Capacity
Traffic capacity, (e g 2,500 Erlang), tells how many simultaneous calls a unit can handle. One Erlang corresponds to one busy line. If a subscriber calls 25 mErlang during busy hour, he is in average calling 25/1000 of the hour (=25*60*60/1000 = 90 seconds).
Erlang can be limited by for example the group switch, available speech trunks, transcoders etc. But this does not give any idea about the processor loading as well as nor about non call activities.
Continued…..
Section 7 – Dimensioning
Call capacity, (e g 100,000 BHCA), tells how many call attempts a
unit can handle during busy hour. This figure is a better measure of processor capacity but still, this measure does not take into account non-call related activities.
Subscriber capacity, (e g 60,000 subscribers), tells how many subscribers that can be served by a unit. This figure is strongly depending on subscriber behavior.
Continued…..
Section 7 – Dimensioning
Addressing capacity, (e g 1020 TRXs), tells how many HW or SW
devices that can be connected / defined. This is also known as system limits. Here, no considerations to real-time processing needs or amount of traffic are made.
Section 7 – Dimensioning
Traffic Load Distribution
Section 7 – Dimensioning
In the default traffic load distribution for a GMSC/MSC/HLR the call part
takes about 70% of the capacity of the traffic load, the location updating part about 25%, the SMS part 3% and supplementary services approximately 2%.
If one look into the traffic part (70% of traffic load) the actual basic load part is 53% of the usage load, a gate way load part is 7.5%, a charging part 5%, a handover part 3% and a part used for authentication about 1%
Section 7 – Dimensioning
Section 7 – Dimensioning
Numbering Plan
The MSISDN is a number which uniquely identifies a mobile telephone subscription in the public switched telephone network numbering plan. These are the digits dialed when calling a mobile subscriber.
In GSM 900/GSM 1800, the MSISDN consists of the following:
MSISDN = CC + NDC + SN
Section 7 – Dimensioning
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
Section 7 – Dimensioning
International Prefix
Country Code
National Destination Code
Subscriber Number
0091 98 113 23448
The digits ‘113’ identify the GSM 900/GSM 1800 PLMN area code.
The digits ‘23448’ define the five digits, which identify the
mobile subscriber.
Section 7 – Dimensioning
A NDC is allocated to each PLMN. In some countries, more than
one NDC may be required for each PLMN.
The international MSISDN number may be of variable length.The maximum length is 15 digits, prefixes not included.
Example: Singapore PSTN subscriber is calling to an Indian GSM PLMN subscriber
Continued…..
Section 7 – Dimensioning
International Mobile Subscriber Identity (IMSI)
The IMSI is the information which uniquely identifies a sub in a GSM PLMN. It is used in all the signaling in the PLMN.
It will be stored in the in the Subscriber Identity Module (SIM), as well as in the HLR and in the serving VLR.
It consists of three different parts
Section 7 – Dimensioning
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
MSIN= Mobile Station Identification Number
All network related subscriber information is connected to the IMSI.
Section 7 – Dimensioning
In GSM 1900, the MSISDN consists of the following:
MSISDN = CC + NPA + SN
CC = Country Code
NPA = Number Planning Area
SN = Subscriber Number
Section 7 – Dimensioning
The NPA is allocated to each GSM 1900 PLMN. The length of MSISDN
depends on the structure and operating plan of each operator. The maximum length is 15 digits, prefixes not included.
Section 7 – Dimensioning
Examples:
xyz = operator code
abcde = Subscriber number
STD code = PSTN area code (11 for delhi)
• Call from PSTN to PLMN
Local Call 98 xyz abcde
Outside area call 0 98 xyz abcde
• Call from PLMN to PSTN
Local Call 0+STD code+SN
Outside area call 0+STD code+SN
Section 7 – Dimensioning
GPRS Core Network Planning
Section 7 – Dimensioning
Circuit Vs Packet Data
Circuit Switched Service: • 2G system (primarily voice and data on circuit switched air interface)
• Call charging based on channel holding time.
• Maximum number of users per TDMA channel is 8
• Suitable for constant bit rate applications
• Resource allocation is done such that UL and DL are paired.
Section 7 – Dimensioning
Packet Switched Service:
• Several users can share the same channel.
