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