ECE 5221 Personal Communication Systems

1. ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 17: Traffic planning Spring 2011

2. Outline • Traffic in communication networks • Circuit switched versus packet switched traffic • Queuing system • Elements of queuing system • Traffic in erlangs Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Traffic in communication networks • Traffic - flow of information messages through a communication network • Generated as a result of • phone conversations • data exchange • audio, video delivery • signaling • Communication networks are designed to provide service to many users • At any instant of time not all users are active • network resources are shared • resource sharing may result in temporary service unavailability • Traffic planning allows sharing of resources with minimum performance degradation Modern communication networks carry mixture of voice and data traffic

4. Cellular network consists of many connected elements Analysis of the entire network is complicated Common practice - analyze each link individually Traffic dimensioning has two aspects Dimensioning the network elements to have enough processing power Dimensioning the connecting lines to have sufficient capacity Traditionally, traffic bottleneck - Air interface Outline of a cellular network

5. First and second generation provides connection oriented services to the users A dedicated channel is allocated over the entire duration of the call In the case of voice communication this is “only” 50% wasteful This mode of communication is called “circuit-switching” Circuit switching is very inefficient for data communication (major driver of 3G cellular systems) Circuit switching is abandoned in 4G Circuit switched communication services Interpretation of term circuit for various cellular technologies

6. Virtual path packet switching Virtual path (sequence of network nodes) is established through the network Implemented within ATM networks Datagram packet switching Every packet travels independently Implemented within IP based networks Transport layer has to assure the proper order of the packets Packet switched communication services Virtual path switching Datagram switching Note: Modern packet data networks are using datagram switching

7. Types of traffic in cellular networks • Cellular networks support • circuit switched (CS) voice • dispatch voice (push to talk) • circuit switched data • packet data (PD) • Communication resources may be • Shared between CS and PS • Separated resources may be set for CS and PS • First and second generation - dominated with circuit switched voice • Third generation and beyond - dominated by data ITU vision for cellular services Traffic planning in heterogeneous cellular networks of the future takes central stage

8. Description of queuing systems • Queuing systems • Mathematical abstraction • Used to develop the traffic analysis and planning methodology • Elements of a queuing system • source population • queue • servers • distributions of interarrival times, service times, queuing discipline, etc. Outline of a queuing system • Queuing system – cell site • Servers – channel resources – trunks • Population – users connecting to cellular network

9. Source population • Consists of all users that are eligible for service • The most important property - size • infinite population - arrival rate does not depend on the number of users in the system • finite population - arrival rate depends on the number of users in the system • if the population is large relative to the number of servers we routinely assume that its is infinite • In cellular systems population are all eligible users within the coverage area of the cell • It is assumed that the number of eligible users is much greater than the number of the users using the system at any given moment • Over a course of day, the size of population changes • Traditionally cellular systems are dimensioned for a good performance during the busiest hour Example of a call stats benchmarking map

10. Arrival rate and interarrival times Example: The average number of call arrivals in two figures is the same: 20 arrivals per minute. The traffic pattern in second figure requires more resources to accommodate for higher demand peaks. • Arrival rate - number of service requests per unit time • The ability of the queuing system to provide effective service depends on distribution of arrival rates • Standard way of specifying arrival rate is through probability density function of interarrival times

11. Service time (call holding time-CHT) Example: Duration of CHT at a cell • Service time-period of time that the resource is allocated to individual user • Usually specified through its distribution • Most commonly, CHT is exponentially distributed Exponential distribution T – average call holding time Note: Exponential distribution is a good model for demand generated by humans (voice, SMS, email,..)

12. Average resource occupancy - traffic in erlangs Definition • Erlang - unit for measuring of traffic intensity • Defined as a fraction of time that the resource is occupied • Occupancy does not have to continuous • Specified relative to some averaging time • Maximum traffic carried by a single resource - 1 erlang • Total traffic carried by service facility cannot exceed number of servers [erlang] Resource occupancy time Averaging time Example

13. Alternative interpretation of erlangtraffic Traffic in erlangs for multi-server system • Traffic in erlangs = average number of simultaneously occupied servers • Can be measured easily • regular poling of service facility and logging the number of occupied resources Sum of times during exactly n out of C servers are held simultaneously Number of servers Averaging time Example of traffic measurements. Averaging time is 60 min. Poling time is 1 min.

14. Offered traffic - traffic that would be served if the number of resources is unlimited Lost traffic - traffic that could not be served due to finite resources Served traffic - difference between offered and lost traffic Attempt to serve all offered traffic results in allocation of large number of resources Note : Communication systems are frequently designed to operate with a certain percentage of lost traffic Offered, carried and lost traffic Relation between offered, carried and lost traffic