# Would Wi-Fi Benefit from the GSM Model? Tom Hewer CCS Seminar - 6th Feb 2008 with many thanks to Dr. Jochen Schiller, Free University of Berlin.

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Would Wi-Fi Benefit from the GSM Model? Tom Hewer CCS Seminar - 6th Feb 2008 with many thanks to Dr. Jochen Schiller, Free University of Berlin

Contents Intro to Radio GSM and WiFi Differences of Control Comparison of Implementation Cost Analysis What is Wi-MAX? Discussion

Introduction to Radio A radiowave is an electromagnetic wave propagating through space A radiowave has a phase, wavelength and amplitude The wave carries signals for reception by a receiving node The frequencies discussed here fall between 900MHz and 5GHz Power restrictions are applied by the regulating bodies for transmission Coding methods exist to make more efficient use of the available spectrum Radio waves fall at the lower end of the EM spectrum

How to Transmit a Signal Signals can be represented in three ways: Frequency Amplitude Phase State f [Hz] A [V] I= M cos Q = M sin A [V] t[s] To carry data a wave must have a recognisable state that changes The reception of the state allows for a decision that translates to a result The simplest form of state is: ON or OFF The state can be changed over frequency, amplitude and phase and these can be combined to allow multiple access to the spectrum

OSI Model DLL - Medium access control and error handling PHY - Puts the data on the medium

Multiplexing k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c Frequency Time Time and Frequency Code

Propagation EM waves propagate predictively in free space as per the extended Friis equation: Models exist to calculate propagation in real environments The more complex the model, the more computationally expensive the calculation Propagation models can be fitted by taking real readings of received signal strength and then converging the model until it shows the real data Massively complex propagation models are only suited to the environment for which they are designed and can take into account: Weather, Particulates, Urbanisation, Land Density, Living Matter and others where P r is the received power at distance d P t is the transmitted power, G t and G r are the antenna heights λ is the wavelength of the transmitted signal and L is the system loss

Development of GSM and WiFi cellular phonessatellites wireless LANcordless phones 1992: GSM 1994: DCS 1800 2001: IMT-2000 1987: CT1+ 1982: Inmarsat-A 1992: Inmarsat-B Inmarsat-M 1998: Iridium 1989: CT 2 1991: DECT 199x: proprietary 1997: IEEE 802.11 1999: 802.11b, Bluetooth 1988: Inmarsat-C analoguedigital 1991: D-AMPS 1991: CDMA 1981: NMT 450 1986: NMT 900 1980: CT0 1984: CT1 1983: AMPS 1993: PDC 2000: GPRS 2000: IEEE 802.11a 200?: Fourth Generation (Internet based) 2002: IEEE 802.11b 2004: 3rd Generation 2002: UMTS/WCDMA 2004: IEEE 802.11g 2006: IEEE 802.11n 2006: IEEE 802.16 WiMAX

GSM System Components MS (Mobile Station) BSS (Base Station Subsystem): consisting of BTS (Base Transceiver Station): sender and receiver BSC (Base Station Controller): controlling several transceivers Interfaces U m : radio interface A bis : standardized, open interface with 16 kbit/s user channels A: standardized, open interface with 64 kbit/s user channels UmUm A bis A BSS radio subsystem network and switching subsystem MS BTS BSC MSC BTS BSC BTS MSC

WiFi System One thing to note here is that we are discussing INFRASTRUCTURE based wireless rather than AD-HOC networks! Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS 2 802.11 LAN BSS 1 Access Point STA 1 STA 2 STA 3 ESS Role of the access point: Time Sync Passes ALL data between network and nodes Decide coding technique Decide who can transmit and when It is the BOSS of the network STA - Station ESS - Extended Service Set BSS - Basic Service Set

Hidden Node Problem Node 1 sends request to send (RTS) to router Router checks to see if anybody transmitting If not, router returns a clear to send (CTS) message This allows control when wireless nodes cannot see each other BAC Why is this problem less apparent in GSM networks?

Control GSM control comes from the Broadcast Control Channel (BCCH) which is permanent and dedicated Nodes are assigned channels for transmission and programmed to hop frequencies every 0.2s Frequency hopping follows a pseudo-random sequence with a random seed This is secret and difficult to intercept (if you listen to GSM frequencies they sound like noise, unless you follow the same FHS) It is possible to leave one base station and join another. The handover is controlled by the BCCH measuring signal levels WiFi control comes from the router All nodes must use the same channel in the network Nodes must join the network and provide any require credentials before transmission can begin Security is provided by WEP (weak) or WPA (hard) and keys can be session, server created or user created Once WEP/WPA key entered the data (not the packet headers) is encrypted as in a PKI system Handover to another base-station is currently not possible. The connection must be dropped and then reconnected to the new router (even if they are on the same network with the same settings)

Antennae WiFi antennae are usually omni- directional as : GSM antennae operate in clusters of shielded dipole arrangement : No in UK: ?? No in UK: 350,000 Maximum separation of antennae in Manhattan: 30m!!

Cost Cost to erect one GSM antenna: approx. £75k Cost of standard wireless router: £40 Approx number of antennae required to match GSM coverage by WiFi: 7,000,000 so cost (£2,800M) Antennae in UK split between operating companies, who cross-charge other networks for using their equipment Free use and no separation rules for WiFi so can put routers in any location Who would own the routers, as they would have be located on private premises? Free use and no separation rules for WiFi so can put routers in any location WOULD IT BE WORTH IT?

WiMAX and the Last Mile Last mile is the term to describe the connection from private property to the nearest access point (usually a BT exchange) WiMAX is covered in IEEE802.16 and is a new(er) technology than WiFi (802.11), Ethernet (802.3) and Optic fibre Coverage 31 miles at 70Mbps 10-66GHz Burst Technology Efficient CDM High QoS and Guaranteed Frame Rate Includes protocols for Mesh networks and Synchronous data access, and DOES have handover between networks http://www.ieee802.org/16/tutorial/index.html

Discussion: Over the past 5 years, the number of mobile phones and WiFi networks have increased massively. The technology that runs WiFi networks has moved forward quickly and efficiently (no 3G dead-ends!). Cellular systems require a great level of central control and inter-device operability is limited (currently). Cellular systems offer more guaranteed quality and data transfer (but 802.16 should improve this for WiFi).

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