UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT  Advanced Course on Networking    Introduction to Cellula...
Mobile Communications system Evolution
Outline <ul><li>GSM </li></ul><ul><li>GPRS </li></ul><ul><li>UMTS </li></ul><ul><li>CDMA </li></ul><ul><li>WCDMA </li></ul...
What is GSM?? ( Global System for Mobile Communications)
GSM Architecture <ul><li>Three broad parts </li></ul><ul><ul><li>Mobile Station (MS):  carried by the subscriber </li></ul...
Mobile Station <ul><li>ME( mobile equipment )   </li></ul><ul><ul><li>the terminal </li></ul></ul><ul><li>SIM  (Subscriber...
Base Station Subsystem <ul><li>BTS  (Base Transceiver Station) </li></ul><ul><ul><li>handle the radio link protocols with ...
Network Subsystem <ul><li>MSC  (Mobile Services Switching Center) </li></ul><ul><ul><li>acts like a normal switching node ...
Network Subsystem  cont. <ul><li>VLR  (Visitor Location Register)   </li></ul><ul><ul><li>contains selected information fr...
GSM Features <ul><li>Multiple access:  use TDMA/FDMA to share the limited radio spectrum </li></ul><ul><ul><li>The FDMA pa...
GSM Features  cont. <ul><li>SMS: Short Message Service  </li></ul><ul><ul><li>is a bi-directional service for short (up to...
GPRS System ( General Packet Radio Service)
GPRS Architecture  <ul><li>Introduce two new nodes into GSM network </li></ul><ul><ul><li>SGSN (the Serving GPRS Support N...
GPRS Features <ul><li>Log on to GPRS </li></ul><ul><ul><li>A GPRS-capable terminal communicates with GSM base Stations, bu...
GPRS Features  Cont. <ul><ul><li>SGSN and GGSN use GPRS tunnel protocol (GTP) which operates over the top of TCP/IP to enc...
GPRS Features  Cont. <ul><ul><li>pay for the amount of data they actually communicate, and not the idle time  </li></ul></...
GPRS Features Cont. <ul><li>Spectrum Efficiency </li></ul><ul><ul><li>users can share the resource (Radio link),it is used...
GPRS Features Cont. <ul><ul><li>a channel that is 200kHz wide, is divided into 8 separate data streams, each carrying maxi...
Scenario for Migration  from 2G to 3G
What are 3G Technologies? <ul><li>UMTS  (Universal Mobile Telecommunications Service) </li></ul><ul><ul><li>EDGE can co-ex...
What does UMTS provide? <ul><li>Circuit- and Packet-Oriented Services </li></ul><ul><li>Seamless Global Roaming </li></ul>...
UMTS coverage vs. bit rate Outdoors Vehicle Walk Fixed Indoors 155Mb/s 2.0 0.5 GSM UMTS Broadband Radio
UTRAN- UMTS Terrestrial Radio Access Network Core Network RNC Node B Node B RNC Node B Node B RNC Node B Node B MH lu lu l...
Frequency Allocation <ul><li>1920-1980 MHz paired with 2110-2170 MHz </li></ul><ul><li>Total 35 MHz unpaired band </li></u...
FDD vs. TDD <ul><li>Both FDD and TDD  are available in UMTS </li></ul><ul><li>TDD has been designed for use in high densit...
CDMA (Coded Division Multiple Access) <ul><li>In CDMA, every user assigned a unique </li></ul><ul><ul><li>Code </li></ul><...
WCDMA <ul><li>Radio Parameters </li></ul><ul><li>Performance Improvement </li></ul><ul><li>Channels </li></ul><ul><li>Chan...
WCDMA Radio Parameters <ul><li>Group 200KHz bands into 4.2-5.0 MHz carriers </li></ul><ul><li>Chip Rate is 4.096 Mchips/se...
WCDMA Performance Improvement <ul><li>Capacity Improvement </li></ul><ul><li>No Frequency Planning </li></ul><ul><li>Multi...
HCS & Hot-Spot HCS-Scenario f1 f1 f2 Macro Macro Micro Handoff between layers  is alwayse needed f1 f1 f1 f2 Hot-Spot Scen...
