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12. State two modes that are used for a subscriber to access an external data network.

13. How does a visited SGSN find its home network?

Transparent and non-transparent. The former is a GPRS operator acting as the ISP. The latter is the external ISP or intranet allocated IP address for the subscriber.

Review

From the access point name (APN) raised by the subscriber or the subscriber‘s application.

The APN identifies a (set of) GGSNs, which offer a connection to the external PDN the subscriber wants to get connected to. Normally, a DNS server in the VPLMN is used for name resolution.

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14. In which cases will the visiting user select or be forced to use the home GGSN and visited GGSN?

Case 1: Selecting the visited GGSN could be faster to connect to a public Internet site, but probably more expensive.

Case 2: Selecting the home GGSN could be the only way to connect to a corporation's network (like Intranet), or a cheaper way to connect to the public Internet.

Review

A subscriber can be restricted by the HLR to use the home GGSN:

Case 1: The user has a permanent IP address allocated by the home GGSN.

Case 2: The home operator would like to control its subscribers.

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15. What are the additional GPRS security features?

16. Define the meanings of LEA and LIG

a) Private IP addressing in the GPRS backbone b) Ciphered links and authentication between

nodes in the GPRS backbone, as well as between network operators

c) Firewalls

Review

LEA- Legal Enforcement Agency. Provides information about a certain pre-defined target subscriber. This information could be sent or received data, location information, etc.

LIG – Legal Interception Gateway. Collects the GPRS data and converts it to the Legal Enforcement Agency format.

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17. How many routes could an SMSC-GMSC choose to deliver a SMS?

18. Which network elements are responsible for collecting CDRs? Which one provides interface to Customer Care and Billing System?

It can route in the "classic" way toward the MSC or choose the "GPRS" way and use the SGSN. Requires Class A or B MS.

Review

SGSN and GGSN are responsible for collecting CDRs. These CDRs are transferred to the Charging Gateway (CG) and then forwarded to the CCBS.

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Review Questions toReview Questions to„GPRS Air Interface“„GPRS Air Interface“

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1. Which fields are used for medium access control and multiplexing multiple users on the uplink and downlink PDCH?

2. How many users can share the same Packet Data Channel (PDCH) timeslot in the uplink direction?

For medium access control, the USF field is used for uplink. There is no need for MAC in the downlink direction since there is only one source in that direction.

The TFI is used to identify the data flow in the uplink and downlink direction.

Review

The USF field has 3 bits; consequently, up to 8 subscribers can share one PDCH physical channel. 111 can be reserved for PRACH, and then only up seven users can share a PDCH with USF, using the values 000 to 110.

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3. How many frames, radio blocks, and bursts are there in a PDCH multiframe?

4. What is the purpose of PTCCH?

One PDCH multiframe that is transmitted in a GPRS timeslot consists of 52 TDMA frames organised into 12 radio blocks of 4 bursts each, 2 idle TDMA frames and 2 PTCCH TDMA frames.

Review

UL: for transmission of RACH bursts to estimate timing advance.(up to 16 MSs)

DL: for transmitting timing advance information for up to 16 MSs.

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5. Which layer is responsible for segmentation and reassembly of LLC PDUs and Backward Error Correction (BEC) procedures?

6. Which coding scheme has adopted the same coding as used for SDCCH?

7. Which layer uses the functionality of USF?

8. Which coding scheme does not use FEC?

9. Which logical channels can be used for resource assignment?

RLC layer (Radio Link Control)

Review

CS-1 (9.05 kb/s)

MAC layer (Medium Access Control)

CS-4 (21.4 kbps, but no redundancy added)

AGCH (PAGCH), PACCH