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cellular telephony
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McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-1
Cellular Telephone
This chapter contains some slides from chapter 17 of Data Communications and networking, 3rd ed. by B. Forouzan. The borrowed slides contain the copyright notice of McGraw-Hill at the bottom of the slide. Most of these slides are either modified or supplemented by additional information. Slides without any copyright notice were taken from the web.
Part II : Wireless Communication
Chapter 5
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-2Cellular System
Mobile station
Base station(each cell has one BS)Also called Mobile Telephone Switching Office (MTSO)
Coordinates communication between all base stations and the telephone central office
Size of cell depends on population(1-12 miles radius)
This network contains telephone central offices (COs)
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-3Cellular System (Cont.)
The neighboring cells can not use the same set of frequencies in order to avoid interference between MS close to cell boundaries. Since the available frequencies are a limited resource, they have to be reused. Patterns below show variousfrequency reuse factors (cells with same numbers can use the same set of frequencies.)
One cell separation Two cells separation
Frequency Reuse
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-4AMPSCellular Bands for AMPS
25 MHz forward and reverse bands832 full-duplex channels (42 for control, 790 for voice) Frequency reuse factor = 7 !!!! 790/7 channels available per cell
25 MHz 25 MHzUplink
Downlink
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-5
Figure 17.4 AMPS reverse communication band
AMPS (Cont.)Reverse Communication Band
Uplink
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-6Second Generation Cellular Systems
Digital AMPS1991
.
824-849 MHz (upload)869-894 MHz (download)
Global System for Mobile Communication
(European standard)1990
.
890-915 MHz (upload)935-960 MHz (download)
Code Division Multiple Access
1993.
824-849 MHz (upload)869-894 MHz (download)
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-7
Total subscribers in 2002 = 249.1 million
Second Generation Cellular Systems
(Published by Lucent Technologies)
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-8D-AMPS
VSELPVector Sum Excited Linear Predictive speech coding
Frequency reuse factor = 7
(Originally defined as IS-54, later revised by IS-136)
D-AMPS uses the same band and channels as AMPS
25 1944-bit frames per second
Control Voice FEC64 bits 159 bits 101 bits
Reverse communication band
Uplink
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-9GSM
= =
Frequency Bands
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-10GSM (Cont.)
RPE-LTPRegular Pulse Excited Long
Term Prediction speech coding(Excellent speech quality at high
complexity, noise immune)
GMSKGaussian Minimum
Shift Keying(used in Europe)
24 traffic + 2 control frames
890-915 MHz(935-960 MHz downlink)
260-bit samples each 20ms = 13 kbps
Uplink
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-11GSM (Cont.)Multiframe Components
A lot of overhead in TDM framesBecause of complex error correctionGSM allows a r frequency reuse factor of 3
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-12IS-95 (Cont.)
Downlink
Forward Transmission
CELPCode excited linear predictive speech coding (very complex, very high quality)
1 bit of 64(not DSSS, for privacy)
64 chip Walsh code (Orthogonal spreading)Code assigned to MS during the call setup
For synchronization of demodulator/CDMAand for signal strength monitoring for handoff
64 CDMA channels(9 control and sync, 55 voice)
Channels 1-7 for pagingchannel 32 for info to MS about the system
869-894 MHz
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-13
Electronic Serial Number (ESN).
The unique 32-bit identification number embedded in a wireless phone by the manufacturer. Each time a call is placed, the ESN is automatically transmitted to the base station so the wireless carrier's mobile switching office can check the call's validity. The ESN cannot easily be altered in the field. The ESN differs from the mobile identification number (MIN), which is the wireless carrier's identifier for a phone in the network. MINs and ESNs can be electronically checked to help prevent fraud.
Mobile Identification Number (MIN).
10-digit number (which is represented by 34 bits) that uniquely identifies a mobile unit within a wireless carrier's network. The MIN often can be dialed from other wireless or wireline networks. The number differs from the electronic serial number (ESN), which is the unit number assigned by a phone manufacturer. MINs and ESNs can be checked electronically to help prevent fraud (Example: 6195947898, meaning (619) 594-7898.)
IS-95 (Cont.)
MFR code reserved Serial Number31 24 23 18 17 0
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-14IS-95 (Cont.)Reverse Transmission
Uplink
The diagram for access channels is identical, only the data bit rate is fixed to 4.8 kbps
242-1 period length
94 channels (32 access, 62 voice)Control channels use for:
- call setup- response to paging message- location update
824-849 MHz
32-bit
Similar to CCK, only with Walsh codes-each group of 6 bits from the 28.8 kbps streamreplaced by a 64-bit Walsh code from the lookup table(28.8 ksps*64/6 = 307.2 kcps, 307.2/64 = 4.8 ksps)Improves reception at BS since the Walsh codes are orthogonal. ( Form of block error-correcting code)
Long code mask- unique to MS(4 PN chips per one Walsh chip)
Rates 4.8, 2.4 and 1.2 also available
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-15IS-95 (Cont.)Multiple Access
Forward transmission uses CDMA (based on Walsh codes) to resolve the contention.This is possible because BS transmits to all MSs simultaneously and therefore does multiplexing of all channels. In addition, BS transmits the pilot signal for all MS, so they can synchronize their CDMA demultiplexers.