WCDMA Channels <ul><li>Transport channels are the services which the physical layer provides to higher layers. </li></ul><...
WCDMA Channels.. Cont. <ul><li>The following common channels are provided: </li></ul><ul><ul><li>BCH  (Broadcast Channel):...
WCDMA Channels.. Cont. <ul><li>There are certain physical channels which are not associated with a transport channel. They...
WCDMA Channels.. Cont. <ul><li>Physical Channels: are typically based on the following structure: </li></ul><ul><ul><li>Ra...
Power Control In WCDMA <ul><li>Near-Far Problem in CDMA </li></ul><ul><ul><li>Different Performance for Subscriber Links <...
Power Control In WCDMA <ul><li>How to do power control </li></ul><ul><ul><li>Force all users to transmit the minimum amoun...
Power Control In WCDMA <ul><li>Open Loop vs. Close Loop </li></ul><ul><ul><li>Open Loop Power Control </li></ul></ul><ul><...
WCDMA Soft Handoff Active set = BS1 BS1 BS2 Active set = BS1 & BS2 BS2 SS > add threshold BS1 BS2 Active set = BS2 BS1 SS ...
Active Research Topics <ul><li>Cellular system architecture </li></ul><ul><li>UMTS air interface </li></ul><ul><li>Power c...
Reference <ul><li>http://www.europe.alcatel.fr/telecom/rcd/keytech/ </li></ul><ul><li>http://www.comms.eee.strath.ac.uk/~g...
Reference <ul><li>http://www.umts-forum.org/reports.html </li></ul><ul><li>http://www.itu.int/imt/ </li></ul><ul><li>http:...
Reference <ul><li>A. Samukic,  UMTS Universal Mobile Telecommunications System:Development of standards for the third gene...
Thank you !!!
UNESCO PROJECT  Advanced Course on Networking  Professor Khalid Al-Begain UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATIC...
Upcoming SlideShare
Loading in …5
×

UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT ...

1,148 views
1,081 views

Published on

Published in: Technology, Business
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
1,148
On SlideShare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
11
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT ...

  1. 1. UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT Advanced Course on Networking Introduction to Cellular Wireless Networks On the Road to UMTS
  2. 2. Mobile Communications system Evolution
  3. 3. Outline <ul><li>GSM </li></ul><ul><li>GPRS </li></ul><ul><li>UMTS </li></ul><ul><li>CDMA </li></ul><ul><li>WCDMA </li></ul><ul><li>Summery </li></ul>
  4. 4. What is GSM?? ( Global System for Mobile Communications)
  5. 5. GSM Architecture <ul><li>Three broad parts </li></ul><ul><ul><li>Mobile Station (MS): carried by the subscriber </li></ul></ul><ul><ul><li>Base Station Subsystem: control radio link with MS </li></ul></ul><ul><ul><li>Network Subsystem: its main part is MSC </li></ul></ul><ul><li>Interfaces: </li></ul><ul><ul><li>Um Interface : known as air interface or radio link. </li></ul></ul><ul><ul><li>Abis Interface: between BTS and BSC </li></ul></ul><ul><ul><li>A Interface: between BSC and MSC </li></ul></ul>
  6. 6. Mobile Station <ul><li>ME( mobile equipment ) </li></ul><ul><ul><li>the terminal </li></ul></ul><ul><li>SIM (Subscriber identity Module) </li></ul><ul><ul><li>provides personal mobility . </li></ul></ul><ul><ul><li>Can insert the SIM card into another GSM terminal and use </li></ul></ul>
  7. 7. Base Station Subsystem <ul><li>BTS (Base Transceiver Station) </li></ul><ul><ul><li>handle the radio link protocols with the Mobile Station </li></ul></ul><ul><ul><li>many BTSs in a large urban area </li></ul></ul><ul><li>BSC (Base Station Controller) </li></ul><ul><ul><li>manages the radio resources for one or more BTSs </li></ul></ul><ul><ul><li>handles such as: channel setup , frequence hopping and handovers . </li></ul></ul><ul><ul><li>connection between MS and MSC </li></ul></ul>
