1 | 49 communication systems 14 th lecture chair of communication systems department of applied...

1 | 49

Communication Systems14th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2008

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Communication SystemsLast 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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Communication SystemsLast 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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Communication SystemsLast lecture – UMTS – main network components

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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)

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Communication SystemsThis 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 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

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Communication SystemsUMTS – 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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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 as multiplexing method4

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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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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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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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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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Communication SystemsUMTS – WCDMA – chip computation

User A sends Ad=1

Key Ak = (0,1,0,0,1,1)

Non return to zero computed of A

d & A

k

Chips sent: A

s = A

d * A

k

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 B

d & B

k

Chips sent: B

s = B

d * B

k

Results in

(-1,+1,-1,-1,+1,+1)

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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

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Maximum spreading factor of 256 used in uplink Scrambling for the complete code tree needed

4

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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 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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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

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Communication SystemsUMTS – 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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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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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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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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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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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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Communication SystemsUMTS – 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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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 ...)

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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)

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Communication Systemswireless technology - introduction

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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.

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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

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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

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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

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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

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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

Communication Systemswireless LAN – media acess

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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

Communication Systemswireless LAN – media access

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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 1 talks to 2 3 thinks medium is free for use - “hidden station problem”

Communication Systemswireless LAN – access protocols

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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 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

Communication Systemswireless 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

Communication Systemswireless LAN – access protocols: MACA

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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

Communication Systemswireless 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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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


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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 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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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


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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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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.)

Communication Systemswireless LAN – 802.11 operation mode

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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)

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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.

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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. 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.

Communication Systemswireless LAN – 802.11 (in)security

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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 ...

Communication Systemswireless LAN – 802.11 (in)security

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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

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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.

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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."

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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.

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Communication Systems802.1X - Typical 802.1X exchange on 802.11

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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

frame4. 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 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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1. 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

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 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.

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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

Communication SystemsEnd/Literature

1 | 49 communication systems 14 th lecture chair of communication systems department of applied...

Documents

frequency division duplex

ccmpcommunication systemsumts

mobile equipment

usimcore network functionality

terrestrial radio access

lectureumts core network

link layer encryption

protocols packet