Transcript Communication Systems 13th lecture - uni

Communication Systems
14th lecture
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2008
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Communication Systems
Last lecture – UMTS infrastructure
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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:
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8th, 11th July – starting at 1:30pm (to catch up with the time of
emitted courses in the beginning of the lecture)
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practical exercises in the computer center seminar room -114 (first
day on IPv6 and SIP, second on QoS)
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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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Communication Systems
Last lecture – UMTS infrastructure
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Last session introduced UMTS
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Network architecture and interfaces, similarities and differences to
GSM/GPRS
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User equipment and USIM
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Core network functionality and protocols (packet switched and circuit
switched domain)
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UTRAN – UTMS radio network subsystem
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RNS, RNC, Node B
 Specific functions of the air interface (cell breathing, rake
receiver)
Network based and connection based functions
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Power control and hand-over
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Communication Systems
Last lecture – UMTS – main network components
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Communication Systems
Last lecture
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UMTS Core Network (CN) migrates from 2G circuit switching to
packet switching as introduced with GPRS to mobile networks
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Thus many components and interfaces taken from GPRS, like the
different GPRS support nodes (GSN)
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Communication Systems
This lecture – UMTS, Authentication, W-CDMA, encoding
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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...
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First WLAN standards tried to introduce layer 2 security
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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
Insecurity in WEP. 802.1x for AAA
802.1i – Link layer encryption, TKIP and CCMP
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Communication Systems
UMTS – the physical layer
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After introduction of physical layer components (Node B) and
principles (rake receiver and macro diversity)
Explanation of the Code Division Multiple Access
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“Chips” instead of combined TDM, FDM
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TDD and FDD frame structure
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...
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Communication Systems
UMTS - WCDMA
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UTMS uses two methods for Terrestrial Radio Access: Frequency
Division Duplex of two paired 5MHz bands
 Wideband CDMA
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Channels are divided via frequency distribution
Time Division Duplex
 A single 5MHz frequency band
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Alternating
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WCDMA und TDMA as multiplexing method4
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Communication Systems
UMTS - WCDMA
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Code Division Multiple Access (CDMA) has some advantages
over the GSM methods
 FDMA, TDMA, CDMA compared in their principles
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Communication Systems
UMTS - WCDMA
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Code Division Multiple Access (CDMA) has some advantages
over the GSM (FDMA, TDMA) methods
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More efficieny in frequncy band usage
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Higher data rates (on demand)
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Longer standby and operation for mobile equipment (less transmit
power needs to be generated)
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Greater ranges between mobile phones and Node Bs (for voice)
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Flexible adjustment of radio traffic onto the demands – voice gaps of
active participants could be used for other traffic channels and users
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Disruption of signal not neccessarily disrupts a session
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Switching from physical to mathematical methods
4
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Communication Systems
UMTS - WCDMA
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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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Communication Systems
UMTS - WCDMA
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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)
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4
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Communication Systems
UMTS – WCDMA – chip computation
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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)
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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)
4
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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)
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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
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Result should be a 6 or -6 which equals to a „1“ set bit or „0“
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WCDMA uses a fixed chiprate of 3,84 MChips/s
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Important is the variable spreading factor
 Different code lengthes are used for up and downlinks
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Spreading factor of 512 in Downlink, thus every Node B uses
the complete code tree
4
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Communication Systems
UMTS – WCDMA, OVSF code tree
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Maximum spreading factor of 256 used in uplink
 Scrambling for the complete code tree needed
4
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Communication Systems
UMTS – WCDMA, OVSF code tree
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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
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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
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Equal spectral distribution
4
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Communication Systems
UMTS – WCDMA
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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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Communication Systems
UMTS – security and authentication
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Security in GSM is weak by our todays standards, mostly broken
and only one way (client-to-network auth)
Authentication in UMTS
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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
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The VLR or SGSN which should authenticate the user requests from
the HLR/AuC 1..n AV(Auth Vectors)
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Each AV is a 5-tupel consisting of
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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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Communication Systems
UMTS – security and authentication
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Communication Systems
UMTS – security and authentication
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RAND and AUTN are sent to the UE/USIM, which checks
AUTN and computes the response RES to the challenge
RAND
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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
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Encryption takes place on the RLC layer and prevents forgery
of data and encryption
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Communication Systems
UMTS – security and authentication
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Functions for authentication and key agreement (AKA)
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f1: computation of MAC (Message Auth. Code)
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f2: computation of MAC, probably shortened
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f3, f4, f5: computation of a key from a random number
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 XOR, || concatenation
Generation of AV (within HLR/AuC)
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Generation of random Sequence Number (SEQ, once at the
beginning)
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Generation of random challenge RAND (per AV)
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AMF (Authentication Key Management Field) to distinguish several
different algorithms
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Communication Systems
UMTS – security and authentication
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Computation of the several values (within HLR/AuC)
 MAC=f1 (SQN || RAND || AMF)
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XRES=f2 (RAND)
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CK=f3 (RAND)
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IK=f4 (RAND)
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AK=f5 (RAND) , anonymity key to anonymize SQN
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AUTN= ((SQN  AK) || AMF || MAC)
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AV= (RAND || XRES || CK || IK || AUTN)
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Communication Systems
UMTS – security and authentication
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Computation of the several values (within USIM)
 Reception of RAND and AUTN from VLR or SGSN
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AK=f5 (RAND)
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SQN=(SQN  AK)  AK
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XMAC=f1 (SQN || RAND || AMF) (eXpected MAC)
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Comparison of XMAC and MAC (from AUTN)
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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
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RES=f2 (RAND)
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Communication Systems
UMTS – security and authentication
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Computation of the several values (within USIM, cont.)
