1. Communication Systems 14th lecture
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2008
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2. Communication Systems Last lecture – UMTS infrastructure
Please hand in the exercise sheet #6, next will be handed out in the next practical exercise
Sheet #7 is due for the 15th July (next lecture)‏
Next two dates:
8th, 11th July – starting at 1:30pm (to catch up with the time of emitted courses in the beginning of the lecture)‏
practical exercises in the computer center seminar room -114 (first day on IPv6 and SIP, second on QoS)‏
please grab your older exercise sheets there to have a reference for exam preparation (we got quite a pile of papers by now :))‏
Type of exam still in discussion – request for a written version pending ...
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3. Communication Systems Last lecture – UMTS infrastructure
Last session introduced UMTS
Network architecture and interfaces, similarities and differences to GSM/GPRS
User equipment and USIM
Core network functionality and protocols (packet switched and circuit switched domain)‏
UTRAN – UTMS radio network subsystem
RNS, RNC, Node B
Specific functions of the air interface (cell breathing, rake receiver)‏
Network based and connection based functions
Power control and hand-over
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4. Communication Systems Last lecture – UMTS – main network components
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5. Communication Systems Last 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)‏
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6. Communication Systems This lecture – UMTS, Authentication, W-CDMA, encoding
Start with network authentication
UMTS physical layer: 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
a/b/g standards and other standards
operation mode, components, services
First WLAN standards tried to introduce layer 2 security
Insecurity in WEP x for AAA
802.1i – Link layer encryption, TKIP and CCMP
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7. Communication Systems UMTS – the physical layer
After introduction of physical layer components (Node B) and principles (rake receiver and macro diversity)‏
Explanation of the Code Division Multiple Access
“Chips” instead of combined TDM, FDM
TDD and FDD frame structure
...
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8. Communication Systems UMTS - 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 as multiplexing method4
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9. Communication Systems UMTS - WCDMA
Code Division Multiple Access (CDMA) has some advantages over the GSM methods
FDMA, TDMA, CDMA compared in their principles
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10. Communication Systems UMTS - 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
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11. Communication Systems UMTS - 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
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12. Communication Systems UMTS - 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
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13. Communication Systems UMTS – 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
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)‏
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14. Communication Systems UMTS – 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 = = 6
Be = (-2, 0, 0,-2,+2, 0) * Bk = – = -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
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15. Communication Systems UMTS – WCDMA, OVSF code tree
Maximum spreading factor of 256 used in uplink
Scrambling for the complete code tree needed 4
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16. Communication Systems UMTS – 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
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17. Communication Systems UMTS – 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
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18. Communication Systems UMTS – security and authentication
Security in GSM is weak by our todays standards, mostly broken and only one way (client-to-network auth)‏
Authentication in UMTS
Base is a common secret key K, which is only known by the USIM (User Services Identity Module) in the UE and by the HLR/AuC of the provider
The VLR or SGSN which should authenticate the user requests from the HLR/AuC 1..n AV(Auth Vectors)‏
Each AV is a 5-tupel consisting of
RAND (random challenge) and XRES (expected response) for the user authentication
CK (cipher key) for protection of confidentiality, IK (integrity key) for protection of integrity, AUTN (auth token) for network authentication
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19. Communication Systems UMTS – security and authentication
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20. Communication Systems UMTS – security and authentication
RAND and AUTN are sent to the UE/USIM, which checks AUTN and computes the response RES to the challenge RAND
RES is sent to the VLR/SGSN which compares it to XRES
Integrity and confidentiality
By request of MSC/VLR or SGSN the communication can be encrypted with CK or IK between UE and RNC
Encryption takes place on the RLC layer and prevents forgery of data and encryption
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21. Communication Systems UMTS – security and authentication
Functions for authentication and key agreement (AKA)‏
f1: computation of MAC (Message Auth. Code)‏
f2: computation of MAC, probably shortened
f3, f4, f5: computation of a key from a random number
 XOR, || concatenation
Generation of AV (within HLR/AuC)‏
Generation of random Sequence Number (SEQ, once at the beginning)‏
Generation of random challenge RAND (per AV)‏
AMF (Authentication Key Management Field) to distinguish several different algorithms
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22. Communication Systems UMTS – security and authentication
Computation of the several values (within HLR/AuC)‏
MAC=f1 (SQN || RAND || AMF)‏
XRES=f2 (RAND)‏
CK=f3 (RAND)‏
IK=f4 (RAND)‏
AK=f5 (RAND) , anonymity key to anonymize SQN
AUTN= ((SQN  AK) || AMF || MAC)‏
AV= (RAND || XRES || CK || IK || AUTN)‏
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23. Communication Systems UMTS – security and authentication
Computation of the several values (within USIM)‏
Reception of RAND and AUTN from VLR or SGSN
AK=f5 (RAND)‏
SQN=(SQN  AK)  AK
XMAC=f1 (SQN || RAND || AMF) (eXpected MAC)‏
Comparison of XMAC and MAC (from AUTN)‏
If this procedure fails the authentication of network does not succeed and the UE sees the cell as forbidden
Check if sequence number is from the expected range
RES=f2 (RAND)‏
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24. Communication Systems UMTS – security and authentication
Computation of the several values (within USIM, cont.)‏
Send response to VLR or SGSN with RES
CK=f3 (RAND
IK=f4 (RAND)‏
