medium Access Control(MAC)
Motivation for a specialized MAC
Types of MAC

3. Cellular System:
 Cellular systems for mobile communications implement SDM.
Each transmitter typically called a base station.
 Cell radii can vary from tens of meters in buildings, and
hundreds of meters in cities, up to tens of kilometers in the
countryside.

5. Advantages:
1. Higher Capacity:
 If one transmitter is far away from another, i.e., outside the
interference range, it can reuse the same frequencies.
 But frequencies are a scarce resource and, the number of
concurrent users per cell is very limited.
 Huge cells do not allow for more users. On the contrary, they
are limited to less possible users per km2.

6. 2. Less Transmission Power:
 While power aspects are not a big problem for base stations,
they are indeed problematic for mobile stations.
 A receiver far away from a base station would need much more
transmit power than the current few Watts.
 But energy is a serious problem for mobile handheld devices.

7. 3.Local Interference Only:
 Having long distances between sender and receiver results in
even more interference problems.
4.Robustness:
 Cellular systems are decentralized and so, more robust against
the failure of single components.
 If one antenna fails, this only influences communication within
a small area.

8. Disadvantages:
1.Infrastructure needed:
 Cellular systems need a complex infrastructure to connect all
base stations.
 This includes many antennas, switches for call forwarding,
location register to find a mobile station etc, which makes the
Whole system quite expensive.

9. 2. Handover needed:
 The mobile station has to perform a handover when changing
from one cell to another.
 Depending on the cell size and the speed of movement, this
can happen quite often.
3.Frequency planning:
 To avoid interference between transmitters using the
same frequencies, frequencies have to be distributed carefully.

11. This phenomenon with a user transmitting a high bit rate stream
within a CDM cell.
This additional user lets the cell shrink with the result that two users
drop out of the cell.
In a real-life scenario this additional user could request a video stream
(high bit rate) while the others use standard voice communication
(low bit rate).

12. Medium Access Control (MAC):
 Medium access control comprises all mechanism that regulate
user access to medium using SDM,TDM,FDM or CDM.
 MAC is thus similar to traffic regulations in the highway
multiplexing.
 MAC belongs to Data link Layer(DLC).

13. Motivation for a specialized MAC:
 Carrier sense multiple access with collision
detection,(CSMA/CD) which works as follows.
 A sender senses the medium (a wire or coaxial cable) to
see if it is free.
 If the medium is busy, the sender waits until it is free.

14.  If the medium is free, the sender starts transmitting data and
continues to listen into the medium.
 If the sender detects a collision while sending, it stops at once
and sends a jamming signal.
Types of MAC:
There are two types of MAC. There are
1. Hidden and exposed terminals
2. Near and far terminals