AgendaErlangGOSBusy HourTrunkCall Holding timeAverage traffic per subscriberDeposition of lost callsTraffic planning AssignmentRelieving traffic congestion

ErlangA single device occupied continuously or intermittently for a total time t during a period of T, carries t/T erlang, and has a maximum possible load of 1E.

Grade Of Service GOSTo keep the resources within reasonable bounds, traffic engineers must plan for a small percentage of blocked or delayed calls during the busy hour. This percentage is termed the system's Grade of Service (GOS).

Grade Of Service ContOffered TrafficTraffic available Carried TrafficTraffic intensity servedLost TrafficThe amount of traffic that cannot be handled

Busy HourFlat Busy HourThe period of sixty minutes during which the traffic is maximumRemains constant from day to day

Carried traffic per day

TrunkA trunk or voice channel is considered to be the smallest traffic resource capable of handling the required voice or data traffic. In TDMA, a trunk is considered to be one pair of time slots from a pair of carrier frequencies.

Call holding timeThe period of time during which a trunk is continuously busy including setup, conversation, and disconnect times can be dynamic with respect to time and demographics.

Average call holding timeCall holding time will also vary with respect to the time of the day. Traffic engineers rely on the average call holding time The average call holding time (H) is defined as the total time during which a trunk is busy divided by the number of calls, which have utilized that trunk during the period of time and normally expressed in seconds

Number of Subscribers (#Subs)It is important variable used to calculate the number of resources required (offered Traffic) at cellProjections based on growth and historical traffic data gathered from the switchCan be estimated as a percentage of the population if switch data is not available

Average busy hour traffic per subscriber (Tsub)Expressed in Erlangs, or Milli-ErlangsCalculated by taking the total number of Erlangs during a busy hour dividing by the total number of subscribers

Required Offered load (ROL)The average busy hour traffic that a particular cell must carryRequires knowledge ofThe number of subsAverage traffic per subscriber

ROL = (#subs)(Tsub)

The Disposition of "Lost Calls"Lost calls cleared -Erlang BBlocked calls are instantly cleared from the system and do not reappearLost calls delayed -Erlang CBlocked calls are put in a queue and held until a resource becomes available

Erlang B Curve

Relieving Traffic Congestion Optimization of system parametersAdd channelsOmni vs. SectorCell splittingMicro cellsOverlay / UnderlayRF changesSmart antennas

Relieving Traffic CongestionAdding Channels Increases traffic resourcesLimited by equipment and frequency planning IssuesEngineering a system wide retuneOrganize the channels in groupsGives some additional channels

Relieving Traffic Congestion2. Modifying System ParametersNeighbor listHandoff hysteresisHandoff threshold

Relieving Traffic CongestionOmni vs. Sector Antenna ConfigurationsSectorization provides less capacity for a given number of channelsDecrease in trunking efficiencySectorization provides an increase in system capacity through channel reuse efficiency

Relieving Traffic Congestion4. Cell splittingProvides additional traffic resources to the same geographical areaMaintains a sound reuse planAdd a cell between 1 and 2 to relieve traffic between the two sectors

Relieving Traffic Congestion4. Cell splitting cont.Example Sectors 1 and 2 in the diagram have excessiveblockingSector 1 has 100 usersSector 2 has 80 usersAssume uniform traffic distribution over faces 1 and 2Relieve the traffic by performing a cell split

Relieving Traffic Congestion4. Cell splitting cont.

Relieving Traffic Congestion5. Micro cellsProvides additional traffic resources to the samegeographical areaInserting a small site in an area with high capacityRequires new frequency planSimilar to cell splitting, only not following the grid

Relieving Traffic Congestion6. Overlay/Underlay Configurations Off-Loads traffic from congested cellsRequires an additional set of channelsSecond set of channels is used at a lower powerReduces the opportunity for out of cell interferenceRequires adjustment of optimization parameters to tailor handoff between layersCan solve congestion problems for a whole group of cells

Relieving Traffic Congestion6. Overlay/Underlay Configurations cont.

Relieving Traffic Congestion7. RF ChangesDecrease cell footprintDown tilt / Up tiltAzimuth ChangeAntenna ChangePower modificationBy pulling in the coverage from a high capacity cell, other cells can pick up the traffic

Relieving Traffic Congestion7. RF changes cont.

Relieving Traffic Congestion8. Smart AntennasMain Feature: improving C/I ration

ExampleIf The following information has been provided find number of sites and its configurations:Designed with tri-sectored sites throughout the network2% GOS requirement5% market penetration by end of year oneFocus on dense urban, urban, and suburban areas onlyCoverage design provides 90% area reliabilitySixty percent of subscribers active during busy hourBusy hour call attempts per subscriber is oneAverage call holding time is 2 minutes (exponential distribution)Blocked calls are cleared from the system50 ARFCNs, 4x3 reuse PatternAssume a uniform distribution of subscribers over the service areaRepeat if the % subs active during Busy hour is 60 for dense urban, 40 for urban and 20 for suburban.

Solution Calculate the average busy hour traffic per subscriber (Tsub)

Solution cont2. Determine the number of subscribers per area type

Solution cont3. Determine what is required offered load per area type

Solution cont4. Determine the number of Erlangs the each carrier can supplyUse erlang B table with 2% GOS

Solution cont5. Determine the number of cells required to cover each morphological areaUsing the nominal cell radii and the demographic data

Solution cont6. Determine the required traffic resources for each sector

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