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Communication Systems 13 th lecture

Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006. Communication Systems 13 th lecture. 1 | 49. Communication Systems Last lecture – UMTS infrastructure. Last session devoted to UMTS

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Communication Systems 13 th lecture

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  1. Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006 Communication Systems13th lecture 1 | 49

  2. Communication SystemsLast lecture – UMTS infrastructure • Last session devoted to UMTS • Network architecture and interfaces, similarities and differences to GSM • User equipment and USIM • Core network functionality and protocols (packet switched and circuit switched domain) • UTRAN – UTMS radio network subsystem • RNS, RNC, Node B • Network based and connection based functions • Power control and hand-over • Authentication and security 2 | 49

  3. Communication SystemsLast lecture – UMTS – main network components 3 | 49

  4. Communication SystemsLast lecture • UMTS Core Network (CN) migrates from 2G circuit switching to packet switching as introduced with GPRS to mobile networks • Thus many components and interfaces taken from GPRS, like the different GPRS support nodes (GSN) 4 | 49

  5. Communication SystemsThis lecture – UMTS, W-CDMA, encoding, WLAN • Start with UMTS Frequency Division Duplex and WCDMA • Explanation of the code duplexing • Then switch over to other wireless technologies used for packet switched networks (IP) • Wireless LAN, widely deployed technology at consumers homes, unversities, companies... • Rather short overview on different WLAN standards • modulation, media access protocol MACA • 802.11 a/b/g standards and other standards • operation mode, components, services • First WLAN standards tried to introduce layer 2 security • Insecurity in WEP. 802.1x for AAA • 802.1i – Link layer encryption, TKIP and CCMP 5 | 49

  6. Communication SystemsUMTS - WCDMA • UTMS uses two methods for Terrestrial Radio Access: Frequency Division Duplex of two paired 5MHz bands • Wideband CDMA • Channels are divided via frequency distribution • Time Division Duplex • A single 5MHz frequency band • Alternating • WCDMA und TDMA als Multiplexverfahren • 4 6 | 49

  7. Communication SystemsUMTS - WCDMA • Code Division Multiple Access (CDMA) has some advantages over the GSM methods • FDMA, TDMA, CDMA compared in their principles • 4 7 | 49

  8. Communication SystemsUMTS - WCDMA • Code Division Multiple Access (CDMA) has some advantages over the GSM (FDMA, TDMA) methods • More efficieny in frequncy band usage • Higher data rates (on demand) • Longer standby and operation for mobile equipment (less transmit power needs to be generated) • Greater ranges between mobile phones and Node Bs (for voice) • Flexible adjustment of radio traffic onto the demands – voice gaps of active participants could be used for other traffic channels and users • Disruption of signal not neccessarily disrupts a session • Switching from physical to mathematical methods • 4 8 | 49

  9. Communication SystemsUMTS - WCDMA • WCDMA: • Codemultiplex vs. Frequency / time multiplex • Multiple signale on just one frequency • Demultiplexing independent of channel bundling • Per participant a binary channalization code is used • 4 9 | 49

  10. Communication SystemsUMTS - WCDMA • Channalization code is used for and decoding and is spread with a vector of a length of e.g. 128Bit • No bits but so called chips are used • The Codes have to be orthogonal • Example for a chipping length of 6 User code A: (0,1,0,0,1,1) User code B: (1,1,0,1,0,1) • 4 10 | 49

  11. User B sends Bd=1 Key Bk = (0,1,0,0,1,1) Non return to zero computed of Bd & Bk Chips sent: Bs = Bd * Bk Results in (-1,+1,-1,-1,+1,+1) Communication SystemsUMTS – WCDMA – chip computation • User A sends Ad=1 • Key Ak = (0,1,0,0,1,1) • Non return to zero computed of Ad & Ak • Chips sent: • As = Ad * Ak • Results in (-1,+1,-1,-1,+1,+1) • 4 11 | 49

  12. Communication SystemsUMTS – WCDMA, OVSF code tree • Addition of all chips: As + Bs = (-1,+1,-1,-1,+1,+1) + (-1,-1,+1,-1,+1,-1) = (-2, 0, 0,-2,+2, 0) • Decoding check all received chips with Ak / Bk (NRZ) Ae = (-2, 0, 0,-2,+2, 0) * Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6 Be = (-2, 0, 0,-2,+2, 0) * Bk = -2 + 0 + 0 - 2 – 2 + 0 = -6 • Result should be a 6 or -6 which equals to a „1“ set bit or „0“ • WCDMA uses a fixed chiprate of 3,84 MChips/s • Important is the variable spreading factor • Different code lengthes are used for up and downlinks • Spreading factor of 512 in Downlink, thus every Node B uses the complete code tree • 4 12 | 49

