Cellular Wireless Networks
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Transcript Cellular Wireless Networks
Cellular Wireless Networks
Example of a Cellular Wireless
Network
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1G Cellular Networks
1st
generation cellular networks are purely
analog cellular systems.
The transmission of data is sent via a
continuously variable signal
2G Cellular Networks
2nd
generation cellular networks refer to
digital cellular and PC wireless systems.
voice and low speed data services.
They consist of digital traffic channels,
perform encryption, error detection &
correction
Users share channels dynamically
3G Cellular Networks
3rd generation refers to the next generation of
wireless systems.
This is digital with high speed data transfer
It is voice quality comparable with a switched
telephone network.
Data transmission rates can be asymmetric or
symmetrical
It provides support for circuit switched and
packet switched data services
Cellular Operation
Three basic devices
A mobile station
A base transceiver
A Mobile Telecommunications Switching Office
(MTSO)
Cellular Operation
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Cellular Network Organization
Base Station (BS)
includes an antenna, a controller, and a number of receivers
Mobile Telecommunications Switching Office (MTSO)
connects calls between mobile units
Two types of channels available between mobile unit and
BS
•
Control channels
•
•
used to exchange information having to do with setting up and
maintaining calls (out-band or in-band through stealing bits)
Traffic channels
•
carry voice or data connection between users
Cellular Operation
Public Land Mobile Network (PLMN) refers to a
cellular network that has land and radio based
sections.
This network consists of:
•
•
Mobile station (MS): A device used for communication
over the network.
Base station transceiver (BST): A transmitter/receiver
used to transmit/receive signals over the network.
Cellular Operation
Mobile switching center (MSC): Sets up and
maintains calls made over the network.
Base station controller (BSC): Communication
between a group of BSTs and a single MSC is
controlled by the BSC
Public switched telephone network (PSTN): Section
of the network that is land based
Cellular Operation
Outgoing from mobile
input
phone number and press send
mobile links to base transceiver via control
channel
base to MTSO to PSTN
MTSO routes connection back to mobile via
voice channel
mobile shifts from control to voice
Cellular Operation
Incoming to mobile
call
goes from PSTN to MTSO
on control channel, MTSO searches for mobile
by PAGING every active mobile
If found, MTSO rings it and establishes voice
channel connection
uses transceiver with strongest signal from
mobile
Cellular Network Organization (Cells)
Cells use low powered transmitters.
Each cell is allocated a band of frequencies, and
is served by its own antenna as well as a base
station consisting of a transmitter, receiver and
control unit.
Hexagon Reuse Clusters
Cellular Coverage Representation
Frequency Reuse
Each
colour/letter
uses the same
frequency band
Picture: netlab.cis.temple.edu/~jmulik/teaching/8550s03-slides/ 8550-Cellular-14.sxi.pdf
3-cell reuse pattern (i=1,j=1)
4-cell reuse pattern (i=2,j=0)
7-cell reuse pattern (i=2,j=1)
12-cell reuse pattern (i=2,j=2)
19-cell reuse pattern (i=3,j=2)
Relationship between Q and N
Factors limiting frequency reuse
Co-channel interference
Adjacent channel interference
Adjacent Channel Interference
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Adjacent channel interference can be controlled with transmit and
receive filters
Coping with increasing capacity
Adding new channels
Frequency borrowing
frequencies are taken from adjacent cells by
congested cells
Picture: www.its.bth.se/courses/etc019/handouts/ ch10_Cellular_wireless_netw.pdf
Coping with increasing capacity
Cell splitting
Cell sectoring
cells in areas of high usage can be split into
smaller cells
cells are divided into a number of wedgeshaped sectors, each with their own set of
channels
Microcells
antennas
posts
move to buildings, hills, and lamp
Cell Splitting
Site Configurations
Handoffs
Network protocols must refresh and renew
paths as a mobile station host moves
between cells.
