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 2110  DB 25 Original standard  Now more common to use DB 9  Most 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 –+++

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 –1.0 12Mb/s – Mb/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