5211 Data Comm 1 Paul Weingartner 7-2004 Presentation #2

Cabling & Coax • Termination is a very important topic often not covered.

• When signals are sent down a wire/cable if they are not fully absorbed in the load at the end, they can be reflected back.

• This reflected signal will collide with other signals coming down the line. The result is garbled data.

• Which has to be re-transmitted

The problem • Is called SWR – Standing Wave Ratio • People in RF communications know it as VSWR (voltage SWR) and use RF devices to match the impedance

The fix • Many devices have the appropriate termination (resistors) included in them.

• Other times a “terminator” must be installed.

• Terminators are also in unused Rambus memory slots, some mother board expansion busses, etc.

Fiber optic cable • Laser light emitters generate light at a very narrow wave length • White light has all colors (wavelengths in it) – play with a prism • Secure communications • Impervious to noise (RF)

Fiber modulation techniques • Single mode (wave length) • Wave division multiplexing (WDM) • Dense WDM (DWDM)

Wireless comm • Simply put, speed is bandwidth • To send high speed data, you need wide channels to transmit the data.

• This effectively means VHF and above • 800 MHz is the lower limit of what is considered “microwave”

Point to point links • Bell telephone has used microwave links for many years.

• Referred to as “Terrestrial” links

Satellite comm • There are two popular flavors that are being used – For home/small biz users, “Direct PC” is an option – For businesses, VSAT (Very Small Aperature Terminal Sat equipment) • Issues

• Analog • Digital Cellular phones

Analog cell

•Who cares!

Digital cell – mod types •CDMA •GSM

Cell – data speeds • Analog – 14,400 b/s (good day) • Digital – 1 st gen – 14.4k

• Dig – 3 rd gen – 144k – What is the real throughput???

• Future – 1Mb/s

Wi-fi – Wireless computer access • There are a series of standards – 802.11a 54Mb – 802.11b Wi-fi 11Mb – 802.11g 54Mb – 802.11i (find out) – 802.16 – Wi-Max • Range – up to 1000’ with standard antennas (2+ miles with beams)

Bluetooth • Short range – up to 30 ft • 2.45 GHz • How fast?

MODEM – Chap 4 • The only way to send data over any distance is to use some type of modulation technique • Audio – Acoustic can be used with telephones • Signals are often converted to different levels (voltages/currents) in order to send them over long distances)

Devices • Analog modems • Cable modems • CSU/DSU • FRAD • ISDN