View
390
Download
1
Category
Tags:
Preview:
Citation preview
Chi-Cheng Lin, Winona State University
CS 313 Introduction to Computer Networking &
Telecommunication
Mobile Telephone System and Wireless
LANs
2
Topics
Mobile Telephone System
CDMA
Wireless LANs
3
Mobile Telephone System
First-Generation Mobile PhonesAnalog Voice
Second-Generation Mobile PhonesDigital Voice
Third-Generation Mobile PhonesDigital Voice and Data
4
Advanced Mobile Phone System
Area is divided into cells with an antenna control by a cell office in each cell
Cell offices communicate with MTSO Transmission frequencies cannot be
the same in adjacent cells Cell size is not fixed
Smaller cells used in higher populated area
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Cellular System
6
Advanced Mobile Phone System
(a) Frequencies are not reused in adjacent cells.(b) To add more users, smaller cells can be used.
7
Frequency reuse patterns
©The McGraw-Hill Companies, Inc., 2004
8
Cellular Transmission
Traditionally analogFM used to minimized noise
Digital transmissionCDPD (Cellular Digital Packet Data)
Low-speed digital service over existing cellular network
Based on OSI ModelModem needed
9
Cellular System
HandoffWhen a mobile telephone leaves a cell1. Its base station notices the signal
fading out2. The base station asks all the
surrounding base stations how much power they are getting from it
3. Ownership is transferred to the neighbor base station that receives strongest power
4. The telephone is informed of its new boss
5. If a call is in progress, it will be asked to switch to a new channel
10
Channels
832 full-duplex channelsEach channel consists of 2 simplex
channelsTransmission channels
(849-824)MHz/30KHz 832
Receiving channels(894-869)MHz/30KHz 832
Typically, actual number of voice channel per cell 45
11
Channel Categories
The 832 channels are divided into four categoriesControl (base to mobile) to manage
the systemPaging (base to mobile) to alert users
to calls for themAccess (bidirectional) for call setup
and channel assignmentData (bidirectional) for voice, fax, or
data
12
Second-Generation Mobile Phones
D-AMP
GSM
CDMA
13
D-AMPS Digital Advanced Mobile Phone System
(a) A D-AMPS channel with three users.(b) A D-AMPS channel with six users.
14
GSMGlobal System for Mobile
Communications GSM uses 124 frequency channels,
each of which uses an eight-slot TDM system
15
GSM
A portion of the GSM framing structure.
16
Third-Generation Mobile Phones:Digital Voice and Data
Basic services an IMT-2000 network should provideHigh-quality voice transmissionMessaging
Replace e-mail, fax, SMS, chat, etc.
Multimedia Music, videos, films, TV, etc.
Internet accessWeb surfing, w/multimedia
2.5G, 4G, …
17
Code Division Multiple Access (CDMA)
In FDMA, the bandwidth is divided into channels.
In TDMA, the bandwidth is just one channel that is timeshared.
In CDMA, one channel carries all transmissions simultaneously.
18
CDMA
Every station has a chip sequence Bipolar encoding
1 is represented by +10 is represented by -1Silence is represented by 0
The set of chip sequences are orthogonalA chip sequence is represented by a vector of +1’s
and -1’s (for example, 4 sequences A, B, C, & D)A ● B = A ● C = A ● D = B ● C = B ● D = C ● D = 0A ● A = B ● B = C ● C = D ● D = length of chip sequence
Transmission: s = sum of (bit chip_sequence) Receiving: bit = (s ● chip_sequence) /
chip_length● : inner product of two vectors
19
CDMA(a) Binary chip
sequences for four stations
(b) Bipolar chip sequences
(c) Six examples of transmissions
(d) Recovery of station C’s signal
Q: Why does it work?A: _ _ _ _
20
Wireless LANs
(a) Wireless networking with a base station.(b) Ad hoc networking.
The range of a single radio may not cover the entire system
Multipath fading Noisy
21
Wireless LANs Problems
A multicell 802.11 network
ISM band
22
Wireless LANs
802.11 BroadbandCellular
23
The 802.11 Protocol Stack (part of)
54Mbps5GHz ISM band
11Mbps2.4GHz ISM band
54Mbps2.4GHz ISM band
24
The 802.11 MAC Sublayer Protocol
The hidden station problem.
CSMA/CD does not work (why?) Hidden station and exposed station
problems
The exposed station problem.
25
The 802.11 MAC Sublayer Protocol
Two modes of operations Contention mode
DCF: distributed coordination function CSMA/CA
Collision-free mode PCF: Point coordination function Polling w/ beacon frame
NAV: Network Allocation Vector
26
The 802.11 MAC Sublayer Protocol
Noisy, unreliable Smaller frames reduce probability of frame damages
Fragmentation Each fragment, with its own checksum and sequence
number, is acknowledged individually Stop-and-wait Fragment burst: sequence of fragments
27
The 802.11 MAC Sublayer Protocol
ProblemsNAV terminates at end of the first fragmentHow do DCF and PCF coexist
Solution: interframe spacing
28
The 802.11 Frame Structure
Three classesData, control, management
4 addresses in the frame
Recommended