• Charges based on channel usage (actual usage of byte transferred).
• Well suited for bursty traffic.
• Resource allocation done independently on UL and DL (good for applications with asymmetrical bit rate)
• Dynamic allocation of resources
• Can multiplex traffic (voice, data, video).
Section 7 – Dimensioning
2
Offered GPRS Traffic0
1
2
3
4
5
6
7
8
9
10
TCH
Circuit Switched Traffic0
2
4
6
8
10
12
14
TCH
GSM
capacity
6
8
10
12
14
0
2
4
Speech traffic leaves some capacity for packet data
Section 7 – Dimensioning
GPRS System feature
• Variable quality of service.
• Independent packet routing.
• Protocol transparent (encapsulation & tunneling)
• Slotted ALOHA for random access procedure
• Provides IP connectivity to mobile subscriber.
• Build on existing GSM infrastructure with added nodes for supporting packets.
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Section 7 – Dimensioning
Conceptual View on GPRS
Shared GSM and GPRS Infrastructure
Internet Corporate Intranet
GPRS Core BSC BTS
GSM Voice
Access Point
GPRS Infrastructure IP World
Section 7 – Dimensioning
Air Interface - Mobile Terminal
• Type C GPRS only (or manually switched between GPRS and speech modes)
• Type B GPRS and Speech (not at same time) (Automatically switches between GPRS and speech modes)
• Type A GPRS and Speech at the same time BSC
BTS
Section 7 – Dimensioning
GPRS Attach / Detach
• Attach
Performed when the MS indicates its presence to PLMN for the purpose of using GPRS service
Carried out between MS and SGSN
MS identifies itself with its GSM identity
GPRS subscription necessary for successful attach
• Detach
Performed when the MS indicates to the PLMN that it no longer be using GPRS services
MS identifies itself with its GSM identity
Section 7 – Dimensioning
Section 7 – Dimensioning
System Architecture
BTS
BTS
BTS
BSC SGSN GGSN
Data Networks
Um Abis
HLR
Gb Gn
Gr Gc
Gi
Section 7 – Dimensioning
SGSN
• Responsible for delivery of packets to mobile subscribers in its service area.
• Mobility Management
• Logical link management, authentication
• GPRS user- related data needed by SGSN to perform routing and transfer functionality stored in GPRS Register eg current cell, current VLR, user profile including IMSI and its address in PDN.
• Interface point between core and Radio networks
Section 7 – Dimensioning
• Acts as an interface between GPRS network and external PDNs
• Mainly responsible for packet routing, transfer and mobility management
Converts packets from SGSN into appropriate PDP format and sends them out to corresponding PDN
PDP addresses of incoming data packets from PDN are converted to IMSI of the destination user and sent to the responsible SGSN.
Tunneling
GGSN
Section 7 – Dimensioning
• Circuit Switched traffic has priority
• In each cell Circuit Switched & Packet Switched territories are defined
• Territories consist of consecutive timeslots
TRX 1
TRX 2
CCCH TS TS TS TS TS TS TS
TS TS TS TS TS TS TSTS
Circuit Switched Territory
Packet Switched Territory
Territory border moves Dynamically based on Circuit
Switched traffic load
Default GPRS
Capacity
Dedicated GPRS
Capacity
TS TS
Additional GPRS
Capacity
TS TS
GPRS and GSM Resource sharing
Section 7 – Dimensioning
Capacity Management
• Dedicated GPRS Capacity
TCHs reserved exclusively for GPRS use.
• Default GPRS Capacity
TCHs always allocated to the GPRS when circuit switched load permits.
Keeps GPRS timeslots consecutive (important for multislot operation)
Section 7 – Dimensioning
PDP Context Activation - 1 Accessing the HLR
BTS BSC
SGSN
GGSN
Intranet
GPRS Backbone IP Network
SS7
HLR
DNS
• Access Point Name = Reference to an external packet data network the user wants to connect to
Internet
APN= "Intranet.Ltd.com"
1.
2.
Access Point
• (1) MS sends "Activate PDP Context Request" to SGSN
– Access Point Name
– PDP Type (IP)
– PDP Address (empty == dynamic)
– QoS & other options
(2) SGSN checks against HLR Access Point Name Dynamic / static IP address QoS
Section 7 – Dimensioning