Cellular_Network

Review & intRoduction

cellulaR netwoRks & etc.

on optimal call admission contRol by Ramjee et al. undeRstanding the effects of hotspots by jobin et al. field expeRience fRom citycell & gRameen telecom

Rahat

an impoRtant technology “So called Face of 21st century communication�

Cellular telephony is one of the fastest growing technologies on the planet.

Presently, we are starting to see the third generation of the cellular phones coming to the market.

New phones allow users to do much more than hold phone conversations.

beyond voice Store contact information Send/receive email Send/receive pictures E-commerce Music Make task/to-do lists Keep track of appointments Calculator Send/receive video clips Get information from the internet Play games Integrate with other devices (PDA’s, MP3 Players, etc.)

the cellulaR concept inteRnet & fRom 6000 level couRse

basic concept Cellular system developed to provide mobile telephony: telephone access “anytime, anywhere.�

First mobile telephone system was developed and inaugurated in the U.S. in 1945 in St. Louis, MO. This was a simplified version of the system used today.

fiRst mobile telephone system One and only one high power base station with which all users communicate.

Normal Telephone System

Entire Coverage Area Wired connection

modeRn teRminologies: bs & ms • Base station (BS)

downlink

– Access point (AP)

• Mobile station (MS) – SS (Subscriber station) – MT (mobile terminal) – MN (mobile node)

• Downlink – Forward link – BSMS

• Uplink – Reverse link – MSBS

uplink BS

MS

teRminologies: cell and sectoR • Cell – Coverage area of a BS

• Sector – Partial area of a cell that is served by a directional antenna

teRminologies: tRansfeR factoR • Handoff – MS changes serving BS due to movement or radio channel variation – handover

teRminologies: cellulaR system Base station are connected by wire to a call center and call center are connected to PSTN Mobile Station needs a frequency (channel) to connect to the base station but channels are limited

teRminologies: 1g, 2g and 3g cellulaR systems Generation • 1st • 3rd Generation – AMPS

• Analog • Analog FM modulation • FDMA

• 2nd Generation – DAMPS(IS-54)

• U.S. • Digital PSK modulation • FDM/TDMA

– GSM • Europe, Asia • Digital PSK modulation • FDM/TDMA

– IS-95 CDMA • U.S. • Digital PSK modulation • FDM/CDMA

– UMTS – CDMA 2000

• Pre- 4G – – – – –

iBurst HiperMAN WiMAX WiBro GAN (UMA)

some otheR teRms • Uplink & Downlink • GSM & different bands 1900 & 850 mainly in USA 1800 & 900 mainly in Europe, Asia Uplink

1850

Ch14

Ch155

Control Gap 1910

Downlink

Ch14 1930

Ch155

1990

• Channel Separation – usually 0.2 MHz • Channel distance spacing – around 80 MHz • No of Channels – Exp: In GSM 1900 around 299 channels

some otheR teRms • EGSM - Extended GSM

Uplink

1850

Ch14

Ch155

Control Gap 1910

Downlink

Ch14 1930

Ch155

• Channel Reuse – Sub-band • New Call Blocking

– If all channels are used up, new call blocking happens

• Hand off dropping – If out of range of BS1 and into BS2, but BS2 is full, called is dropped

•Dual band - 1800 & 900 or 1900 & 850

•Tri band Duplex distance = fDL - fUL Band

Duplex Distance

No of Channels

1900

80 MHz

299

1800

95 MHz

374

900

45 MHz

124

850

45 MHz

124

- 1800, 1900 & 850

•Quad band - All 1800, 900, 1900 & 850

1990

example of fRequency Reuse

Cells using the same frequencies

channel Reuse: macRo,micRo and pico cell

Macro

(Small City)

Micro (City)

Pico

(Downtown)

SB1

BS1

BS2

BS3

SB1

SB2

SB3

SB2

SB3

SB1

SB2

SB3

SB1

SB2

SB3

SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB1 SB3

BS gets 2 types of request – new call and handoff request SB tells which mobile uses which channel

teRminologies: tessellation • Some group of small regions tessellate a large region if they over the large region without any gaps or overlaps. • There are only three regular polygons that tessellate any given region.

teRminologies: tessellation (cont’d) • Three regular polygons that always tessellate: – Equilateral triangle – Square – Regular Hexagon

Triangles

Squares

Hexagons

teRminologies: ciRculaR coveRage aReas • Original cellular system was developed assuming base station antennas are omnidirectional, i.e., they transmit in all directions equally.

nal g i s k Wea

ng o r St

al n sig

Users located outside some distance to the base station receive weak signals.

Result: base station has circular coverage area.

teRminologies: ciRcles don’t tessellate • Thus, ideally base stations have identical, circular coverage areas.

• Problem: Circles do not tessellate.

