Lecture 05 cellularsatellitenetwork wap

  • 48 trang
  • file .pdf
Cellular & Satellite Network
Dr. Nguyen Tuan Nam
History of Cellular Network
• First telephone system:
– A single radio transmitter
– A single channel was used
– Half-duplex: press button to talk, and release
it to listen
• Cellular radio system
– Analogue
– Digital
Nguyen Tuan Nam/WNC 2011 2
The First-generation System
• Analogue
• Started in the 1980s
• No worldwide coordination for the
development of technical standards for the
system
• Roaming was not possible
• Efficient use of the frequency spectrum
was not there
Nguyen Tuan Nam/WNC 2011 3
The Second-generation System
• Mid-1980s, the European commission started a series of activities to liberalize the
communication sector, including mobile communication
• Global System for Mobile Communication (GSM)
– First specifications
– The network is based on digital technology
– 1990s
– Capable of providing all the basic services such as speech and data services
• GSM and VAS (Value Added Service)
– Voice Mail System (VMS)
– Short Message Service Center (SMSC): proved to be incredibly commercially successful
• GSM and GPRS (General Packet Radio Services)
– Possible to send packet data on the air-interface
• GSM and EDGE (Enhancement Data rates in GSM Environment)
– More sophisticated coding methods over the Internet to increase the data rate
• 2G phone systems were characterized by
– Digital circuit switched transmission
– Introduction of advanced and fast phone-to-network signaling
• Smaller phone. Why?
Nguyen Tuan Nam/WNC 2011 4
The Third-generation Networks
• In EDGE
– Packet transfer on air-interface behaves like a circuit
switch call à low efficiency
– Standards for developing the networks were different
for different parts of the world
• Standardized in the IMT-2000 standardization
processing
– Not standardize on a technology
– Instead, standardize in a set of requirements (data
rate)
• WCDMA for UMTS
Nguyen Tuan Nam/WNC 2011 5
The Fourth-generation Networks
• Not discussed here
Nguyen Tuan Nam/WNC 2011 6
Cellular Network
• Radio network made up of a number of radio
cells (cells)
• Each cell served by at least one fixed-location
transceiver (known as cell site or base station)
• These cells cover different land areas à provide
radio coverage over a wider area than a cell
• A variable number of portable transceivers can
be used in any one cell and moved through
more than one during transmission
Nguyen Tuan Nam/WNC 2011 7
Cellular Network Architecture
• Mobile station (MS): A device used to communicate over the cellular
network.
• Base station transceiver (BST):
– A transmitter/receiver used to transmit/receive signals over the radio
interface section of the network.
– Fair amount of computing power, correcting errors in the received
signal and encrypting the conversation
• Mobile switching center (MSC): The heart of the network which sets
up and maintains calls made over the network.
• Base station controller (BSC): Controls communication between a
group of BSTs and a single MSC.
• Public switched telephone network (PSTN): The land based section
of the network.
Nguyen Tuan Nam/WNC 2011 8
Cellular Network Architecture
Hierarchical fashion. Why?
Nguyen Tuan Nam/WNC 2011 9
Switches
• The very first switches
– Human manning a switchboard
– You phoned the switchboard
– Told them which line you wanted to be connected
– They plugged your phone line into the appropriate
socket
• Automatic switch
– Invented by Strowger
– Relay
Nguyen Tuan Nam/WNC 2011 10
The Structure of a Simple Switch
Nguyen Tuan Nam/WNC 2011 11
Frequency reuse (FDMA)
• Problem:
– Only one transmission can be used on any given frequency
– Number of available frequencies is very limited
• “Spectrum is like real estate – they just don’t make it anymore”
– A cellular operator has typically been given
• 25MHz of radio spectrum
• Each individual requires 25KHz in order to make a call
à Maximum 1,000 subscribers
• Solution:
– Same frequency can be reused in a different area for a completely
different transmission
– But some level of interference from cells sharing the same frequencies
à There must be at least a one cell gap between cells which reuse the
same frequency
Nguyen Tuan Nam/WNC 2011 12
Frequency reuse
• The frequency reuse factor:
– The rate at which the same frequency can be used in
the network.
– It is 1/K (or K according to some books) where K is
the number of cells which cannot use the same
frequencies for transmission.
– Common values for the frequency reuse factor are
1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12
depending on notation).
• N sector antennas
– One base station can have N sector antennas, each
with different direction
Nguyen Tuan Nam/WNC 2011 13
Examples
F1 F1
F1
F3
F2 F2 F3
F4 F2
F1 F1
F3
F2 F2
F4
Nguyen Tuan Nam/WNC 2011 14
CDMA
• Adjacent base station sites use the same
frequencies
• The different base stations and users are
separated by codes rather than frequencies
• Requires certain signal-to-noise ratio to operate
• Near-far problem:
– As receiver moves away from transmitter à the
power transmitted is reduced à signal becomes
corrupted and unusable
Nguyen Tuan Nam/WNC 2011 15
Advantages of Cellular Network
• Reduce power usage
• Larger coverage area
• Increase capacity
• Reduce interference from other signals
Since most mobile phones use cellular network à cell phones
In cities, cell site range is shorter compared to rural areas
Nguyen Tuan Nam/WNC 2011 16
Setting up a Call
• Polling scheme
– A control channel exists permanently between mobile stations and base station
– Base station send a request for a call setup to mobile station using the control
channel
– Mobile station continually poll channel for connection request
– If mobile station and radio resources are available àa dedicated voice circuit is
setup à voice circuit only exist when necessary, and then get destroyed
• Original analogue representation of speech is encoded into digital
representation using a speech coder:
– Well known pulse code modulation (PCM) encoding scheme used in most public
switched telephone networks (PSTN) output data at a rate of 64 kbps
– GSM only 34kbps àRPE-LC encoding scheme at 25kbps (added overhead)
• GSM systems operate in a discontinuous transmission mode:
– No data is transmitted during a user's silent period
– Each person on average speaks no more than 40% of the time à effective
Nguyen Tuan Nam/WNC 2011 17
Handover/Handoff
• Mobile transceiver moves from one cell to
another during ongoing continuous
communication à switch from one cell
frequency to a different cell frequency
• Intra-cell handoff vs. inter-cell handoff
• Soft-handoff vs. Hard handoff
Nguyen Tuan Nam/WNC 2011 18
Registering
Per-user location caching
Pointer forwarding
Local anchoring
Nguyen Tuan Nam/WNC 2011 19
Roaming
• A service that enables customers of a particular
network operator to make calls from areas not
served by that network operator
• Network operator within the area of the call
initiation contacts a Gateway MSC (GMSC),
which links the user to its own network operator
• Using mobile station ISDN number (MSISDN),
which uniquely identifies a mobile station
• MSISDN consists of a country code (CC),
national destination code (NDC) and a subscriber
number (SN).
Nguyen Tuan Nam/WNC 2011 20