Dimensioning and Deployment of GSM Networks

Transcription

1 Case Study: Dimensioning and Deployment of GSM Networks Acknowledgement: some of these slides are based on originals and information kindly provided by Ian O Shea of Altobridge Ltd GSM System Architecture Um Handset BTS A-bis SGSN Gb BSC OMC Gr A GGSN IP (Operation & Maintenance Center) MSC Gi HLR/ VLR SS7 ISUP PSTN Internet Radio sub-system Network sub-system 2 1

2 GSM System Architecture (Voice) 16kbit/s Um Handset 16kbit/s 64kbit/s BTS BSC A-bis (E1) A-Int (E1) MSC Radio sub-system 3 Dimensioning of a New Site Radio Interface (MS to BTS) How many Traffic Channels (TCHs) are required? How many Standalone Dedicated Control Channels (SDCCH) are required? Abis Interface (BTS to BSC) [2Mbit/s E1] How many 64Kb/s time slots are required? A-Interface (BSC to MSC) How many 64Kb/s time slots are required to carry voice to MSC? 4 2

3 Traffic Dimensioning The Traffic Analysis Process (Data used for Dimensioning) Demographics, estimate population and business data Market Projections Target Coverage Expected traffic per sub; SMS Voice Providing the Service, Voice and Data (GPRS/EDGE) Traffic measurements Erlangs and Blocking (GoS) 5 Dimensioning Of GSM Networks (Radio Interface) Dimension for the Busy Hour Traffic period Typically at Fid Friday evening 17:00/18:00 Typical Assumptions At Busy Hour every subscriber; Generates 90 seconds of voice traffic (TCH) =(90/3600)= 25mErl One Location Update (SDCCH) One Mobile Originated SMS (SDCCH) One Mobile Terminated SMS (SDCCH) Ignore GPRS/EDGE for this example 6 3

4 Traffic Measurement Unit of traffic measurement: Erlang (E) Traffic in erlangs is the number of call hours per hour: A = C T / 3600 A = Traffic in Erlang C = number of calls during the hour T = mean holding time per call in seconds One channel in continuous use is carrying a traffic of 1 erlang Network of 1 million subscribers ~ 25,000Erl at Busy Hour (assuming 25mErl/subscriber) 7 Case Study Case study based on real world deployment by Irish Company Altobridge in Indonesia Assumptions population of 5000 people 20% penetration = 1000 subscribers Each subscriber ~20mErl of traffic (NB: merl = 1/1000 th Erlang) Ignore GPRS in this example Assume 2 % GoS (standard assumption) Thus, total traffic to be carried is 1000 subscribers* 20mErl = 20 Erlang 8 4

5 Case Study (Cont d) Use a three sector cell (very common configuration in real cell sites) Assume traffic is evenly distributed across the threesector cell, so 20Erlang/3sectors = ~7 Erlang per sector Thus 13 traffic channels (TCH) per sector needed (see next slide) 9 Erlang B Table 10 5

6 Case Study (Cont d) Need to decide on number of Transceivers (TRX) per sector Each TRX supports a GSM carrier of 8 slots but 1 st TRX uses 2 slots for signalling (BCCH & SDCCH), leaving just 6 for traffic channels (TCHs) so we need two TRXs per sector to provide our 13 TCHs Usually a single SDCCH slot is sufficient for up to 2 TRXs each SDCCH slot also allows up to 8 SMS messages need more SDCCH slots if large SMS demand expected 11 Radio Dimensioning 2 TRX TRX 5 TRX 6 2TRX TRX 1 TRX 2 Sector_A Sector_C 2 TRX TRX 3 TRX 4 Sector_A Sector_B Sector_B 3 Sectors; 2 Transceivers (TRX) per sector (8 channels per TRX) Sector_C 12 6

7 Abis Dimensioning 6 TRXs over Abis Link Typically E1 running over microwave from BTS to BSC Speech traffic on each TRX is 8 * 13Kb/s Each TRX occupies 2 * 64Kb/s for speech 1 * 32Kb/s signalling Abis E1 (32*64Kb/s channels) A B C 13 GSM System Architecture 16kbit/s UM Handset BTS 16kbit/s A-bis BSS 64kbit/s A MSC A-Ter Radio sub-system 14 7

8 A Interface Dimensioning Typically in A Interface dimensioning use 0.1% GoS Our example of 20 Erlang at 0.1% GoS => 35 channels required (see next slide) Typically use an E1 Interface between BSS and MSC A single E1 typically has 30 usable channels, so require 2 E1s in this example SS7 (BSSAP) typically carried on TS

9 Summary Case study for GSM dimensioning Air interface and backhaul Inspired by real world network Involves key concepts Frequency reuse Traffic trunking Cell sectoring Channel assignment 17 9