• The most circular of the regular polygons that tessellate is the hexagon.

• Thus, early researchers started using hexagons to represent the coverage area of a base station, i.e., a cell.

thus the name cellulaR • With hexagonal coverage area, a cellular network is drawn as:

Base Station

• Since the network resembles cells from a honeycomb, the name cellular was used to describe the resulting mobile telephone network.

on optimal call admission contRol

by Ramjee et al

handoff & teRmination of a call As the Mobile Subscriber moves from one cell to another, any active call needs to be allocated a channel in the destination cell This event, termed the handover or handoff, must be transparent to the end subscriber If the destination cell has no available channels, a call is terminated and this disconnection in the middle of a call is highly undesirable

the tRade-off: hand off oR new call One of the goals of the network designer is to keep the probability of such occurrences to low, also termed the handoff blocking probability On the other hand, reserving channels for handoff traffic could increase blocking for new calls.

So there is a trade-off between the two QoS measures, the handoff and the new call blocking probabilities

call admission contRol

can be defined as a process for managing the arriving traffic based on some predefined criteria.

cac is usually used to: Guarantee the signal quality Guarantee the call dropping probability Giving priority to some classes Maximize revenue Fair resource sharing Guarantee transmission rate

In general CAC is used to guarantee the QoS

call admission contRol: how?

Channel allocation.

Basestation assignment.

Scheduling or buffer management.

Power control.

Bandwidth reservation.

Available CAC algorithms focus on only some of the QoS measures

the thRee pRoblems In this paper, the authors consider the optimal admission control policies for three problems based on these two QoS measures:

MINOBJ: Minimizing a linear objective function of the two blocking probabilities

MINBLOCK:For a given number of channels, minimizing

the new call blocking probability subject to a hard constraint on the handoff blocking probability

MINC: Minimizing the number of channels subject to hard

constraints on the new and handoff call blocking probabilities

the guaRd channel policies The notion of guard channels was introduced in the mid-80s as a call admission mechanism to give priority to handoff calls over new calls.

In this policy, a set of channels called the guard channels are permanently reserved for handoff calls.

The authors then introduce the Fractional guard channel policy which effectively reserves a non-integral number of guard channels for handoff calls by rejecting new calls with some probability that depends on the current channel occupancy.

Also they show that a restricted version of this Fractional policy is optimal for the MINBLOCK and MINC problems.

basic guaRd channel policy Consider a cellular network with C channels in a given cell. The Guard channel policy reserves a subset of these channels C -T for handoff calls.

Whenever the channel occupancy exceeds a certain threshold T, the Guard channel policy rejects all new calls until the channel occupancy goes below threshold.

if (NEW CALL) then if (NumberOfOccupiedChannels < T) else

admit call; reject call;

if (HANDOFF CALL) then if (NumberOfOccupiedChannels < C) else

admit call; reject call;

basic guaRd channel policy

λ 0

λ 1

μ

λ 2

2μ

αλ

T

T+1

αλ

αλ

T+2

Tμ (T+1)μ (T+2)μ

αλ

C-1 (C-1)μ Cμ

the Guard channel policy rejects all new calls until the channel occupancy goes below threshold

C

fRactional guaRd channel policy In the Fractional Guard channel policy, new calls are accepted with a certain probability that depends on the current channel occupancy. Thus for each state we have a randomization parameter which denotes the probability of accepting a new call

if NEW CALL) then if (random(0,1) < = proability (NumberOfOcupiedChanels)) else

admit call; reject call;

if (HANDOFF CALL) then if (NumberOfOccupiedChannels < C) else

admit call; reject call;

/* random(0,1) returns a uniformly generated random number in the interval [0,1] */

fRactional guaRd channel policy γ1λ 0

γ2λ 1

μ

2 2μ

γTλ

γT+1λ γT+2λ T

T+1

γC-1λ T+2

Tμ (T+1)μ (T+2)μ

γCλ C-1

(C-1)μ Cμ

For each state we have a randomization parameter which denotes the probability of accepting a new call

C

limited fRactional guaRd channel policy For Limited Fractional Guard Policy, when the system is in T state, new calls are accepted with a probability. From state T+1 to C state, hand off call is accepted and in state 0 to T-1, both calls are accepted.

if (NEW CALL) then if (NumberOfOccupiedChannels < T) then admit call; else if (NumberOfOccupiedChannels == T) AND (random(0,1) <= probability_) else

admit call; reject call;

if (HANDOFF CALL) then if (NumberOfOccupiedChannels < C) else

admit call; reject call;

/* random(0,1) returns a uniformly generated random number in the interval [0,1] */

limited fRactional guaRd channel policy λ 0

λ 1

μ

λ 2

2μ

α+(1- α) β)λ αλ

T

T+1

αλ

T+2

Tμ (T+1)μ (T+2)μ

αλ

C-1 (C-1)μ Cμ

From state T+1 to C state, hand off call is accepted and in state 0 to T-1, both calls are accepted.