  13. Communication SystemsUMTS – WCDMA, OVSF code tree • Maximum spreading factor of 256 used in uplink • Scrambling for the complete code tree needed • 4 13 | 49

  14. Communication SystemsUMTS – WCDMA, OVSF code tree • If code on a node in the code tree is assigned, the subsequent codes could not be assigned to other (not orthogonal then) • Scrambling of signals is the following • Multiplication of a code sequence of 1 and -1 (NRZ) into the signal • Assigned identity via the scrambling code is nearly 100% orthogonal • Advantages time shifited sending (Position within a cell) • Deliniation toward bordering cells • Equal spectral distribution • 4 14 | 49

  15. Communication SystemsUMTS – WCDMA • Chips instead of bits has some advantages and disadvantages • Negative is that you have to send e.g. 128 times more data and reduces the data rate extremely • Positive is to increase the transmission qualitty • More codes means more orthogonals thus 128 users on one Node B • WCDMA allows a reduced signal/noise ratio, thus • Reduced transmit power needed, Processing Gain achieved • Tradeoff: High spreading factor (SF) allows high processing gain but low data rate, low Sfgets low processing gain but high data rates 15 | 49

  16. Communication SystemsUMTS – end of mobile telephony part • At this point cut of the wired and mobile telephony part and return to infrastructures mainly developed for internet protocol services • During the next periods we will see a move of both networks IP and telephony into a more merged on – principles and ideas o the other one could be found in each network (IP in UTMS infrastucture, ideas of telephony in VoIP services) • Have a look on some other wireless technologies for data transmission • Same idea: The user should be able to move around but keep a network connection • Increasingly possible with the development of the broad range of mobile devices and technology 16 | 49

  17. Communication SystemsWireless LAN technology - introduction • Ethernet defines LAN standards for relatively short distances over coaxial cable, twisted pair copper wire or fiber optics • ADSL extends the data rates achievable on customers telephone connections over (old) two wire copper • But: • Cable may not present everywhere • Cabling may be very expensive (crossing streets or rivers) or impossible (historical buildings, prohibition of owners, ...) • Desire for ad-hoc LANs • Wish for cable-less offices • Changing number of connections needed in an office (desktop pc, laptop, other devices ...) 17 | 49

  18. Communication Systemswireless technology - introduction • Problems to be solved • which differences exist in comparison to wired LAN • which data rates are achievable • security issues (wired network connectors are not easily misusable if office is locked, but wireless LANs may cross office/building boundaries easily) • ranges of different wireless technologies • how to organize network access of multiple stations (especially if they are not “see” each other) 18 | 49

  19. Communication Systemswireless technology - introduction 19 | 49

  20. Communication Systemswireless LAN - history • 1997 the IEEE approved 802.11, which specified the characteristics of devices with a signal rate of 1 and 2 Mb/s. • The standard specifies the MAC and the physical layers for transmissions in the 2.4 GHz band. • 1999, the IEEE ratified a new amendment, called IEEE 802.11b, which works at additional signal rates of 5.5 and 11 Mb/s. • Hereinafter, to the IEEE 802.11 standards as Wi-Fi(Wireless-Fidelity), certifying device interoperability. • 1999, the IEEE approved the specifications of 802.11a, which uses the 5 Ghz band. The signal rates are 6, 9, 12, 18, 24, 36, 48 and 54 Mb/s. • In 2003, the IEEE approved 802.11g as a further evolution of the 802.11 standard. • 802.11g provides the same performance as 802.11a, while working in the 2.4 GHz band. Compatible with 802.11b devices. 20 | 49

  21. Communication Systemswireless LAN – basics • Moving electrons send out waves, which spread in free space, vacuum • Frequency(f): number of oscillations per second measured in Hertz (Hz) • Wavelength (λ) is the distance between two maxima • Speed of wave spreading in vacuum • c = 3  108 m/s = 30 cm/ns 21 | 49