  8. 8. Network Subsystem <ul><li>MSC (Mobile Services Switching Center) </li></ul><ul><ul><li>acts like a normal switching node of the PSTN or ISDN </li></ul></ul><ul><ul><li>provides the connection to the fixed networks (such as the PSTN or ISDN). </li></ul></ul><ul><li>HLR (Home Location Register ) </li></ul><ul><ul><li>contains information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. </li></ul></ul><ul><ul><li>logically one HLR per GSM network </li></ul></ul>
  9. 9. Network Subsystem cont. <ul><li>VLR (Visitor Location Register) </li></ul><ul><ul><li>contains selected information from the HLR, necessary for call control and provision of the subscribed services, </li></ul></ul><ul><ul><li>each mobile currently located in the geographical area controlled by the VLR. </li></ul></ul><ul><li>EIR (The Equipment Identity Register) </li></ul><ul><ul><li>a database that contains a list of all valid mobile equipment on the network, </li></ul></ul><ul><li>AuC (The Authentication Center) </li></ul><ul><ul><li>is a protected database:secret key of SIM </li></ul></ul>
  10. 10. GSM Features <ul><li>Multiple access: use TDMA/FDMA to share the limited radio spectrum </li></ul><ul><ul><li>The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. </li></ul></ul><ul><ul><li>Each of these carrier frequencies is then divided in time, using a TDMA scheme. </li></ul></ul><ul><li>GSM is a digital network </li></ul><ul><ul><li>Based on Circuit-switch </li></ul></ul>
  11. 11. GSM Features cont. <ul><li>SMS: Short Message Service </li></ul><ul><ul><li>is a bi-directional service for short (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. </li></ul></ul><ul><ul><li>an acknowledgement of receipt is provided to the sender. </li></ul></ul><ul><ul><li>can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval </li></ul></ul>
  12. 12. GPRS System ( General Packet Radio Service)
  13. 13. GPRS Architecture <ul><li>Introduce two new nodes into GSM network </li></ul><ul><ul><li>SGSN (the Serving GPRS Support Node): </li></ul></ul><ul><ul><ul><li>Keep track of the location of the mobile within its service area and send/receive packets from the mobile , passing them on, or receiving them from the GGSN </li></ul></ul></ul><ul><ul><li>GGSN (Gateway GPRS Support Node): </li></ul></ul><ul><ul><ul><li>convert the GSM packets into other packet protocols (e.g.IP or X.25) and send them out into another network. </li></ul></ul></ul>
  14. 14. GPRS Features <ul><li>Log on to GPRS </li></ul><ul><ul><li>A GPRS-capable terminal communicates with GSM base Stations, but unlike circuit-switched data calls which connects to MSC, GPRS packets are sent from the base station to SGSN, SGSN communicates with GGSN. </li></ul></ul><ul><ul><li>Establishes a Packet Data Protocol (PDP) which is logical connection between the mobile and GGSN </li></ul></ul><ul><ul><li>now visible to the outside fixed networks </li></ul></ul>
  15. 15. GPRS Features Cont. <ul><ul><li>SGSN and GGSN use GPRS tunnel protocol (GTP) which operates over the top of TCP/IP to encapsulate the packets </li></ul></ul><ul><ul><li>Tunnels: information may be encrypted and additional data is added to each packet to prevent tampering </li></ul></ul><ul><li>Packed based </li></ul><ul><ul><li>No dial-up, just as with a LAN connection. </li></ul></ul><ul><ul><li>No delay for sending data </li></ul></ul>
  16. 16. GPRS Features Cont. <ul><ul><li>pay for the amount of data they actually communicate, and not the idle time </li></ul></ul><ul><ul><li>users need to confirm their agreement to pay for the delivery of content from the service. This is performed by using WAP (Wireless Application Protocol) </li></ul></ul><ul><ul><li>unsolicited packets may not be charged </li></ul></ul><ul><ul><li>voice and Data Communication at the same time </li></ul></ul><ul><ul><li>can be viewed as a sub-network of the Internet </li></ul></ul>