C

concluding RemaRks The authors showed that the Fractional guard channel policy resulted in significant (20-50%) savings in the new call blocking probability over the integral Guard channel policy Furthermore, they showed that the algorithms developed in the paper were computationally inexpensive and have potential for use as a real-time control mechanism in cellular networks.

undeRstanding the effects of hotspots by jobin et al

hotspots Hotspots can occur whenever there is contention among users for the bandwidth resources at some location in a network.

This could potentially lead to blocked and dropped users and thus impact the performance So understanding and modeling hotspots is important for conducting realistic simulations.

hotspots In the paper the authors identified some causes for the formation of hotspots and categorized hotspots into three different types. Also they showed how these types have different effects on network performance Most of the previous studies simply assume that a hotspot cell is more loaded than the normal cells by some arbitrary factor. In this paper the authors describe three different types of hotspots that can arise due to different reasons.

thRee types of hot spot: delay based Delay based: A hotspot based on delay can happen if there is an accident on some street which is holding up traffic and delaying all the users in that cell. They implement this type of a hotspot by increasing the average time spent by the users in one or more cells that have been identified as hotspots.

thRee types of hot spot: capacity based Capacity based: This type of a hotspot can happen in a network when the base station of a cell is undergoing some technical problems and therefore, can only support a lower number of users. They implement this by choosing a cell as a hotspot cell and decreasing the capacity in that cell. Here, capacity is the number of users the cell can support.

thRee types of hot spot: pRefeRential mobility based Preferential mobility based: Such a hotspot could occur when there is an event and people are moving towards a given location thereby increasing the number of users in that spot. To implement such a hotspot, they make the users select the hotspot cell as the preferred destination with a greater probability than the other cells

the expeRiments In the experiments, the authors use a square cell instead of the traditionally used hexagon cell.

A square cell will have only four neighbors as compared to six neighbors for a hexagon cell. However the shape of the cell does not make a difference in the above three scenarios In the experiments, they select one or more cells to be hotspots and implement the hotspot(s) using one of the three methods

the Results Hotspots can be of different types and their impact on the system performance varies with the type More hotspots lead to lower utilization in the delay based hotspot case. This is until the point where exactly half the cells are hotspots; after that utilization rises. The worst case is where exactly half the cells are hotspots In the capacity based type, worst case is when all cells are hotspots In the preferential mobility hotspot, the worst performance is when there is a single hotspot.

The authors use utilization as a measure of performance.

They define network utilization as the ratio of the number of users in the network to the number of users that the network can support .

concluding RemaRks Clustering of hotspots does not make a difference in the delay based or capacity based hotspots. However, in the preferential mobility based hotspot, fewer clusters lead to lower utilization; indeed, the worst case is when there is only a single cluster.

The delay based and capacity based hotspots affect only the users in the hotspots whereas the preferential mobility model impacts the network on a global scale with cells closer to the hotspot being affected more.

field expeRience fRom citycell & gRameen telecom

countRy pRofile: bangladesh 1. Area

: 144000 Sq.Km

2. Population

: 153,546,896 (July 2008 est.)

3. Population density

: about 880/ sq. Km.

4. Divisions

:6

5. Districts

: 64

6. Sub Districts (Upazila) : 465 7. Unions (rural)

: 4484

8. Villages (rural)

: 87319

9. Municipalities (urban ) : 223 10. City Corporation

:6

11.Per capita GDP

: About USD 1400

• • • • • • • • • • •

telecom infRastRuctuRe in bangladesh

Fixed line Telephone : 0.93 Million, (digital: 0.79 M) (Public Sector) Digitalized District : 64 Digitalized Sub district : 465 out of 465, 100% Coverage will be within 2005) Mobile Telephone : 30.00 Million (Private Sector) Teledensity : 29.6 line per 100 people (PSTN & Mobile) Bangladesh Telecommunication Regulatory Commission established in 2002 Telephone call charge reduced significantly Recently fixed line phone sector has been opened for private sector investment.

Mobile telecoverage reaches most of the rural areas 182000 Grameen Phone serving remotest areas Recently VOIP legalized

telecom infRastRuctuRe in bangladesh 1800 KM fibre optic cable under Bangladesh Railway is being utilizad by the private mobile telephone operator Fibre optic link between few districts existing under BTTB 

Packet Switching Exchange in 9 big District HQ

established for data communication by BTTB BTTB’s 1 million mobile phone project is yet to be

implemented in 2005 Fibre optic link between all district is planned by

BTTB

field woRk in dhaka at citycell & gRameen phone Citycell (Pacific Bangladesh Telecom Limited) is the first mobile communications company of Bangladesh. It is the only CDMA network operator in the country. As of 1st March, 2008, Citycell's total mobile subscriber base is 1.56 million, up 137 per cent or 680,000 from two years ago, giving it the best growth rate of the company till date.