  22. Communication Systemswireless LAN – modulation FHSS • different protocols available • frequency hopping spread spectrum (FHSS) • 79 channels of 1MHz bandwidth within the 2.4GHz band • a pseudo random generator initiates each hop • the minimum hopping distance is 6MHz • the maximum of 26 participants could share the medium without bandwidth restriction (but max. bandwidth is 2Mbits) • if collision occurs the data is simply transferred again • low power consumption -> used for Bluetooth 22 | 49

  23. Communication Systemswireless LAN – modulation DSSS • different protocols available • direct sequence spread spectrum (DSSS) • bundles the 79 channels of 1MHz into broader channels of 5MHz • a minimum distance of 5 channels should be adhered • within modulation the signal is spreaded • the channels may overlap, so the maximum of three independent services sets are possible • extension is high rate DSSS • b standard uses HR-DSSS 23 | 49

  24. Communication Systemswireless LAN – modulation OFDM • different protocols available • orthogonal frequency (OFDM) • multi carrier modulation technology • 52 frequency bands, for of them for synchronization • small bands are less susceptible for disturbance and noise • avoiding of the use of directly neighbored frequencies • used for the g and a,j,h standards 24 | 49

  25. Communication Systemswireless LAN – media acess • wireless LANs need more complex media access protocols than wired LANs • restricted range of signals makes it more difficult to have a global signal detection • move from cell to cell should be possible (roaming), so a mobile station could communicate during transit • OSI layer 2 is split up once more • a special MAC sublayering is introduced 25 | 49

  26. Communication Systemswireless LAN – media access • this layer handles • cyclic redundancy check (CRC) • fragmentation (no to be confused with IP fragmentation) • authentication • WEP encryption • auto roaming • with the latter a unified network over more than one station becomes possible • other layer is physical layer convergence protocol • e.g. defines modulation: FHSS, DSSS, HR-DSSS, OFDM, IrDA 26 | 49

  27. Communication Systemswireless LAN – access protocols • would think of CSMA/CD first (carrier sense multiple access with collision detection seems to solve our problem) • but see picture below • restricted range of signals • 1 talks to 2 • 3 thinks medium is free for use - “hidden station problem” 27 | 49

  28. Communication Systemswireless LAN – access protocols: MACA • or inefficient use of given bandwidth • if 1 transfers to 2 (or vice versa), 3 could think that medium is blocked and does not transfer to 4 • give away of bandwidth - “exposed station problem” • therefore new access protocol: MACA (multiple access with collision avoidance) • before data is transferred send out a short test sequence: RTS (ready to send) – sender asks if medium is available for transferring data packets • destination stations of data exchanges answers with CTS (clear to send) • all stations which received RTS have to remain silent for a given time period 28 | 49

  29. Communication Systemswireless LAN – access protocols: MACA • There is an optimization of this protocol: MACA (W), W for wireless • Other protocol (but rather different) using collision avoidance – TokenRing, FDDI 29 | 49

  30. Communication Systemswireless LAN standards – 802.11 overview • 802.11 is a member of the IEEE 802 family, including several standards • The standards define transmission protocols and brutto bandwidth 30 | 49

  31. Communication Systemswireless LAN standards – 802.11 overview • b – available several years, 11Mbit/s, 2.4GHz, 13 channels in Germany • but only 3 non overlapping channels • utilize the free 2.4GHz band → it is rather packed, no exclusive use • no guarantees (if you mess with other bands someone of the RegTP will stop you, only excess of output power is prohibited) • special protocol implementations needed to cope with noise, fading, ... • not standardized 22Mbits in 2.4GHz band of several vendors (sometimes called b+, channel bundling) • g – defined 2003, 54Mbits, 2.4GHz, OFDM • most of the hardware sold at the moment confirms to this standard • backward compatible to “b”, but then more overhead compared to “clean” g standard networks (preamble an initialization sequence must be handled within b standard) 31 | 49

  32. Communication Systemswireless LAN standards – 802.11 overview • a – 54Mbit/s standard for the 5GHz band, 12 non-overlapping channels, OFDM (orthogonal frequency division multiplexing), restricted output power • introduction of transmit power control (TPC) and dynamic frequency selection (DFC) • DFS should reduce the transmission power so it is sufficient for a given connection but does not spread farther than needed • it checks if the used frequency is free and sufficient, if not tries to switch over to another frequency with DFC • band is reserved for WLAN only • range is more restricted than with 802.11b • bandwidth is increased up to 54Mbit/s 32 | 49

