Transcript Cellular Phone Networks

Cellular Phone Networks
Some slides are based on: Computer
Networking: A Top Down Approach Featuring
the Internet, 3rdedition. Jim Kurose, Keith
RossAddison-Wesley, July 2004.
Cellular Phone Networks
• Compared to WLANs
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Longer range
Less speed
Higher mobility
More for voice services (evolving!)
• Overview
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Divides the area into cells.
Base stations in each cells.
User have cellular phones.
The phones talks to the base station directly in
wireless.
GSM
• Global System for Mobile (GSM)
– Used by more than 3 billion people
– 2nd Generation (2G), because everything is digital,
compared to the analog mobile phones which is 1G
– Operates in 900MHz and 1800MHz band, or 850M
and 1900M bands in the US
– Uplink and down link both 25MHz wide. In GSM900,
uplink band is 890-915M, downlink is 935-960M.
– A band is divided into 124 channels with 200KHz
spacing. The data rate is about 270Kbps for a channel.
GSM
• GSM continued
– The time in a channel is divided into 8 slots. Each slot is 577us and is
allocated to a user.
– A mobile phone is allocated two channels, one for uplink and the
other for downlink. Separated by 45MHz.
– The uplink and downlink slot numbers are separated by 3 – a phone
never transmits and receives at the same time
– FDMA/TDMA (Frequency Division Multiple Access / Time Division
Multiple Access).
– Different from In wireless LAN, in which a node is given the entire
bandwidth for the time it needs to send a packet. Difference due to
the nature of the application.
– The peak transmission power of a GSM phone is 2W in 900M band
and 1W in 1800M band. In contrast, your wireless router transmits at
20dBm, which is 0.1W. Difference due to the distance.
GMSK modulation
• GSM uses Gaussian minimum-shift keying
(GMSK).
– MSK: basically replaces {1,-1} with a positive half
sine wave or a negative half sine wave, and
multiply it with the carrier.
– GMSK: first pass the digital waveform through a
Gaussian low pass filter.
– Reduces interferences
Frequency Reuse
• Adjacent cells should not use the same
frequency, because it will cause interference.
• However, non-adjacent cells may use the same
frequency.
• Due to frequency reuse, the actual number of
channels used in an area is larger than 124.
• Given an area, reducing the size of cells (reducing
the transmit power) increases the frequency
reuse, hence increasing the total capacity.
• Cell planning is a major issue.
Frequency Reuse
• Cell sizes in GSM:
– Large microcell: 3-30km
– Small microcell: 1-3km
– Microcell: 0.1-1km
– Picocell: 0.01-0.1km
– Nanocell: 0.001-0.01km
• Typically, there are lots of small cells plus
several large cells. The small cells carries the
majority of the traffic. The large cells fills the
holes.
Capacity
• Given the number of channels in a cell, the
maximum number of users supported is fixed.
– With GSM, if there are C channels, we can support
8C users at most
• The number of users can be much larger than
the maximum number of slots. That is why
some time you see “emergency calls only.”
• The hope is that not all users want to make
calls at the same time.
The Erlang-B Formula
• The phone company wants to allocate enough
channels such that the probability of dropping
a call is below a threshold.
• There is an old formula, the Erlang-B Formula,
to calculate this. (It is probably 100 years old.)
• Check: http://wireless.per.nl/reference/chaptr04/erlang/erlangb.htm
The Erlang-B formula
• Given N channels, where each channel supports a
user. A new call can be served if and only if there
is an available channel; otherwise it is dropped.
• Assume the calls arrive following a Poisson
process, that is, the inter-arrival time between
two calls follows exponential distribution with
parameter . Assume the duration of a phone
call follows the exponential distribution with
parameter .
• What is the probability of call dropping?
Exponential Distribution
• The p.d.f. of exponential distribution
• A unique feature. Suppose you are waiting for a
bus. The bus is following a random schedule, and
the inter-arrival time follows the exponential
distribution. If you just missed a bus, the
probability you have to wait t seconds is given
above. But, given that you have waited t0
seconds, what is the probability you have to wait
another t seconds?
Exponential Distribution
• The probability that you have to wait for t
seconds given you have waited t0 seconds is
• We know that
and
• So,
• That is, no matter how long you waited, you look
like you didn’t wait at all! Memoryless.
The Erlang-B Formula
• Consider the phone system. It can be fully
described by the number of current users, if
the inter-arrival time of calls and the call
duration both follows the exponential
distribution.
– Because regardless of how long you have waited
for the next arriving call or for a call to finish, it
looks like you didn’t wait for any time at all
– In other words, by assuming the exponential
distribution, the system is memoryless.
Markov Chain
• The number of current phone calls will change with time:
– When a new call arrives
– When a call finishes
• Given that there are i ongoing calls, or, when the system is
in state i, the next state is either i-1 or i+1.
– Time is fine-grained and no two events happen at the same
time.
• The system is a Markov Chain if the probability to go from
the current state to a next state is only determined by the
current state, but not previous states.
• Because the system is memoryless, it is a Markov chain.
The Erlang-B formula
• Probability to go from state i to state i-1
– Event happens when a call finishes before a new
call arrives
• Probability to go from state i to state i+1
– Event happens when a new call arrives before an
existing call finishes
The Erlang-B formula
• The probability that none of the i existing calls
finishes after time t is
• Therefore, the probability that the first call
finishes at time t among the i calls is
The Erlang-B formula
• The probability that a new call arrives after the
first existing call finishes is
• So, at state i, with probability
, the next state
is i-1; with probability
, the next state is
i+1.
The Erlang-B formula
• Suppose you know that the probability that
there is no existing call is p0. What is the
probability that there is one ongoing call?