Citycell is currently owned by Singtel with 45% stake and the rest 55% owned by Pacific Group and Far East Telecom.

Grameenphone is a GSM-based cellular operator in Bangladesh.

Grameenphone started operations on March 26, 1997. It is partly owned

by Telenor(62%) and Grameen Telecom (38%).

Grameenphone is the largest mobile phone company in Bangladesh with 20.84 million customers as of August, 2008. It is also one the fastest growing cellular telephone network in Bangladesh.

field woRk in dhaka at citycell & gRameen phone In the first few days of my internship they taught me the basics of the whole process in a BTS station.

I used to go out with the engineers team for survey of the field and through some complicated process they tried to establish new BTS station and improve performances.

Also they try to find out the congestion area of the cities and out skirt of the city. Try to check the landscape and available signal in the gadgets. Usually we used roam from city area to city area to cover the most dense locality.

field woRk in dhaka at citycell & gRameen phone Usually the engineers had a GPRS enable car with lot of antenna and gadget inside.

When we traveled, we used to get the map of the city and try to go along the roads to cover the telecom area. Also we tend to find the market places, stadiums, mosques and other busiest area to find more congested places

This picture is the City Cell main office where most of the main activity goes around.

field woRk in dhaka at citycell & gRameen phone Also the Dhaka has become a big high rise capital and for that it was always helpful to find buildings which are very noticeable and huge. As the big building usually block coverage area. As for the engineers, they tried to get to the commercial building from the map and try to find out the sustainability of establishing new BTS in the area.

Usually the top most building of city parts had always been identified and leased.

At Grameen phone I had learnt some basic introductory stuffs such as Switch control mechanism, handoff criteria, SMS technology, BTS introduction, Switching properties, etc.

Next few Slides are brief animation on the whole handoff process which is really useful to know

types of handoff â&#x20AC;&#x201C; hieRaRchy of hlR/vlR/msc (1) (2) (3)

Intra-MSC handoff Inter VLR-handoff Inter MSC handoff + Inter VLR handoff

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gsm handoff 1. Basic handoff 2. Subsequent 3. Mobile-assisted handoff

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Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III BS

Prepare Subsequent HO All. HO#

Handoff Command MS Ack Handoff Complete

MSC I

Prepare Ho Ack IAM ACM

Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III BS

Prepare Subsequent HO All. HO#

Handoff Command MS Ack Handoff Complete

MSC I

Prepare Ho Ack IAM ACM

Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III

MSC I BS

Prepare Subsequent HO All. HO#

Handoff Command MS Ack Handoff Complete Clear

Prepare Ho Ack IAM ACM

Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III

MSC I BS

Prepare Subsequent HO All. HO#

Prepare Ho Ack IAM ACM

Handoff Command MS Ack Handoff Complete Clear

Send End Signal

Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III

MSC I BS

Prepare Subsequent HO All. HO#

Prepare Ho Ack IAM ACM

Handoff Command MS Ack Handoff Complete Clear

Send End Signal REL

Anchor

BS MS

VLR MSC II

Channel Meas.

MSC III

MSC I BS

Prepare Subsequent HO All. HO#

Prepare Ho Ack IAM ACM

Handoff Command MS Ack Handoff Complete Clear

Send End Signal REL RLC

gsm handoff - maho (moblie assisted handoff)

MSC

Old BS

MS

New BS

MSC

Old BS Measurement Report

MS

New BS

MSC

Old BS Measurement Report

MS

New BS

MSC

Old BS

MS

Measurement Report Measurement Report

New BS

MSC

Old BS

MS

Measurement Report Measurement Report

New BS

MSC

Old BS

MS

Measurement Report Measurement Report Measurement Report

New BS

MSC

Old BS

MS

Measurement Report Measurement Report Measurement Report

New BS

MSC

Old BS

MS

Measurement Report Measurement Report Measurement Report

New BS

MSC

Old BS

MS

Measurement Report Measurement Report Measurement Report Handoff Order

New BS

MSC

Old BS

MS

New BS

Measurement Report Measurement Report Measurement Report Handoff Order Handoff Access

MSC

Old BS

MS

New BS

Measurement Report Measurement Report Measurement Report Handoff Order Handoff Access Handoff Access

MSC

Old BS

MS

New BS

Measurement Report Measurement Report Measurement Report Handoff Order Handoff Access Handoff Access

Handoff Complete

MSC

Old BS

MS

New BS

Measurement Report Measurement Report Measurement Report Handoff Order Handoff Access Handoff Access

Handoff Complete

some thoughts Sky is still the limit but cell phone did not reached half of its limit yet