CS 898T Mobile and Wireless Networks - Homework 3 - Name:_________________ Assigned: Wednesday, March 31 Due: Monday, April 12 (in class) Please show all work on a separate sheet attached to this sheet. For each question write key points. (50 points - 10 points for each question) 1. Give reasons for a handover in GSM and the problems associated with it. What are the typical steps for handover, what types of handover can occur? Which resources need to be allocated during handover for data transmission using HSCSD or GPRS respectively? What about QoS guarantees? Ans: o The typical reason for a handover is a weaker signal from the current base station compared with a neighbouring base station. Another reason could be the current load situation: the network could decide to offload some users from a crowded cell. o For the typical steps and types of handover see figures 4.11-4.13. o For HSCSD to succeed the same resources are needed in the new cell as were available in the old one. i.e., there must be enough time-slots available to handle the same number of simultaneous connections. Otherwise the available bandwidth will decrease. Sure the probability of having several channels available is much lower than having a single channel. For GPRS data rates fluctuate anyway depending on the current load. The same happens during and after handover. Without pre-reservation neither HSCSD nor GPRS can give any QoS guarantees. o There is not even a QoS guarantee for a voice call. if the next cell is already completely booked the connection will break upon entering this cell. 2. How can higher data rates be achieved in standard GSM, how is this possible with the additional schemes HSCSD, GPRS, EDGE? What are the main differences of the approaches, also in terms of complexity? What problems remain even if the data rate is increased? Ans: o The classical data rate of GSM is 9.6 kbit/s. Using less FEC 14.4 kbit/s are available, too. These data rates are achievable using a single time-slot per frame in a certain channel. o HSCSD combines several time-slots but leaves coding untouched. GPRS can dynamically use several time-slots per frame plus offers 4 different coding schemes that allow for higher data rates per slot. EDGE finally introduces another modulation scheme (PSK) in addition to GMSK, which offers even higher data rates under good propagation conditions. Only EDGE can really increase the capacity of a GSM cell. o Independent of the coding and modulation schemes the complexity of handover signalling, handover delay and high delay due to coding/interleaving remain. 3. Compare the current situation of mobile phone networks in Europe, Japan, China, and North America. What are the main differences, what are efforts to find a common system or at least interoperable systems? Ans: Currently, the situation is not absolutely clear as the different countries are in different stages implementing 3G systems. Right now no one believes in a common worldwide system, not even the same frequencies are available everywhere: o Europe: After a much discussed licensing process (beauty contests and auctions) many operators are currently deploying 3G systems. Some operators already dropped out, some filed bankruptcy. All operators for 3G will use UMTS, in the beginning the UTRA/FDD mode only (no one knows when and if UTRA/TDD will be deployed). Although licensing did not prescribe the usage of UMTS, there were only a few operators thinking of different systems in the beginning. Start of the system was 2002, 50% of the population should have access to UMTS in 2005 (in Germany). o Japan: Two different 3G systems are available in Japan. NTT DoCoMo uses a variation of UMTS in their W-CDMA system marketed as FOMA. KDDI deploys a cdma2000 system, which is 3G from the version 1xEV-DO on. o China: While most 2G users today use GSM creating the biggest national market for this system, it may be speculated that UMTS will be a major 3G system in China, too, as this system can easily reuse the existing core network in its Release 99. The Chinese development TD-SCDMA was incorporated into UMTS (UTRA/TDD, slow chipping option, Release 4). However, it is currently not clear when and if this system will be deployed. There are also some cdma-operators in China which might opt for cdma2000. o North America: The situation in the US and Canada is quite unclear. Already today many systems exist in parallel without a clear winner (compared to GSM in Europe). Furthermore, licensing of 3G spectrum takes a long time and the availability of spectrum is not clear yet. Thus, it could be the case that EDGE enhanced systems (TDMA and GSM) will be deployed offering higher data rates with EGPRS compared to today's networks. The cdma-operators will go for cdma2000. 4. With a focus on security, what are the problems of WLANs? What level of security can WLANs provide, what is needed additionally and how far do the standards go? Ans: WLANs introduce the air interface which is very simple to eavesdrop. Thus, many WLAN standards introduce more or less strong encryption mechanisms. The most famous one, WEP, has been cracked soon after introduction. Furthermore, the most prominent WLAN family, 802.11, does not provide powerful authentication mechanisms. New standards introduce more security (802.11i), however, users should always use an additional VPN on top of the WLAN to protect privacy and data integrity. WLANs following Bluetooth or HiperLAN2 offer more advanced security functions compared to 802.11. 5. Compare 802.11b and 802.11g. Ans: Wireless 802.11b 802.11g Standard Frequency 2.4 GHz 2.4 GHz Heavily used 2.4 GHz band. Heavily used 2.4 GHz band. Interference from other Interference from other 2.4 GHz devices such as 2.4 GHz devices such as cordless phones, microwave cordless phones, microwave ovens, etc. may occur ovens, etc. may occur Modulation HR-DSSS OFDM Speed 11 Mbps 54 Mbps 5X faster than 802.11b Average Actual 4-5 Mbps 20-25 Mbps Throughput # Channels/ 11 / 3 11 / 3 Non-overlapping Range 30 - 50 m indoors 30 - 50 m indoors (Range will depend 2.4 GHz signal travel 2.4 GHz signal travel on antenna gain, Better range than 802.11a. Better range than 802.11a. transmit power, farther, and can work farther, and can work the receive through walls and floors through walls and floors sensitivity of more effectively than 5 GHz more effectively than 5 GHz the radio card signals signals and any obstacles between path ends.) Compatibility Widely adopted. Will work Backwards compatible with in 802.11g networks 802.11b networks (at 11 Mbps); Incompatible with 802.11a Popularity Currently has the largest Latest ratified standard. user base. 802.11b is With speeds up to 5 times currently used in most hot faster than 802.11b, spots including airports, Expect this standard to hotels, campuses, and public overtake 802.11b as the areas. Wide selection of standard of choice. 802.11b equipment. Relative Cost inexpensive Relatively inexpensive Since this standard's ratification, prices have dropped significantly. Pricing is competitive with 802.11b. Cheaper than 802.11a Benefits Largest user base, cheapest, The speed of 802.11a with used in most public hot the range of 802.11b, spots, largest user base, compatible with 802.11b wide selection of equipment networks and hotspots, affordable