Telecoms Network Design

T-110.300Timo Kiravuo, partially from slides by

Olli Knuuttila & Hannu H. Kari

Design Issues

• Telecoms networks are expensive– Installation in physically locations

– Sometimes access rights and physical access is difficult– High availability hardware– Time to market is an important factor– Heavy investment prior to returns– Long payback time

• Income is based on usage– If the available resources can not fulfill a demand, income is

lost– Lacking customer satisfaction increases churn (loss of

customers)• Wrong investments eat profit margins and can destroy the

company

Planning and Dimensioning• Inputs

– User needs– Current statistics– Future estimates– Competition– Technology, standards, products– Visioning

• Internals– Capacity needs– Network element requirements– Modeling network / user behavior

• Outputs– Strategic planning, company policy– Installation/purchase schedule– Configuration setting– Prioritizing, pricing rules

Dimensioning Examples (GSM)

• Capacity– Number of users supported in the network– Number of lines between BSC and MSC– Number of carriers per cell– Number of modem lines for GSM data– Disk space and CPU capacity for voice mail

• Location– Where to install new equipment

– Central or distributed service topology– Location of the base stations– Location of the modem pool

Load Control

• In the circuit switched world service is black & white– Call is established or blocked– Packet networking is different

• How to reduce load in the network– Blocking -> unhappy customers– Pricing -> happy customers

• In mobile networks increasing load decreases quality– In TDMA other mobiles are monitoring same frequency and

time slot– In CDMA other mobiles are monitoring all other mobiles in the

same cell– Fixed line telephone networks are more simple

Increasing Capacity (GSM)

• Radio capacity– More base stations, smaller cell radius

– Costly, usually possible– More frequencies / channels

– Depends on regulation or base station hardware– Better radio frequency utilization through different coding

– Depends usually on handset hardware and standards• More efficient utilization of resources

– Circuit vs. packet switched services– Dedicated vs. multiplexed channels

• Parallel servers (pizza boxes)– GGSN, SGSN, MSC, HLR– requires algorithms that parallelize

Busy Hour

• The sliding 60 minute period, during which occurs the maximum total traffic load in a given 24 h period

• Examples:– Morning: 9-11– Afternoon 13-17– Evening: 19-21

• Operator network design usually focuses on the Busy Hour– The load of off-peak hours is not significant for network design

• If traffic can somehow be moved from peak hours to off-peak hours, operators can save on network costs– Pricing

Traffic Measurements

• Traffic is measured in Erlangs– Strict definition is related to one voice path

• In network design Erlang is usually used to measure traffic volume in one hour– If a set of subscribers makes 30 calls in one hour and

the average call is 5 minutes, that is 2.5 Erlangs

• Named after A. K. Erlang, who did the basic research on this in the early 1900's

• Applies to services as well as trunk lines

Customer Profiling

• E.g. if an average residential customer creates 25 mErl during the busy hour, 50 000 subscribes would generate 1250 calls (Erlangs) on the average

• Profiles depend on culture and area, change with time, are affected by mobility etc.

• Traffic patterns can be studied from the logs of carried traffic– Notice that blocked calls are not shown

• Sometimes traffic is difficult to predict– Major accidents– TV-competitions– National holidays

Blocking

• Blocking means how many uncompleted calls do we accept– Usually during the Busy Hour

• Measured usually as a percentage– 1% blocking during the Busy Hour is often acceptable

– ITU-T E.543– Means that 1 out of 100 calls can not be completed,

because the network is full and no free path can be found

– 1% is the design target, most of the time performance is much more better

• Blocking can be caused by lack of trunk lines, congestion in signaling, unavailable services etc.

The Erlang B Formula

• The Erlang B Formula calculates the probability of a call being blocked, based on the number of links (trunks) or other available services and offered load

• The call arrivals are assumed to be random (Poisson distribution)

• c = trunks, operators etc.• a = offered load• Pb = Probability of call being blocked

??

?? c

k

k

c

b

ka

caacBP

0

)!/(

!/),(

The Erlang B Formula in Practice

• Increasing the amount of trunks decreases the blocking probability in non-linear fashion

3.128

2.501

1.909

1.361

0.869

0.455

0.153

0.010

1%

2.730

2.157

1.622

1.132

0.701

0.349

0.105

0.005

0.5%

3.6278

2.9357

2.2766

1.6575

1.0924

0.6023

0.2232

0.0201

2%– This table shows the Erlang-value as a function of trunk lines and the blocking factor

– To carry 1 Erl at 1% blocking, requires 5 lines

– 8 lines can carry 3 Erls– Accepting greater blocking

likelihood allows some more traffic – Note that the amount of users is

not important, traffic generated is

Practical Network Dimensioning

• Amount of traffic can be estimated from– Past experience– Known demographics– City planning– etc.

• In fixed line networks the number of trunks is one major issue– These come usually in PDH multiplexes or SDH containers– 30 trunks can carry 20 Erl at 1% blocking– 120 trunks can carry 103 Erl at 1% blocking

• There is also the switching equipment and services

Network Throughput

• The issue of interest to users is the quality of the total service– The end to end traffic – Calls cross the telephone network over many trunks and

switches, any of which may be blocked

• Some network patterns are easy to analyze– A link, series of links, a tree

• The actual mesh network is hard to analyze• One method is to analyze all the major paths

– E.g. all trunks have a blocking factor of 1%, if a call crosses three trunks, the total factor is 1-(1-0.01)3 = 3%

– ITU-T has standardized both the reference network and QoS requirements

More Network Design Issues

• Signaling network capacity should also be calculated

• And switching capabilities

System capabilities

Traffic volumeErlangs

Quality of ServiceB < 1%

Network Topology• Fixed line networks are usually meshes built of

rings– Redundancy and alternative routes– Alternatives can also carry excess traffic– Land rights and equipment center permissions are

important

• Cellular networks provide a completely different challenge– Urban areas

– Dense population, buildings– Rural areas– Mobility

– Dense lines across rural areas– Unpredictable traffic

Cellular Network Topology

• Cellular network design is focused on the – Transceiver antenna locations and directions– Radio frequencies and channels– Transmission power

• Trunks from the base station to backbone are also an issue, but more manageable one– Dual band GSM requires roughly 2-4 Mbps– Microwave links

• Cells can be macro, micro or indoor cells– Smaller cells enable better re-use of frequencies but

cost more

Regulation

• Government assistance requirements• Emergency services• Consumer protection

– QoS requirements– Monopoly regulation– Mandatory service provisioning

– Customers– Competition

• Resources– Frequencies– Right of way

• No control over terminal equipment or service usage