  17. 17. GPRS Features Cont. <ul><li>Spectrum Efficiency </li></ul><ul><ul><li>users can share the resource (Radio link),it is used only when users are actually sending or receiving data </li></ul></ul><ul><li>Speed: Based on GMSK </li></ul><ul><ul><li>a modulation technique known as Gaussian Minimum-shift keying. </li></ul></ul><ul><ul><li>Theoretical Max speeds up to 171.2kbps.(GSM:9.6Kbps) </li></ul></ul>
  18. 18. GPRS Features Cont. <ul><ul><li>a channel that is 200kHz wide, is divided into 8 separate data streams, each carrying maximum 20kbps(14.4kbps typical), GSM only use one channel, GPRS combine up to 8 of these channels. </li></ul></ul><ul><li>complement rather than replace the current data services available through today’s GSM </li></ul><ul><ul><li>doesn't require new radio spectrum </li></ul></ul><ul><ul><li>supports TDMA: also use for IS-136 </li></ul></ul>
  19. 19. Scenario for Migration from 2G to 3G
  20. 20. What are 3G Technologies? <ul><li>UMTS (Universal Mobile Telecommunications Service) </li></ul><ul><ul><li>EDGE can co-exist with UMTS </li></ul></ul><ul><ul><ul><li>e.g. Edge provide high speed services for wide-area coverage while UMTS is deployed in urban hot spots. </li></ul></ul></ul><ul><li>Cdma 2000 </li></ul><ul><ul><li>based on the cdma One standard, two air modes: </li></ul></ul><ul><ul><ul><li>one based on the parallel use of 3 contiguous cdmaOne carriers (multi-carrier approach), </li></ul></ul></ul><ul><ul><ul><li>the other one on the use of the corresponding 3 carriers width spreading (direct spread approach) </li></ul></ul></ul>
  21. 21. What does UMTS provide? <ul><li>Circuit- and Packet-Oriented Services </li></ul><ul><li>Seamless Global Roaming </li></ul><ul><li>Capacity and Capability to serve more than 50% population </li></ul><ul><li>A Wide Range of Services </li></ul><ul><ul><li>Voice, low-rate data and high-rate data </li></ul></ul><ul><ul><li>144kb/s, 384kb/s , 2Mb/s </li></ul></ul>
  22. 22. UMTS coverage vs. bit rate Outdoors Vehicle Walk Fixed Indoors 155Mb/s 2.0 0.5 GSM UMTS Broadband Radio
  23. 23. UTRAN- UMTS Terrestrial Radio Access Network Core Network RNC Node B Node B RNC Node B Node B RNC Node B Node B MH lu lu lu lur lur Uu lub
  24. 24. Frequency Allocation <ul><li>1920-1980 MHz paired with 2110-2170 MHz </li></ul><ul><li>Total 35 MHz unpaired band </li></ul>1920 1980 2010 2025 C C A MSS B A’ 2110 2170 2200 MSS
  25. 25. FDD vs. TDD <ul><li>Both FDD and TDD are available in UMTS </li></ul><ul><li>TDD has been designed for use in high density areas </li></ul><ul><li>The highest bearer rate: </li></ul><ul><ul><li>TDD-2.048Mb/s </li></ul></ul><ul><ul><li>FDD-384kb/s </li></ul></ul>
  26. 26. CDMA (Coded Division Multiple Access) <ul><li>In CDMA, every user assigned a unique </li></ul><ul><ul><li>Code </li></ul></ul>
  27. 27. WCDMA <ul><li>Radio Parameters </li></ul><ul><li>Performance Improvement </li></ul><ul><li>Channels </li></ul><ul><li>Channel Generation </li></ul><ul><li>Power Control </li></ul><ul><li>Handoff </li></ul>
  28. 28. WCDMA Radio Parameters <ul><li>Group 200KHz bands into 4.2-5.0 MHz carriers </li></ul><ul><li>Chip Rate is 4.096 Mchips/sec </li></ul><ul><li>System Capacity of 128 channels per cell provided by 5 MHz bandwidth </li></ul>
  29. 29. WCDMA Performance Improvement <ul><li>Capacity Improvement </li></ul><ul><li>No Frequency Planning </li></ul><ul><li>Multiple Services per Connection </li></ul><ul><li>Frequency Handoff </li></ul><ul><ul><li>HCS </li></ul></ul><ul><ul><li>Hot Spot </li></ul></ul>
