The Physical Layer : Transmission Media common communications media include: wireless –line of sight transmission –eg, satellite wired metallic: –twisted pair –baseband coax –broadband coax wired non-metallic: –optical fibre physical interfaces : RS-232-C and EIA-232-D

2.3 2 Baseband and Broadband: Definitions Baseband dedicates entire bandpass of line to one channel media usually driven from a single-ended voltage source often used for digital transmission Broadband divides line bandwidth into multiple channels independent users can use line simultaneously often used for analog transmission

2.3 3 The Baseband vs Broadband tradeoff baseband cheaper simpler to work with adequate for most LANs (1km, 10Mbps) broadband multiple channels data, voice, video

2.3 4 Magnetic media magnetic storage device (tapes, floppies) physically transported high bandwidth, eg St Bernard with 6250 bpi magnetic tape transported across town St Bernard takes one hour, tape holds 180 Megabytes, data rate in bits per second: = 180,000,000 bytes * 8 bits / 3600 secs = bits per second! good for large data transfers "reliable" transport poor latency

2.3 5 Twisted pair most common medium backbone of telephone network, workhorse for intra-building communication two twisted insulated copper wires twists reduces electrical interference parallel wires become antennas signals can run several kilometres without repeaters for amplification analog: amplifiers every 5-6 km digital: repeaters every 2-3 km

2.3 6 Twisted pair, cont’d many pairs tightly bundled together into cable bandwidth up to four Mbps over several kilometers eg T1 carrier (phone system) Mbps using 22-gauge wire

2.3 7 Baseband Coaxial Cable "Coax" digital transmission: 50-ohm cable central copper wire, insulting material, outer conductor, plastic covering high bandwidth 1km cable gives ~10Mbps excellent noise immunity local area networks, long distance telephone connections require: T junction: cut wire and add T connector, vampire tap: connector drilled into uncut cable

2.3 8 Broadband Coax analog transmission: 75-ohm cable used in cable television 150 Mbps over 100 km analog signals multiplexed in 3Mbps divisions analog amplifiers: transmit signals only one way two-way transmission via dual cable or different frequencies

2.3 9 Optical Fibre digital data transmitted by pulses of light enormous bandwidth (~1-2 Gbps for 10s kms) system composed of: light source (LED or laser diode) detector (photodiode) transmission medium (ultra thin fibre of glass ~2-125  m) used for long distance telephone lines LANs (just coming in)

Optical Fibre cont’d other advantages not affected by electro-magnetic interference thousands of cables can be bundled together disadvantages unfamilliar difficult to work with –eg cuts must be very clean, tapping has substantial light loss unidirectional

Line of Sight: infra-red, lasers, microwave, radio used for between building communication (laser, infra- red) long-haul telecommunication (microwave radio with antennas -> 100 km) advantages no cables, digging, (eg 100 km trench) cheap laser & infra-red immune to tapping/jamming no maintainance of transmission medium! disadvantages affected by rain/fog/thunderstorms

Communication satellites high bandwidth (eg ~50 Mbps) good medium for high-use int’nl trunks acts as a large microwave repeater multiple antennas and transponders transponders listen to a portion of a spectrum amplifies signal rebroadcasts at another frequency (to avoid interference) broadcast beams can be very narrow or broad

Communication Satellites cont’d LEOs Irridium disadvantages satellites expensive unless amortized over many users geo-synchronous orbits: limited #satellites possible (~90 at same operating frequency) propogation delays due to speed of light (~ msec)

Point to Point Guided Media Twisted Pair4 Mb/s250kHz2-10 km Coaxial Cable500 Mb/s350 MHz1-10 km Optical Fibre2000 Gb/s2000 GHz km

Characteristics of Unguided Media BandNameRateApplications kHzLF.1-100b/snavigation.3-3MHzMF b/sAM radio 3-30 MHzHF b/sshortwave MHzVHFto 100 kb/sFM radio, TV.3-3 GHzUHFto 10 Mb/sUHF TV 3-30 GHzSHFto 100 Mb/ssatellite GHzEHFto 750 Mb/spoint-point

