COMP1321 Digital Infrastructures Richard Henson November 2015

Week 6: Peripherals, Storage & Device Communication Objectives: Objectives:  explain the difference in communication involving peripheral and intelligent devices  produce a simple algorithm describing point- point communication between intelligent devices

What is a “Peripheral” anyway? Digital… Digital… Generally not “intelligent” Generally not “intelligent” Controlled from the CPU, via connection through motherboard Controlled from the CPU, via connection through motherboard CPU needs unique instructions for each peripheral… CPU needs unique instructions for each peripheral…  system software known a “driver”

Control of Peripherals Input (e.g. keyboard): Input (e.g. keyboard):  communication between motherboard and peripheral to establish »electrical connection »logical connection  communication channel opened… »operating system call (e.g. INT 21) »data received into RAM… »flow rate & processing controlled by CPU »data passed on the requesting application CPU Motherboard keyboard ASCII code

Control of Peripherals Output (e.g. screen):  communication between motherboard and peripheral to establish »electrical connection »logical connection  communication channel opened… »data sent into RAM… »operating system call (e.g. INT 21) »flow rate & processing controlled via CPU »data passed from RAM to remote peripheral CPU Motherboard screen

Intelligent/Dumb Devices Intelligent can process data independently  communication channel with CPU more sophisticated  control software at each end of the communication Dumb controlled from CPU  dependent on driver software installed on the computer…

Principle of “Handshaking” First stage of communication:  makes sure devices in electrical & logical contact  if so… establishes an agreed common set of rules (protocol) for sending and receiving data

Why “handshaking”? Replicates what humans do…  equivalent of two multi-lingual people walking up to one another, greeting each other, and deciding which language they will use to communicate: »English, French, Chinese? »Smoke Signals, Semaphores, Morse Code?

Establishing a point-point Communications Channel For any two computers to communicate digitally, they need to “understand” each other’s signals $%^&£(? eh?

Establishing a point-point Communications Channel Need a common “language”:  i.e. a programmed set of rules and procedures Incorporated into software  known as a “communications “protocol” $%^&£(? eh?

Establishing a point-point Communications Channel Each computer needs a copy of the protocol rules:  to send valid data  to understand what the other computer is sending $%^&£(? eh?

What is needed for a Communications Protocol? All the instructions for the CPU to organize the sending/receiving of data…  and ways to manage that data even when in electronic format (i.e. 1= higher voltage, 0= lower voltage) At communication endpoints…  synchronizing speed of sending/receiving  error checking  data encryption/compression & vice versa

Essentials for a Communications Protocol Agree a standard set of codes First standard… ASCII, early 1960s  set of control codes built into ASCII characters

Instructions, ASCII characters and control codes  Rules and procedures for effective and reliable communication between devices converted into a language the CPU can understand…  The ASCII control codes then need to be incorporated into code written in that language  code can be shared between the sender and receiver

Point-Point Protocol Device A Device B conversions Electrical signals Protocol for Comms mgt Protocol for Comms mgt

Development of Communication Protocols Problem…  Different manufacturers have historically produced their own unique protocols,  serve their own specific purposes  some common ground… »mostly used ASCII for control codes »IBM used a system called EBCDIC (!) Gradually, attempts at universal protocols started to emerge…  e.g. the people creating the Internet produced TCP/IP in 1972

A simple point-to-point communications protocol Basis of a very simple algorithm that could be translated into a program for managing communications between individual devices  ASCII control codes (0-31) for communications between devices  ASCII character codes (32-127) to carry data Half duplex transmission Data sent in “blocks” or “packets” Programs written in “C”

Point-to-point algorithm - Stage 1 Purpose: Sender needs to make sure that the receiver is actually “there” (i.e. switched on and working properly) Action: sends a small amount of “standard” data - usually one byte, known in ASCII terms as SYN  ASCII code 22 (binary )

Point-to-point - Stage 2 Purpose: Receiver needs (if it can!) to reply to the senders ACK signal Action  EITHER replies with a positive acknowledgement ACK, “I’m ready!” ASCII code 6 (binary )  OR replies with a negative acknowledgement NAK, “Not ready!” ASCII code 21 (binary )

Point-to-Point: Stage 3 Purpose  sender reacts to the reply (or not) from the receiver by either closing down or sending a “preamble” Action  IF sender  EITHER does not receive acknowledgement  OR receives a NAK response… THEN it closes down the communication, with an error message  ELSE, it sends out a further set of data, known as a preamble…

Point-to-point - Stage 4 Purpose  synchronise protocols (receiver)  1. processes the preamble  contains data telling the it which comms protocol, error control, flow rate, encrypt, etc. the sender would like to use  2. passes short message back to the sender  indicates whether it will be able to understand and handle data in that form

Point-to-point – Stage 5 Purpose: (sender) establish or close down the communications channel if response received is negative (ie receiver can’t handle data in that format):  THEN the communication is closed down with an error message (e.g. NAK)  ELSE, a communications “channel” is established

Point-to-point - Stage 6 Purpose: sending data & error checking info in blocks/packets according to the format agreed in the preamble  would take much too long to send and receive data one byte at a time! Action:  Sender sends one block/packet followed by error checking info

Point-to-point - Stage 7 Purpose: using results of processing the error checking info to request next packet or resending of same packet Action:  Receiver compares error checking info with that already obtained in the block/packet »if an error is detected request to resend block/packet »else request to send NEXT packet

Point-to-point - Stage 8 Purpose: sending the rest of data… Action: process or sending/resending continues (loops…) until all the data is satisfactorily received Note: a poor communications environment will result in a lot of errors/resending  which will slow down communications…  BUT the data will still get through…

Point-to-point - Stage 9 Purpose: close down the communications channel Action:  when no further data is available to send and the final packet/block has been acknowledged…  the sender sends an EOT (end of transmission) byte ASCII 4 binary  this terminates the communications channel!

Techniques for Detecting/Managing Errors Many error detection and correcting techniques are available  process normally involves »sending of the main block »Sending of further information »calculation based on recreating the additional information from the main block »comparison of the two  choice must be appropriate for speed, cabling, protocol type, etc

Flow Control A feedback mechanism between sender and receiver so that the receiver can inform the sender if it is not keeping up Usually achieved by “synchronization” bits or byte (ASCII SYN)  a pause in the communication can occur if necessary »e.g. if buffer becomes full »gives time for buffer to empty…  sender will not send any more data until it gets the “all clear” from the receiver

Further point-point issues Programming code could be added to the basic algorithm which would enable:  a series of protocols to be tested during handshaking  packets to travel through analogue as well as digital media  packets to be converted from one format to another before/after transmission  extra processing to check for virus footprints

So what about networks? All this is needed just to send data from one computer to another, without any routing! If the computer is on a network, the following additional factors immediately need to be taken into consideration:  naming of computers  addressing (if on a different network or segment)

More factors for a network protocol? Logging on/off and access rights Packet switching or circuit switching Navigation:  type of routing algorithm if circuit switching  creation of packets if packet switching  mopping up lost packets Packet reassembling at destination… CONCLUSION: A NETWORK PROTOCOL IS A COMPLEX PIECE OF PROGRAMMING