  33. Communication Systemswireless LAN standards – 802.11 overview • h – 54Mbit/s standard extension for Europe within the 5GHz band, defined standards for indoor and outdoor use • j – extension of a for Japan • the usable bandwidth for the several standards is much lower than the maximum one • in b standard networks under optimal environment up to nearly 6Mbit/s of higher level protocol data speed is possible • in g standard you might achieve up to 30Mbit/s • the remaining capacity is consumed for WLAN preambles, protocol headers, coordination 33 | 49

  34. Communication Systemswireless LAN standards – 802.11 overview • More standards defining several other aspects of WLANs • c – wireless bridging • d – world mode (combined definitions for different countries) • e – quality of service (QoS on layer 2), packet priorization for real time multimedia and Voice over IP • f – general definition of roaming between access points (of different vendors) • i – authentication and encryption • k – better measurement of WLAN parameters for increase of signal quality, dense networks and location based services (LBS) • m – summarization of extensions to the protocol • n – extension of bandwidth up to 108-320Mbit/s 34 | 49

  35. Communication Systemswireless LAN standards – 802.11 overview • Wi-Fi (wireless fidelity) • certificate of interoperability of wireless devices • each device is marked with a 48bit MAC address as known from the ethernet world • allocation of frequency spectrum • 802.11a,j,h: 8 20-MHz channels in the frequency band from 5,15GHz up to 5,35GHz • 802.11b and g: 14 channels in the 2,4GHz band • distribution of channels different in different countries, not all channels available in every country • with a tight woven network of access points a clever setup of channels is needed 35 | 49

  36. Communication Systemswireless LAN – 802.11 operation mode • more than one access point in a given area possible if channels are at least by a number three away from each other • WLAN of 802.11 offer several operation modes • Ad-Hoc (peer-to-peer mode, no access point) • Managed (point-to-point connection from mobile device to access point) • Access Point (flow control between base station and switch or more than one base station – for roaming etc.) 36 | 49

  37. Communication Systemswireless LAN – 802.11 components and services • In managed mode 802.11 provides nine Services: • Distribution • Integration • Association • Reassociation • Disassociation • Authentication • Deauthentication • Confidentiality • MSDU delivery • Transmit Power Control (TPC) • Dynamic Frequency Selection (DFS) 37 | 49

  38. Communication Systemswireless LAN – 802.11 Frame format • Protocol version: At present, protocol number 0 • Type and subtype: identify the type of frame • ToDS and FromDS bits: whether a frame is destined for distribution system • Retry bit: any retransmitted frames set this bit to 1 • Power management bit: indicates whether the sender will be in a powersaving mode after the completion of the current atomic frame exchange. 38 | 49

  39. Communication Systemswireless LAN – 802.11 (in)security • AAA: Authentication, Authorization, Accounting • 802.11 specification defines Open and Shared Key authentication. • Open authentication is a null authentication algorithm. The AP grants any request for authentication. • Shared Key authentication requires that the client station and the AP have WEP enabled and have matching WEP keys • 802.11 specification defines WEP to provide data encryption. • WEP is based on the RC4 symmetric stream cipher. • Matching WEP keys must be statically configured on both client and infrastructure devices. • You can define up to four keys on a device, but you can use only one at a time for encrypting outbound frames. 39 | 49

  40. Communication Systemswireless LAN – 802.11 (in)security • problems • WLANs are very open • connection secured through WEP (wired equivalent security), works with 64 and 128Bit keys • but: clear text initialization vector (24Bit), which precedes every packet • for that reason WEP key is only of 40 or 104Bit • WEP was cracked four years ago • The 802.11 specification does not specify key-management mechanisms. WEP is defined to support only static, preshared keys. • other solutions ... 40 | 49

  41. Communication Systems802.1X - Network Port Authentication • Port-Based Network Access Control • Provides a framework for user authentication and key management over any LANs, including wireless LAN. • The "port" in 802.1X on wireless LAN is an association between a wireless device and its access point. • Authenticate users rather than machines. • Authentication is at the link layer • It is an IEEE adaptation of the IETF's Extensible Authentication Protocol (EAP). • Can update keys dynamically periodically 41 | 49

