Larry Jordan Buffalo NY
                     The architecture of the cellular networks, Larry Jordan Buffalo NY
A cellular network is made up of some cells. The cell covers a geographical area whose base 
 station conforms to 802.11 AP which helps mobile users to connect to the network. There is 
 an air interface of the physical and link layer protocols between the mobile and the base  
station.
All these base stations are connected to the mobile switching centre which connects the cells  
to the wide area net, manages call setup, and handles mobility. There is a radio spectrum that  
is allocated to base stations and a particular area and which now needs to be shared. There are 
 2 techniques for sharing mobile-to-base station radio spectrum.
1. Combined FDMA/TDMA
It divides the spectrum into frequency channels and divides each channel into time slots.
2. Code Division Multiple Access
This allows the re-use of the same spectrum on all cells. Two frequency bands are used to  
improve net efficiency, one forward channel for the cell site to the subscriber and one reverse  
channel for the cell site to the sub.
Cell Fundamentals
In practice the cells are of arbitrary shape close to a circle as it has equal power on all sides  
and equal sensitivity on all sides but placing two or three circles side by side may cause gaps  
or overlap each other in sequence can keep. To solve this problem, an equilateral triangle,  
square, or regular hexagon is used in which the hexagonal cell is close to the circle used for a 
 system design.
The number of cells in cluster N determines the amount of co-channel interference and the 
 number of frequency channels available per channel.
Cell Splitting
When the number of subscribers in a given area increases it is necessary to allocate more  
channels covered by that channel which is done by cell segmentation. A single small cell  
passage is introduced between the two co-channel cells.
Need for Cellular Hierarchy
 It helps extend coverage to areas that are difficult to cover by a single large cell.
 Also, to increase the capacity of the network for areas that has a high density of 
users.
 The growing number of wireless devices and the communication between them.
Cellular Hierarchy 
 Femto cells
The smallest unit in the hierarchy these cells only need to cover a few meters where all 
 devices are in physical range of use.
Pico cells
The size of these networks is in the range of a few tens of meters, for example, 
WLAN.
Micro cells
Cover a range of hundreds of meters to support PCS in urban areas which is another type of 
 mobile technology.
Macro cells
Cover areas in the order of several kilometres, eg, metropolitan areas.
Mega cells
Cover nationwide areas with a range of hundreds of kilometres, such as those used with 
 satellites.
Fixed Channel Allocation
The frequency band to which a particular channel is attached is fixed. Adjacent radio  
frequency bands are assigned to different cells. In analog, each channel corresponds to one  
user while in digital each RF channel carries multiple time slots or codes (TDMA / CDMA). 
 Simple to implement as traffic is similar.
GSM communications
GSM uses 124 frequency channels, each using an 8-slot time division multiplexing (TDM)  
system. There is a frequency band which is also fixed. Transmitting and receiving do not  
occur in the same time slot because GSM radios cannot transmit and receive at the same time 
 and switching from one to the other takes time. A data frame is transmitted in 547  
microseconds but a transmitter is only allowed to send one data frame in 4.615 microseconds 
 because it is sharing the channel with seven other stations. The gross rate for each channel is  
270, 833 bps divided among eight users, giving a gross of 33.854 kbps.
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