Day 7

Connections

Standards• Unless we had connection

standards nothing would be interchangeable.– There would be different printers for

Macs and Windows and Unix– You’d have to buy a DELL modem, or

an HP sound card.

• Standards are good for everyone– Manufacturers only make one thing– Consumers don’t have to worry about

it

2 Sides• DTE

– Data Terminal Equipment– Your computer

• DCE– Data Communication Equipment– Your modem

Who makes standards?• IEEE

– Institute for Electrical and Electronics Engineers

• ISO– International Organization of Standardization

• ANSI– American National Standards Institute

• ITU – International Telecommunication Union

• EIA– Electronics Industries Association

Interface standards are made of:• Mechanical

– Size, Shape of connector, Number of Pins

• Electrical– Voltage, Resistance etc.

• Functional– How each pin is used

• Procedural– Describes how a plug and connector

work together

RS232 - Serial• One of the first connectors on

computers– Officially called EIA-232F

• Electric: ITU – V.28• Mechanical: ISO 2110• Functional & Procedural: ITU V.24

– Used to connect computer to modem

RS232 Standards• Electrical – ITU V.28 Standard:

0 is sent by having a voltage difference of -3v or more

1 is sent by having a voltage difference of +3 or more

Mechanical – ISO 2110DB 25 Original standard

Now more common to use DB 9Most commonly used wires

Functional Specification

Procedural Sequence

Bell & Hayes Standards• Communicating over a modem has

had many standards– Bell

• 209A– 9600bps– quadratic amplitude modulation

– Hayes• AT command set• Supports all modem functionality• Replaced need for setting parameters by

switches– AT D 123-4567– +++

Modems• Digital Signal -> Analogue

– Modulate/Demodulate– Use phase, amplitude and frequency

shift keying– Speeds up to 56K (56,000 bits per

second)– Speed dynamically decided by both

modems to ensure compatibility and max speed

– Compression and Error Correction• Handled by modem

The 56K myth• POTS transmit an 8K sample 8,000

times per second• 8*8000 = 64,000bits/second

– Some of that speed is reserved for phone use

– FCC standards require lower power for modems which allows noise• 53,000 is max possible

• V.90 and V.92 are 2 standards– V.92 includes call waiting

Other Modem Features• Auto call back

– You dial ISP, it answers authenticates and then hangs up to call you back

• Fax– All modern modems can act as fax

machines.

Connection of modem• Internal

– PCI, ISA, On board

• External– RS232, USB

• Laptop– PCMCIA

Modem pool• 100 employees

– Only 20 online at any time– Buy 20 modems– Have computer shuffle connections to

modems

• Reasons– Cheaper– Less maintenance

• Problems– What if more than 20 want to use at once

Replacements for RS232• RS499

– Faster, built in testing ability (loopback)

– Never caught on

• X11– Fewer pins (15)– Primarily used for connection to ISDN

modems

Faster Alternatives• T1 Line

– CSU/DSU required on both ends– 1.544Mb/s = 24 phone lines (24*64,000)

• Cable Modem– Download speed can be as high as 16Mbps, upload

typically 128k or 256k• Actual speed depends on how busy the network is

• ISDN modem– Digital phone connection end to end.– 64K/channel 2 B channels + D channel

• DSL– Asynchronous or Synchronous– All digital use of unused frequencies on phone

wires

Newer standards• USB

– Can connect up to 128 devices– Supplies 2.5W of power per segment (5v

@.5A)– 1.0

• 12Mb/s– 2.0

• 480Mb/s

• Firewire – Can connect up to 63 devices– Supplies 45W of power– (400)

• 393 Mb/s• 4.5 meters, 16 cable daisy chain

– (800)• 786Mb/s

SCSI, iSCSI, Fiberchannel• SCSI

– Allows connection of hard drives • Up to 16 on a dual channel• Speeds up to 320MB/s (2.56Gbps)

depending on protocol

• iSCSI– Connection using TCP/IP instead of

serial connectors

• SATA– 1.5Gb/s or 2.4Gb/s

• Fiber Channel– Connects hard drives at high speeds– 400MB/s (3.2Gb/s)

Data link layer• Asynchronous

– Character by character• Start bit• Data• Stop bit• Sometimes a parity bit

– High overhead for large transmissions

• Synchronous communication– Many characters at once

Duplex• Half

– Only one side can talk at any given moment

– Think CB Radio

• Full– Both sides can talk at once– Think Phone

• Simplex– Only one side can ever talk– Think Radio

Point to Point vs Multipoint• Point to point

– Each computer is directly connected to mainframe and each connection is handled directly

• Multipoint– All computers and mainframe connect

via a common backbone• Requires polling