  42. Communication Systems802.1X - Architecture and component • 802.1X defines 3 components: • Supplicant: Resides on the WLAN client, e.g., end user machine that seeks access to network resources. • Authenticator: Resides on the AP, controlling network access. It terminates only the link-layer authentication exchange and does not maintain user information. • Authentication server: Resides on the RADIUS server, implementing actual authentication mechanism. 42 | 49

  43. Communication Systems802.1X - Architecture and component • Both the supplicant and the authenticator are referred to as Port Authentication Entities (PAEs). • The authentication exchange is logically carried out between the supplicant and the authentication server, with the authenticator acting only as a bridge. • From the supplicant to the authenticator (the "front end"), the protocol is EAP over LANs (EAPOL), as defined by 802.1X. • On the "back end," EAP is carried in RADIUS packets. Some documentation may refer to it as "EAP over RADIUS." 43 | 49

  44. Communication Systems802.1X - EAPOL Encapsulation • EAPOL messages can be encapsulated in both wired Ethernet and 802.11. • Ethernet Type: two-byte type code assigned to EAPOL: 88-8e. • Version: Version 1 was standardized in the 2001 version of 802.1X; version 2 was specified in 802.1X-2004. • Packet Type: EAP messages, EAPOL messages to adapt EAP to the port-based LAN environment. 44 | 49

  45. Communication Systems802.1X - Typical 802.1X exchange on 802.11 45 | 49

  46. Communication Systems802.1X - Typical 802.1X exchange on 802.11 The supplicant associates with the 802.11 network. The supplicant starts the 802.1X exchange with an EAPOL-Start message (step is optional) The authenticator (access point) issues an EAP-Request/Identity frame The supplicant replies with an EAP-Response/Identity frame, which is passed on to the RADIUS server as a Radius-Access-Request packet The RADIUS server determines the type of authentication that is required, and sends an EAP-Request for the method type. The EAP-Request is encapsulated in a Radius-Access-Challenge packet to the AP. When it reaches the AP, the EAP-Request is passed on to the supplicant. 46 | 49

  47. Communication Systems802.1X - Typical 802.1X exchange on 802.11 The supplicant gathers the reply from the user and sends an EAP-Response in return. The response is translated by the authenticator into a Radius-Access-Request with the response to the challenge as a data field. Steps 5 and 6 repeat as many times as is necessary to complete the authentication The RADIUS server grants access with a Radius-Access-Accept packet, so the authenticator issues an EAP-Success frame and authorizes the port Immediately following receipt of the Access-Accept packet, the access point distributes keys to the supplicant using EAPOL-Key messages Once keys are installed in the supplicant, it can begin sending data frames to access the network When the supplicant is done accessing the network, it sends an EAPOL-Logoff message to put the port back into an unauthorized state 47 | 49

  48. Communication Systems802.1i – Link layer encryption, TKIP and CCMP • 802.1X provides a framework for authentication and key management • The major remaining flaw is the lack of confidentiality provided by WEP encryption. • 802.11i takes a two-track approach to addressing the weaknesses in link-layer encryption. • Its major components are two new link-layer encryption protocols. • Temporal Key Integrity Protocol (TKIP): designed to bolster security to the greatest extent possible on pre-802.11i hardware. (initially called “WEP2”) • Counter Mode with CBC-MAC Protocol (CCMP): a new encryption protocol designed from the ground up to offer the highest level of security possible. 48 | 49

  49. Communication SystemsEnd/Literature • UMTS • Seminar paper: http://www.ks.uni-freiburg.de/download/papers/telsemWS05/UMTS-nextGeneration/UMTS-Seminararbeit-Stefan%20Nagy.pdf • 802.11 WLAN • Kurose & Ross: Computer Networking (3rd): Section 6.3 • Tanenbaum: Computer Networks (4th): Section 2.3.1, Section 4.4 • Matthew Gast: 802.11 Wireless Networks The Definitive Guide, O'Reilly • Seminar paper: http://www.ks.uni-freiburg.de/download/papers/wlanSS06/GrundlagenStandards/Jasinski.pdf • http://dienst.isti.cnr.it/Dienst/Repository/2.0/Body/ercim.cnr.isti/2004-TR-27/pdf?tiposearch=cnr&langver • Security: • Matthew Gast: 802.11 Wireless Networks The Definitive Guide, O'Reilly • Seminar paper: http://www.ks.uni-freiburg.de/download/papers/wlanSS06/AbsicherungWLANs/SeminararbeitSteffenSchott.pdf 49 | 49

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