Reverse transmission can not use CDMA forcontention resolution because there is difficultto synchronize reverse receivers in BS (the pilot signal must be transmitted from only one device). Instead, MS transmitters use DSSSbased on spreading sequences that are uniquefor each MS (ESN).
MS 1
MS n
BS
Forward
Reverse
(DSSS)
Reverse(DSSS)
CDMA
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-16
1100330338Users per cell
137Frequency reuse factor
199319901991Year introduced
9.6 kbps13 kbps7.95 kbpsSpeech coding bit rate
5583Users per channel
CELPRPE-LTPVSELPSpeech coder
20124790Number of duplex channels
1,228 kHz200 kHz30 kHzChannel bandwidth
IS-95GSMIS-136
Summary of 2G
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-17Connection Management in Cellular Systems
Transmitting1. User enters the telephone number (MIN) for destination telephone. 2. MS scans the band for an analog channel that has the strongest signal.3. MS sends the MIN to the closest BS.4. BS relays MIN to MSC5. MSC sends MIN to telephone CO. If the called party is available the
connection is made and CO relays the result back to MSC.6. MSC assigns an unused voice channel to the call.7. MS tunes to the new channel and communication can begin.
Receiving1. When a MS is called, the telephone CO sends MIN to MSC2. MSC searches for the location of MS by sending query signal to each
cell (paging).3. When MS is found, the MSC transmits the ringing signal to MS4. When MS answers, MSC assigns a voice channel to the call, voice
communication begins
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-18Connection Management in Cellular Systems (Cont.)
HandoffMS can move (during conversation) from one cell to another, resulting in a weaker signal. Therefore MSC monitors the level of the signal every few seconds. If the level of the signal diminishes, the MSC seeks a new cell that can better accommodate the communication. MSC then changes the channel carrying the call.
Hard HandoffIn early systems a MS can communicate with only one BS. When MS moves from one BS to another, the communication must first be broken, then established with new BS. Rough transmission.
Soft HandoffNew systems. MS can communicate with two BS at the same time. MS can continue communication via new BS before breaking with the old BS.
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-19
Personal Communication Services(PCS)
The term PCS refers to a wide variety of wireless access and personal mobility services provided through a small terminal, with the goal of enabling communications at any time, at any place and in any form. (PCS doesnt refer to a single technology or standard such as IS-136, GSM, IS-95.)
Some services are short message service (SMS) and limited access to Internet.
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-20
Wireless hand held devices (cell phones, pagers, PDA,) have limitations:
Small displaySmall keypadSmall CPUSmall memoryShort battery lifeLimited data bit rateLonger latencyLower reliability (stability)Browser must be simple (nave users)
WAP is designed to integrate lightweight WEB browser into hand held devices which are bearer independent (D-AMPS, GSM, IS-95, 3G,).
WAP was designed by WAP Forum, established by Ericsson, Nokia, Motorola and Phone.com in 1997. Today, the forum has hundreds of members.
Wireless Application protocol (WAP)
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-21WAP (Cont.)WAP Programming Model
WAEuser agent
Client WAP Gateway Original Server
Encodersdecoders
CGIScripts
Encoded requestsEncoded requests RequestsRequests
Response (content)Response (content)Encoded responseEncoded response
Proxy server for the wireless domain
(DNS, WAP-WWW protocol conversion,HTML-WML translation, encoding, caching)
HTTP content intext format
Requests in compact(binary) format
WML contentin compact
(binary) format
Requests in textformat
Has microbrowser
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-22WAP (Cont.)
GSMD-AMPS IS-95 3G Bluetooth
Wireless Datagram Protocol (WDP)IP
UDP
Wireless Transport Layer Security (WTLS)
Wireless Transaction Protocol (WTP)
Wireless Session Protocol (WSP)
Wireless Application Environment (WAE)
Wireless Markup Language (WML) WML Script
WAP Protocol Stack
Similar to HTMLbut smaller,
less demanding
Similar to JavaScriptbut smaller,
less demanding
Tools for WAP application
development
Based on HTTP(Session
establishment/termination)
Provides end-to-end security between the client and
the WAP gateway( data integrity, privacy,
authentication and denial-of-service detection
- based on TLS)
Bearer
SDSU Marko Vuskovic, 2004
McGraw-Hill The McGraw-Hill Companies, Inc., 2004
5-23
HTTP Hypertext Transfer ProtocolSSL Secure Sockets Layer
WAP (Cont.)WAP gateway Protocol Stack