The Electromagnetic Spectrum twisted pair telephone coax FM radio & TV AM radio terrestrial microwave satellite optical fibre radio microwave infrared visible ultraviolet

Standards for the physical layer various standards describe electrical specs (eg voltages, signals, encoding techniques,...) mechanical (connectors, pin location) functional (what circuits belong to what pins & what a signal on them mean) procedural (protocol of using the standard eg for answering calls...) References: Welcome to the standards jungle : –RS 232c & RS 449 (p 76-77) –X.21 (p 82-84)

Some Jargon Data Terminal Equipment (DTE): computers and terminals transmitting data rarely attach directly to transmission medium Data Circuit Terminating Equipment (DCE) responsible for receiving/transmitting equipment over transmission medium intermediary between DTE and transmission medium eg modem

Jargon Cont’d DTE/DCE pairs must have complementary interfaces usually several wires (interchange circuits) between them with different signals physical layer standards for DTE/DCE communication: –RS232c, RS449, X.21,... Data Set another name for a modem (jargon used by Bell Systems)

RS-232c popular EIA standard for connecting computers and terminals (DTE) to modems (DCE) ie DTE to go over analog line through DCE doesn't support autodialing directly Mechanical specs: 25 pin male and femal connectors

RS 232 Basics serial connection up to 50Kbps max specified distance 50 feet, can go 100 feet originaly for dumb terminals to modems handshaking not used consistently protection: voltage to 25V & short circuit good noise immunity very cheap, single asynchronous chip unbalanced interface for control & data

RS-232c Electrical specs digital signaling –1: voltage < -3V –0: voltage > +3V (both with respect to common ground) reliability –standard guarantees bit rate up to < 20 kbps and cable distance of < 15 meters –greater speeds/distance is possible, but not guaranteed by standard –this is sufficient for connecting to modem

RS232c - Functional specs (9 typically used pins) defines which circuits connect to each of 25 pins 9 pins used for typical modem configurations 20: Data Terminal Ready (DTE to DCE) – tells DCE that DTE is powered up and ready 6: Data Set Ready (DCE to DTE) –tells DTE that DCE is powered up and ready 8: Carrier Detect (DCE to DTE) –tells DTE that it detects a carrier on the (telephone) transmission line 4: Request to Send (DTE to DCE) –tells DCE it wants to send data (usually for half duplex)

RS232c - Functional specs (9 typically used pins) 5: Clear to Send: (DCE to DTE) –tells DTE that it can accept data (usually for half duplex) 2: Transmit (DTE to DCE) –sends data to DCE for it to transmit 3: Receive (DCE to DTE) –sends received data to DTE 1: Protective Ground (for safety) 7: Common Return (reference voltage for detecting signal levels)

RS-232c for DTE/DTE connection also used (but not designed for) connecting terminals to computers Issue 1: wiring isn't set up to connect two DTE's together Null modems used to cross several wires (eg transmit line of one DTE goes to receive line of other DTE) Issue 2: allowable distance & data rate severely limits terminal to computer connection new standard needed

RS-449 using RS-422A or RS-423A EIA successor to RS-232C overcomes defects of RS-232C greater DTE control over modem –but still doesn't support autodialing RS-449 defines mechanical, procedural and functional standard 46 pins over two plugs (but one 9-pin plug rarely required)

RS-449 using RS-422A or RS-423A Two electrical standards used with RS-449: RS-422A: –balanced transmission (2 wires per signal) –doubles wires in cable – m to 10 12m –for high speed operation RS-423A: –unbalanced signal (common ground) 100m to m Backwards compatable can be made to emulate RS-232c by changing various connections

X.21 CCITT standard for direct digital connection to digital telephone network uses only 8 signal lines, on 15 pin connector data rate of at least 56 Kbps instead of more circuits (RS 449 approach) use more logic - transmit and receive line provides user and control information defines both bit and byte synchronization acts as physical layer standard for X.25 X.21bis: interim standard allows X.21 to be used with analog essentially same as RS- 232!

Part 2 The Physical Layer 2.1 Data Transmission 2.2 Encoding & Multiplexing 2.3 Transmission Media