 Send response to VLR or SGSN with RES
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CK=f3 (RAND
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IK=f4 (RAND)
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IK, CK used for RLC encryption
Operation within VLR or SGSN
 Reception of RES from the USIM
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Comparison of RES with XRES (eXpected RES, from AV sent
by HLR/AuC)
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If not equal user authentication failed
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Communication Systems
UMTS – end of mobile telephony part
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Topic switch:
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stay in the mobile network domain but switch from mobile telephony
part
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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
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Increasingly possible with the development of the broad range of
mobile devices and technology
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Communication Systems
Wireless LAN technology - introduction
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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
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Cabling may be very expensive (crossing streets or rivers) or
impossible (historical buildings, prohibition of owners, ...)
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Desire for ad-hoc LANs
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Wish for cable-less offices
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Changing number of connections needed in an office (desktop
pc, laptop, other devices ...)
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Communication Systems
wireless technology - introduction
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Problems to be solved
 which differences exist in comparison to wired LAN
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which data rates are achievable
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security issues (wired network connectors are not easily
misusable if office is locked, but wireless LANs may cross
office/building boundaries easily)
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ranges of different wireless technologies
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how to organize network access of multiple stations
(especially if they are not “see” each other)
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Communication Systems
wireless technology - introduction
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Communication Systems
wireless LAN - history
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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 (WirelessFidelity), 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.
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Communication Systems
wireless LAN – basics
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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
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c = 3  108 m/s = 30 cm/ns
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Communication Systems
wireless LAN – modulation FHSS
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different protocols available
 frequency hopping spread spectrum (FHSS)
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79 channels of 1MHz bandwidth within the 2.4GHz band
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a pseudo random generator initiates each hop
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the minimum hopping distance is 6MHz
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the maximum of 26 participants could share the medium
without bandwidth restriction (but max. bandwidth is 2Mbits)
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if collision occurs the data is simply transferred again
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low power consumption -> used for Bluetooth
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Communication Systems
wireless LAN – modulation DSSS
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different protocols available
 direct sequence spread spectrum (DSSS)
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bundles the 79 channels of 1MHz into broader channels of
5MHz
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a minimum distance of 5 channels should be adhered
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within modulation the signal is spreaded
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the channels may overlap, so the maximum of three
independent services sets are possible
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extension is high rate DSSS
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b standard uses HR-DSSS
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Communication Systems
wireless LAN – modulation OFDM
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different protocols available
 orthogonal frequency (OFDM)
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multi carrier modulation technology
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52 frequency bands, for of them for synchronization
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small bands are less susceptible for disturbance and noise
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avoiding of the use of directly neighbored frequencies
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used for the g and a,j,h standards
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Communication Systems
wireless LAN – media acess
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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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Communication Systems
wireless LAN – media access
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this layer handles
 cyclic redundancy check (CRC)
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fragmentation (no to be confused with IP fragmentation)
authentication
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WEP encryption
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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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Communication Systems
wireless LAN – access protocols
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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
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1 talks to 2
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3 thinks medium is free for use - “hidden station problem”
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Communication Systems
wireless LAN – access protocols: MACA
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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
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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
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destination stations of data exchanges answers with CTS (clear
to send)
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all stations which received RTS have to remain silent for a
given time period
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Communication Systems
wireless LAN – access protocols: MACA
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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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Communication Systems
wireless LAN standards – 802.11 overview
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802.11 is a member of the IEEE 802 family, including several standards
The standards define transmission protocols and brutto bandwidth
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Communication Systems
wireless LAN standards – 802.11 overview
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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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Communication Systems