Handoffs are the function of one cell
handing over the communication link
between itself and a mobile station as the
mobile station moves out of the boundary
of its region into the boundary of an
adjacent cell.
Handoffs
This practice must preserve end-to-end
connectivity in a dynamically reconfigured
network topology.
Handoff Types (cont’d)
Avoiding handoff: Umbrella cells
Encoding:
Modulation(1)
Amplitude
Modulation
Frequency Modulation
Phase Modulation
are the three different methods of encoding
binary information on a regular wave.
Encoding:
Modulation(2)
When using digital signals the methods are
known as Amplitude Shift Keying (ASK),
Frequency Shift Keying (FSK), and Phase
Shift Keying (PSK).
Encoding:
Multiplexing(1)
Multiplexing allows many mobile users to use
cellular radio transmission schemes at the
same time. The different schemes are:
Frequency Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing
Encoding:
Multiplexing(2)
Frequency Division Multiplexing involves a different
frequency channel given to each user
Encoding:
Multiplexing(3)
Time Division Multiplexing involves a channel
with a given number of time slots (per
millisecond) where each user is assigned certain
time interval.
Code Division Multiplexing gives each user a
“code” for differentiation purposes. The receiver
picks out each channel from the “noise” using
the code. Wide frequency band is used. Does
not contain single frequencies or time slots.
Differences between FDMA, TDMA,
and CDMA.
Advantages of
Code Division Multiplexing
better protection against interference
good security
signal difficult to jam
Disadvantages of
Code Division Multiplexing
pseudo-random code sequences
generated by the transmitters and
receivers are not always random
fast power control system needed so that
strong signals don’t overpower weaker
signals.
Analogy:
Multiplexing
Lectures at a learning institute:
Frequency Division: takes place in
different rooms
Time Division: taking turns in a single
room
Code Division: lectures on different
subjects.
Wired vs. Wireless
Wired vs. Wireless (1)
A problem with wireless networks is that
anyone with a wireless network card is able
to access this network and is potentially
harmful since they are able to corrupt and
steal important files.
These networks transmit data over an area
such that the network signals may penetrate
physical areas such as walls.
Wired vs. Wireless (1)
Although this problem is relevant to a wired
network also, it exists to a greater degree
in a wireless network.
With regard to wired networks, the
electromagnetic waves that are given off
from the current traveling through the
network cables.
Advantages of wireless
networks
Flexible
Cost is less (long term)
Mobile user choice
Accesses areas that wired networks
cannot reach
Disadvantages of wireless
networks
Compared to wired networks the data
rates are slower
User location determines performance
Devices such as microwaves, cordless
phones, etc may cause interference
Can be accessed by hackers from the
outside
Wired Equivalent Privacy
Algorithm (WEP) (1)
Wireless networks may include additional
security elements, which are not
supported in wired networks.
For eg. The use of security algorithms such
as WEP (Wired Equivalent Privacy
Algorithm), that uses an encryption
algorithm which deals with unauthorized
access to the network (eavesdropping).
WEP (2)
WEP is a implemented such that a block of
plaintext (input text) is bitwise XORed with an
equal length random key sequence. A
random number generator is used on the
initialization vector and the secret key and
outputs a key sequence of random of octets.
An integrity check value is produced to protect
against data modification.
The key sequence combined with plaintext
combined with the integrity check value gives
the enciphered message. The integrity check
value and the ciphertext is the combination of
the output.
Block Ciphers
Another security algorithm is Block Ciphers which
is the most common of the encryption
techniques. The Block Cipher consists of:
Data Encryption Standard (DES)
Triple Data Encryption (TDEA)
Advanced Encryption Standard (AES).
The GSM Network
The Global System for Mobile
communications.-most widely used digital
cellular communications system
The GSM Network
Picture: http://www.alphaorg.com/cellular/
Mobile Station
Mobile equipment or terminal
3
types: fixed, portable, handheld
Subscriber Identity Module (SIM).