  30. 30. HCS & Hot-Spot HCS-Scenario f1 f1 f2 Macro Macro Micro Handoff between layers is alwayse needed f1 f1 f1 f2 Hot-Spot Scenario Hot-Spot Handoff at Hot-Spot is sometimes needed
  31. 31. WCDMA Channels <ul><li>Transport channels are the services which the physical layer provides to higher layers. </li></ul><ul><li>The number of transport channels is much higher than for GSM as more services are needed. </li></ul><ul><li>Transport channels are grouped into two classes: </li></ul><ul><ul><li>Common channels (where information is transmitted to all mobile terminals without distinction) </li></ul></ul><ul><ul><li>Dedicated channels (where communication takes place towards a single terminal by associating it with a physical channel, i.e. a code and a frequency or, in the case of TDD, also a time slot). </li></ul></ul>
  32. 32. WCDMA Channels.. Cont. <ul><li>The following common channels are provided: </li></ul><ul><ul><li>BCH (Broadcast Channel): used on the downlink to transmit system information in the entire cell. </li></ul></ul><ul><ul><li>FACH (Forward Access Channel): used on the downlink to transmit control information to mobile terminal. It also can be used to transport short data packets (as with GSM Short Message Service). FACH is used when the system knows the cell in which the terminal is registered. </li></ul></ul><ul><ul><li>PCH (Paging Channel): used on the downlink to transmit control information to mobile terminal whose location is not known. Transmission here is associated with paging indicator which informs the mobile terminal that the information is present on the paging channel, thus permitting lower battery consumption. </li></ul></ul><ul><ul><li>SCH (Synchronization Channel): used on the downlink to permit synchronization between the mobile terminal and base station. </li></ul></ul><ul><ul><li>RACH (Random Access Channel): used on the uplink to transport control information transmitted by the mobile terminal. </li></ul></ul><ul><ul><li>CPCH (Common Packet Channel): used on the uplink to transport data packets especially burst traffic. </li></ul></ul><ul><ul><li>DSCH (Downlink Shared Channel): used in the downlink to transport data packets. Access is shared by various users and is regulated by the base station. </li></ul></ul>
  33. 33. WCDMA Channels.. Cont. <ul><li>There are certain physical channels which are not associated with a transport channel. They are used to transport physical layer information that does not need to be sent to higher layers. </li></ul><ul><li>These channels are as follows: </li></ul><ul><ul><li>CPICH (Common Pilot Channel): a downlink channel on which a known un-modulated sequence is transmitted. </li></ul></ul><ul><ul><li>DPCCH (Dedicated Physical Control Channel): physical channel present on both links and used to transport physical layer signalling. </li></ul></ul><ul><ul><li>AICH (Acquisition Indication Channel): present on the downlink and used to inform the mobile terminal that there is a message on the FACH channel in response to an access attempt. </li></ul></ul><ul><ul><li>PICH (Paging indication Channel): present on the downlink and used to inform the mobile terminal that there is a message on the PCH channel. </li></ul></ul>
  34. 34. WCDMA Channels.. Cont. <ul><li>Physical Channels: are typically based on the following structure: </li></ul><ul><ul><li>Radio frame : has a length of 10 ms and consists of 15 time slots. </li></ul></ul><ul><ul><li>Time slot : has a length of 10/15 ms. Each slot consists of a number of symbols which varies according to the bit rate of the service to be transmitted. </li></ul></ul><ul><ul><li>Symbol : this is the information element after the channel encoding operations (i.e. after error correction codes are inserted) </li></ul></ul>