IK, CK used for RLC encryption
Operation within VLR or SGSN
Reception of RES from the USIM
Comparison of RES with XRES (eXpected RES, from AV sent by HLR/AuC)‏
If not equal user authentication failed
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25. Communication Systems UMTS – end of mobile telephony part
Topic switch:
stay in the mobile network domain but switch from mobile telephony part
return to infrastructures mainly developed for Internet protocol / packet switched networks
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
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26. Communication Systems Wireless 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 ...)‏
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27. Communication Systems wireless 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)‏
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28. Communication Systems wireless technology - introduction
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29. Communication Systems wireless LAN - history
1997 the IEEE approved , 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 b, which works at additional signal rates of 5.5 and 11 Mb/s.
Hereinafter, to the IEEE standards as Wi-Fi (Wireless- Fidelity), certifying device interoperability.
1999, the IEEE approved the specifications of a, 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 g as a further evolution of the standard.
802.11g provides the same performance as a, while working in the 2.4 GHz band. Compatible with b devices.
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30. Communication Systems wireless 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
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31. Communication Systems wireless 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
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32. Communication Systems wireless 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
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33. Communication Systems wireless 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
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34. Communication Systems wireless 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
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35. Communication Systems wireless 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
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36. Communication Systems wireless 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”
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37. Communication Systems wireless 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
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38. Communication Systems wireless 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
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39. Communication Systems wireless LAN standards – 802.11 overview
is a member of the IEEE 802 family, including several standards
The standards define transmission protocols and brutto bandwidth
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40. Communication Systems wireless 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)‏
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41. Communication Systems wireless 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 b
bandwidth is increased up to 54Mbit/s
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42. Communication Systems wireless 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
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43. Communication Systems wireless 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 Mbit/s
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44. Communication Systems wireless 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
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45. Communication Systems wireless 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 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.)‏
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46. Communication Systems wireless LAN – 802.11 components and services
In managed mode provides nine Services:
Distribution
Integration
Association
Reassociation
Disassociation
Authentication
Deauthentication
Confidentiality
MSDU delivery
Transmit Power Control (TPC)‏
Dynamic Frequency Selection (DFS)‏
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47. Communication Systems wireless 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.
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48. Communication Systems wireless LAN – 802.11 (in)security
AAA: Authentication, Authorization, Accounting
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
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.
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49. Communication Systems wireless 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 specification does not specify key-management mechanisms. WEP is defined to support only static, preshared keys.
other solutions ...
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50. Communication Systems 802.1X - 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
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51. Communication Systems 802.1X - 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.
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52. Communication Systems 802.1X - 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."
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53. Communication Systems 802.1X - EAPOL Encapsulation
EAPOL messages can be encapsulated in both wired Ethernet and
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.
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54. Communication Systems 802.1X - Typical 802.1X exchange on 802.11
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55. Communication Systems 802.1X - Typical 802.1X exchange on 802.11
The supplicant associates with the 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.
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56. Communication Systems 802.1X - 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
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57. Communication Systems 802.1i – 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 i 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.
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58. Communication Systems End/Literature
UMTS
Seminar paper: freiburg.de/download/papers/telsemWS05/UMTS-nextGeneration/UMTS- Seminararbeit-Stefan%20Nagy.pdf
WLAN
Kurose & Ross: Computer Networking (3rd): Section 6.3
Tanenbaum: Computer Networks (4th): Section 2.3.1, Section 4.4
Matthew Gast: Wireless Networks The Definitive Guide, O'Reilly
Seminar paper: freiburg.de/download/papers/wlanSS06/GrundlagenStandards/Jasinski.pdf
TR-27/pdf?tiposearch=cnr&langver
Security:
Seminar paper: freiburg.de/download/papers/wlanSS06/AbsicherungWLANs/SeminararbeitStef fenSchott.pdf
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