The Base Station Subsystem (BSS)
The Base Station Subsystem connects the
Mobile Station and the Network and
Switching Subsystem. It is responsible for
transmission and reception and can be
divided into two parts:
The
Base Transceiver Station (BTS) or Base
Station.
The Base Station Controller (BSC).
The Base Transceiver Station (BTS)
The BTS corresponds to the transceivers
and antennas used in each cell of the
network. It is usually placed in the center
of a cell. The size of a cell defines the
transmitting power. A BTS can have up to
16 transceivers. It all depends on the
density of users in the cell.
The Base Station Controller (BSC)
The BSC controls a group of BTS.
A BSC is primarily responsible for
handovers, frequency hopping, exchange
functions and control of the frequency
power levels of the BTSs.
The Network and Switching
Subsystem (NSS)
Its main role is to manage the
communications between the mobile users
and other users, such as mobile users,
ISDN users, landline users, etc.
It also has databases that store
information about the subscribers and to
manage their mobility.
The different components of the NSS are
described below.
The Mobile services Switching Center
(MSC)
Central component of the NSS.
It performs switching functions and also
provides connection to other networks.
The Gateway Mobile services
Switching Center (GMSC)
A gateway is a node that interconnects
two networks.
The GMSC is the interface between the
mobile cellular network and the Public
Switched Telephone Network (PSTN).
It is responsible for routing calls from the
fixed network towards a GSM user.
Home Location Register (HLR)
The HLR is an important database as it stores
information about the subscribers belonging to
the covering area of a MSC.
It stores the current location of these
subscribers and the services to which they have
access.
The location of the subscriber corresponds to
the SS7 address of the Visitor Location Register
(VLR) associated to the terminal.
Visitor Location Register (VLR)
The VLR contains information from a
subscriber's HLR.
It is necessary for the provision of subscribed
services to visiting users.
When a subscriber enters the covering area of a
new MSC, the VLR associated to this MSC will
request information about the new subscriber to
its corresponding HLR.
The VLR will then have enough information to
assure the subscribed services without needing
to confirm with the HLR each time a
communication is established.
The Authentication Center (AuC)
The AuC register provides the parameters
needed for authentication and encryption
functions.
These parameters help verify the user's
identity.
The Equipment Identity Register
(EIR)
The EIR is also used for security purposes.
It is a register containing information
about the mobile equipment.
It particularly contains a list of all valid
terminals.
A terminal is identified by its International
Mobile Equipment Identity (IMEI).
The EIR uses this to forbid calls from
stolen or unauthorized terminals.
The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to
various networks for data
communications.
During these communications, the
transmission of speech and data can be
alternated.
The Operation and Support
Subsystem (OSS)
The OSS is connected to the different
components of the NSS and BSC in order
to control and monitor the GSM system.
It also takes care of controlling the traffic
load of the BSS.
In GSM, there are five main functions:
Transmission.
Radio
Resources management (RR).
Mobility Management (MM).
Communication Management (CM).
Operation, Administration and Maintenance
GSM
GSM uses an intriguing method to send
data. By simply sending computer data as
it would send voice data, GSM allows
every phone to be "data enabled."
Since the GSM network is already a packet
network of sorts because of its frequency
hopping, it requires no additional
hardware to support data.
GSM allows data rates in multiples of 300
bits per second, up to 64 kilobits per
second.
Advantages of Cellular Networks
Cellular networks use "small" cells with low
powered transceivers instead of one large area
with a high powered transceiver.
Using cellular networks increases overall call
handling capacity.
Avoids central point of failure.
Allows dynamic distribution of capacity based on
demand.
Less interference with other wireless
communications
Conclusion (1)
Mobile phone contracts provide a useful and
relatively cheap service.
Cellular radio network infrastructures are
growing at a tremendous rate.
Cellular networks are becoming high speed
data networks.
Conclusion (2)
Speeding up the development of mobile
communication technologies is the
focus of the industry. The ideal
communication system where both
voice and data services can be
delivered regardless of location,
network, or terminal.