  35. 35. Power Control In WCDMA <ul><li>Near-Far Problem in CDMA </li></ul><ul><ul><li>Different Performance for Subscriber Links </li></ul></ul><ul><ul><li>A Few Subscribers closest to the BTS may contribute too much multiple Access Interference. </li></ul></ul>
  36. 36. Power Control In WCDMA <ul><li>How to do power control </li></ul><ul><ul><li>Force all users to transmit the minimum amount of power </li></ul></ul><ul><ul><li>Reduce the power transmitted by users closest to the BTS; increase the power transmitted by users farst to the BTS </li></ul></ul>
  37. 37. Power Control In WCDMA <ul><li>Open Loop vs. Close Loop </li></ul><ul><ul><li>Open Loop Power Control </li></ul></ul><ul><ul><li>Subscriber measures the DL power and adjusts its transmission power </li></ul></ul><ul><ul><li>Close Loop Power Control </li></ul></ul><ul><ul><li>BS measures the UL power. MS measures the DL power and reports to the BS. BS instructs the user to raise or lower it transmission power </li></ul></ul>
  38. 38. WCDMA Soft Handoff Active set = BS1 BS1 BS2 Active set = BS1 & BS2 BS2 SS > add threshold BS1 BS2 Active set = BS2 BS1 SS < drop threshold BS1 BS2
  39. 39. Active Research Topics <ul><li>Cellular system architecture </li></ul><ul><li>UMTS air interface </li></ul><ul><li>Power control in CDMA </li></ul><ul><li>Handoff </li></ul><ul><li>Satellite-UMTS traffic </li></ul><ul><li>Integrated All-IP 3G-WLAN </li></ul>
  40. 40. Reference <ul><li>http://www.europe.alcatel.fr/telecom/rcd/keytech/ </li></ul><ul><li>http://www.comms.eee.strath.ac.uk/~gozalvez/gsm/ </li></ul><ul><li>http://www.gsmworld.com/ </li></ul><ul><li>http://www.ibctelecoms.com/ </li></ul><ul><li>http://www.span.net.au/ </li></ul><ul><li>http://www.cdg.org/tech/a_ross/ </li></ul><ul><li>http://www.nokia.com/networks/mobile/ </li></ul><ul><li>http://www.gsmdata.com/ </li></ul><ul><li>http://www.sds.lcs.mit.edu/~turletti/gmsk/ </li></ul><ul><li>http://www.wirelessweek.com/issues/3G/ </li></ul>
  41. 41. Reference <ul><li>http://www.umts-forum.org/reports.html </li></ul><ul><li>http://www.itu.int/imt/ </li></ul><ul><li>http://www.etsi.org/ </li></ul><ul><li>Flavio Muratore: UMTS, Mobile Communications for future, John Wiley & Sons, 2001. </li></ul><ul><li>S. Dutnall, N. Lobley, A. Clapton, UMTS: The mobile part of broadband communications for the next century IEEE Atm Workshop, Proceedings. p242-252,1998 </li></ul><ul><li>S. Breyer, G. Dega, V. Kumar, L. Szabo, Global view of the UMTS concept Alcatel Telecommunications Review. n 3 1999. p 219-227 </li></ul><ul><li>M. Lee, CDMA Network Security , Prentice-Hall, 1998 </li></ul><ul><li>U. Black, Mobile & Wireless Networks , Prentice-Hall, 1999 </li></ul><ul><li>M. Gallagher, W. Webb, UMTS: The next generation of mobile radio, IEE Review. v 45 n 2 1999. p 59-63 </li></ul>
  42. 42. Reference <ul><li>A. Samukic, UMTS Universal Mobile Telecommunications System:Development of standards for the third generation, IEEE Global Telecommunications Conference & Exhibition. v 4 1998. p 1976-1983 </li></ul><ul><li>N. Prasad, GSM evolution towards third generation UMTS/IMT2000, IEEE International Conference on Personal Wireless Communications 1999, p 50-54 </li></ul><ul><li>A. Samukic, UMTS universal mobile telecommunications system: Development of standards for the third generation, IEEE Transactions on Vehicular Technology. v 47 n 4 Nov 1998. p 1099-1104 </li></ul>
  43. 43. Thank you !!!
  44. 44. UNESCO PROJECT Advanced Course on Networking Professor Khalid Al-Begain UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS University of Damascus, Syria, 06 - 15 April 2004

×