1. COMP1321 Digital Infrastructures
Richard Henson
November 2017

2. Week 8: Peripherals, Storage & Device Communication
Objectives:
explain the difference in communication involving peripheral and intelligent devices
produce one simple algorithm describing point-point communication between intelligent devices

3. What is a “Peripheral” anyway?
Digitally controlled…
could be intelligent (own CPU)
could be dumb (no CPU)
controlled from the CPU, via connection through motherboard
CPU needs unique instruction(s)
system software known as a “driver”

4. CPU/control of “dumb” devices (1)
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
ASCII
code
keyboard

5. CPU/control of dumb devices (2)
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

6. CPU-peripheral Communications Protocol?
Centrally
all the instructions for the CPU to organize the sending/receiving of data…
ways to manage that data in electronic format (i.e. 1= higher voltage, 0= lower voltage)
Both communication endpoints…
synchronizing data sending/receiving speed
error checking
data encryption/compression & vice versa

7. Essentials for a Peripheral control Communications Protocol
Agree a standard set of codes between CPU and peripheral…
First standard… ASCII, early 1960s
ASCII – American Standard Code for Information Interchange
set of control codes built into ASCII characters
used for input (keyboard) and output (printer)
still used today!

8. Instructions, ASCII characters and control codes
Stage 1: establish rules and procedures for effective and reliable communication between devices
known as ASCII control codes
Stage 2: ASCII control codes trigger programs wrritten in assembly language for CPU being used
data and control codes can be shared between CPU and peripheral

9. Evolution of ASCII codes
Originally control set 7-bit (128 codes)
ASCII control codes (0-31) for communications between devices
ASCII character codes (32-127) to carry data
Extended ASCII codes 8-bit (256 codes)
codes used for other languages and graphics elements
Unicode: includes 16-bits to cover a wider range of language symbols

10. Intelligent Devices Can process data independently
communication channel with CPU more sophisticated
control software at each end of the communication
Establishment of such inter-device communication known as “handshaking”

11. Principle of Handshaking
First stage of communication…
makes sure devices in electrical & logical contact…
if this is true… establishes an agreed common set of rules (protocol) for sending and receiving data

12. 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?
Overview:

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

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

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

16. Peripherals and Bandwidth
Data transfer needs to be as fast as possible
Protocol code will slow the data down
“KISS”?
With “dumb” peripherals, protocol used in USB v3 enables very fast communication

17. Communications between intelligent devices
Device A
Device B
Both ends have a CPU
both can process instructions
protocol needs to be more sophisticated…

18. Development of inter-CPU Communication Protocols
OK… but… BIG Problem!!!
different manufacturers produced their own unique protocols
serve their own specific purposes
Thankfully, some common ground…
most used ASCII for control codes
IBM used a system called EBCDIC (!)
Gradually, “universal” protocols became available…
e.g. the Internet produced TCP/IP, released in 1972

19. 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
Half duplex transmission
Data sent in “blocks” or “packets”
Programs written in “C”

20. Point-Point Protocol (PPP)
Protocol for
Comms mgt
Protocol for
Comms mgt
Device A
Electrical signals
Device B
Electrical signals
conversions
conversions

21. Writing a 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 16 (hex) (binary) A B 16

22. 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 15 (binary )
6 or 15… A B

23. 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…
&(£^:~ A B

24. 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
$£!* A B

25. 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

26. 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 - block/packet followed by error check
Hello… CRC

27. 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
CRC OK! A B

28. 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…

29. 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!

30. 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
must be appropriate for speed, cabling, protocol type, etc

31. 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

32. Buffering Also used in flow control (FIFO)
data to CPU
Also used in flow control (FIFO)
Receiver saves incoming data to a “buffer” (area of memory for temporary storage)
If receiver CPU can’t process the data yet, it sits there, and other data piles up
Eventually, buffer overflow occurs and data is lost
data in

33. Further point-point issues
Programming code could enhance the basic algorithm so it could (e.g.):
test a series of protocols between sender and receiver during handshaking
allow packets to travel through analogue as well as digital media
convert packets from one format to another before/after transmission
provide extra processing to check for virus footprints

34. 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 (e.g. IP address)
And also….

35. 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!