wireless LAN standards – 802.11 overview
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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)
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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
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band is reserved for WLAN only
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range is more restricted than with 802.11b
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bandwidth is increased up to 54Mbit/s
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Communication Systems
wireless LAN standards – 802.11 overview
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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
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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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Communication Systems
wireless LAN standards – 802.11 overview
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More standards defining several other aspects of WLANs
 c – wireless bridging
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d – world mode (combined definitions for different countries)
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e – quality of service (QoS on layer 2), packet priorization for
real time multimedia and Voice over IP
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f – general definition of roaming between access points (of
different vendors)
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i – authentication and encryption
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k – better measurement of WLAN parameters for increase of
signal quality, dense networks and location based services
(LBS)
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m – summarization of extensions to the protocol
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n – extension of bandwidth up to 108-320Mbit/s
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Communication Systems
wireless LAN standards – 802.11 overview
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Wi-Fi (wireless fidelity)
 certificate of interoperability of wireless devices
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allocation of frequency spectrum
 802.11a,j,h: 8 20-MHz channels in the frequency band from
5,15GHz up to 5,35GHz
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each device is marked with a 48bit MAC address as known
from the ethernet world
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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Communication Systems
wireless LAN – 802.11 operation mode
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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
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Ad-Hoc (peer-to-peer mode, no access point)
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Managed (point-to-point connection from mobile device to access point)
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Access Point (flow control between base station and switch or more than
one base station – for roaming etc.)
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Communication Systems
wireless LAN – 802.11 components and services
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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)
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Communication Systems
wireless LAN – 802.11 Frame format
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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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Communication Systems
wireless LAN – 802.11 (in)security
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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.
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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.
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Matching WEP keys must be statically configured on both
client and infrastructure devices.
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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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Communication Systems
wireless LAN – 802.11 (in)security
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problems
 WLANs are very open
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connection secured through WEP (wired equivalent security),
works with 64 and 128Bit keys
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but: clear text initialization vector (24Bit), which precedes
every packet
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for that reason WEP key is only of 40 or 104Bit
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WEP was cracked four years ago
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The 802.11 specification does not specify key-management
mechanisms. WEP is defined to support only static, preshared
keys.
other solutions ...
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Communication Systems
802.1X - Network Port Authentication
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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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Communication Systems
802.1X - Architecture and component
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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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Communication Systems
802.1X - Architecture and component
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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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Communication Systems
802.1X - EAPOL Encapsulation
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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.
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Communication Systems
802.1X - Typical 802.1X exchange on 802.11
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Communication Systems
802.1X - Typical 802.1X exchange on 802.11
1. The supplicant associates with the 802.11 network.
2. The supplicant starts the 802.1X exchange with an EAPOL-Start
message (step is optional)
3. The authenticator (access point) issues an EAP-Request/Identity
frame
4. The supplicant replies with an EAP-Response/Identity frame, which
is passed on to the RADIUS server as a Radius-Access-Request
packet
5. The RADIUS server determines the type of authentication that is
required, and sends an EAP-Request for the method type. The EAPRequest 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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Communication Systems
802.1X - Typical 802.1X exchange on 802.11
1. The supplicant gathers the reply from the user and sends an EAPResponse 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
2. The RADIUS server grants access with a Radius-Access-Accept
packet, so the authenticator issues an EAP-Success frame and
authorizes the port
3. Immediately following receipt of the Access-Accept packet, the
access point distributes keys to the supplicant using EAPOL-Key
messages
4. Once keys are installed in the supplicant, it can begin sending data
frames to access the network
5. 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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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-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.
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Communication Systems
End/Literature
UMTS

Seminar paper: http://www.ks.unifreiburg.de/download/papers/telsemWS05/UMTS-nextGeneration/UMTSSeminararbeit-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.unifreiburg.de/download/papers/wlanSS06/GrundlagenStandards/Jasinski.pdf

http://dienst.isti.cnr.it/Dienst/Repository/2.0/Body/ercim.cnr.isti/2004TR-27/pdf?tiposearch=cnr&langver
Security:


Matthew Gast: 802.11 Wireless Networks The Definitive Guide, O'Reilly
Seminar paper: http://www.ks.unifreiburg.de/download/papers/wlanSS06/AbsicherungWLANs/SeminararbeitStef
fenSchott.pdf
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