0
1
2
N
• Note that in equilibrium, the number of 0 to 1
transitions is the same as 1 to 0 transitions.
The Erlang-B formula
• You can apply similar arguments to get
where
.
• Then, considering that all probabilities
summing up at 1, you will get pi for all
0<=i<=N:
The Erlang-B formula
• Finally, the probability of blocking is the
probability that a call arrives when the system
is in state N.
• Because Poission traffic is raondom, it is
simply pN.
http://elm.eeng.dcu.ie/~kaszubow/Biography/Lecture5.pdf
Management of GSM
• Base Transceiver Station (BTS)
– Has 1 to 16 transceivers
• Base Station Controller (BSC)
– Controls hundreds of BTS
• Mobile Switching Center (MSC)
– Typical MSC supports up to 100,000 mobiles and 5000 simultaneous
calls
– MSC are connected with each other.
– Gateway MSC connects the GSM system to external networks, e.g.
PSTN.
– Each MSC controls at least one Base Station System (BSS)
• Authentication Center (AUC)
• Operations and maintenance center (OMC)
• …
GSM
• Visitor’s Location Register (VLR): Each MSC connects to a VLR. The VLR is a
data base with the information about cellphones temporarily located in
the area served by particular MSC.
• Home Location Register (HLR): database of all cellphones permanently
registered in the system. Stores
– Current location of the phone.
– All parameters needed by the system to establish a connection with
the user.
– The address of the VLR currently associated with the phone
– Encryption keys for data transmission and user authentication
– …
GSM Call Flow (Simplified)
1.
2.
3.
4.
5.
6.
7.
8.
9.
User enters the phone number and presses the “send” button.
To set up the phone call, the phone needs to send information to the MSC. The phone
sends “Radio Resource Channel Request” to the associated BSS. The phone then waits to
hear from the BSS at the Access Grant Channel (AGCH). The request is sent on the Random
Access Channel (RACH) according to ALOHA.
The BSS allocates a Traffic Channel (TCH), including the frequency and time slot, and
broadcast it in the AGCH. It also contains information about time and frequency
corrections.
The phone applies the corrections and tune to the assigned TCH.
MSC checks whether the phone is authenticated.
The BSS enables ciphering with the phone. At this step the connection has been set up
between the phone and MSC. The BSS just forwards the message.
The phone sends a connection set up request to the MAC with the called phone number.
The MSC connects to the PSTN and allocates the voice communication channel between
the BSS.
Make the conversation.
User presses the “end” button. The MSC releases the voice channel with the BSS. The MSC
informs the PTSN about the call release and the PTSN will inform the call has been released
on its end. The phone then releases the TCH.
GSM: indirect routing to mobile
home
network
HLR
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
correspondent
home
Mobile
Switching
Center
1
3
VLR
Mobile
Switching
Center
4
Public switched
telephone
network
home MSC sets up 2nd leg of call
to MSC in visited network
mobile
user
visited
network
27
call routed
to home network
MSC in visited network completes
call through base station to mobile
GSM: handoff with common MSC
• Handoff goal: route call via new
base station (without interruption)
• reasons for handoff:
VLR
Mobile
Switching
Center
old
routing
old BSS
new
routing
new BSS
– stronger signal to/from new BSS
(continuing connectivity, less
battery drain)
– load balance: free up channel in
current BSS
– GSM doesn’t mandate why to
perform handoff (policy), only
how (mechanism)
• handoff initiated by old BSS
28
GSM: handoff with common MSC
VLR
4
1
8
old BSS
29
5
Mobile
Switching
Center 2
7
3
6
new BSS
1. old BSS informs MSC of impending handoff,
provides list of 1+ new BSSs
2. MSC sets up path (allocates resources) to
new BSS
3. new BSS allocates radio channel for use by
mobile
4. new BSS signals MSC, old BSS: ready
5. old BSS tells mobile: perform handoff to
new BSS
6. mobile, new BSS signal to activate new
channel
7. mobile signals via new BSS to MSC: handoff
complete. MSC reroutes call
8 MSC-old-BSS resources released
GSM: handoff between MSCs
• anchor MSC: first MSC
visited during call
home network
correspondent
Home
MSC
anchor MSC
PSTN
MSC
MSC
MSC
(a) before handoff
30
– call remains routed through
anchor MSC
• new MSCs add on to end of
MSC chain as mobile moves
to new MSC
• IS-41 allows optional path
minimization step to
shorten multi-MSC chain
GSM: handoff between MSCs
u
home network
– call remains routed through
anchor MSC
correspondent
Home
MSC
u
anchor MSC
PSTN
MSC
MSC
(b) after handoff
31
anchor MSC: first MSC
visited during cal
MSC
u
new MSCs add on to end of
MSC chain as mobile moves
to new MSC
IS-41 allows optional path
minimization step to
shorten multi-MSC chain
General Packet Radio Service
(GPRS)
• General Packet Radio Service
– Supports data service.
– GSM with GPRS is often called 2.5G, because it is
providing a moderate level of data service
– Supports IP, PPP (Point to point protocol). The
mobile device can be used as a modem.
– Different coding scheme can be used, CS-1 to CS4, with different over head. Per time slot, the data
rate is 8, 12, 14.4, 20.2 kbps.
GRPS
• Multiple Access
– Users are assigned frequency channels and time
slots.
– Packets are constant length, determined by the
GSM slot.
– Downlink: first come first served
– Uplink: Slotted ALOHA for reserving, dynamic
TDMA for data transmission
GSM SIM
GSM Security
Reading
•
http://www.eventhelix.com/realtimemantra/Telecom/GSM_Originating_Call_Flow.pdf