1. Chapter 9 Transmission Modes
© Bobby Hoggard, Department of Computer Science, East Carolina University
These slides may not be used or duplicated without permission

2. Transmission Modes
Network transmission can be divided into two categories:
Serial – one bit is sent at a time
Parallel – multiple bits are sent simultaneously

3. Parallel Transmission
Transmits multiple bits at the same time, by using multiple wires to connect the sender and receiver (or multiple channels if using wireless)
There are usually additional wires used for the sender/receiver to coordinate the transmission
Example: an older style, LPT (line printer terminal) printer port
sender
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
a network capable of transmitting one byte of data at once, by using one connecting wire for each bit
receiver

4. Parallel Transmission
You can view this circuit simulation under the Supplemental section of our website
4-bit sending
memory device made
of D-flip flops
4-bit receiving
memory device made
of D-flip flops

5. Serial Transmission
Transmits one bit at the same time, by using one wire to connect the sender and receiver (or one channel if using wireless)
There are usually additional wires used for the sender/receiver to coordinate the transmission
Example: USB (universal serial bus)
Extra hardware is needed to convert data between parallel (internal to the device) and serial (for the network connection)
sender
parallel-to-serial converter
a network capable of transmitting one bit of data at a time, by using one connecting wire for each bit
serial-to-parallel converter
receiver

6. Serial Transmission
You can view this circuit simulation under the Supplemental section of our website
4-bit receiving
memory device made
of D-flip flops
4-bit sending
memory device made
of D-flip flops

7. Parallel vs Serial Speeds5 1
wire 1
Clock speeds being equal--- a parallel connection containing N data bit connections can send N-bits in the same amount of time that a serial connection takes to send one bit 1 5 1
wire 2
Example: Send the data bits 1011 5 1
wire 3
sender 1 1 1 1
receiver
Parallel 5
time 1 1
wire 4
Time to send: 1 clock pulse
voltage 5
time
sender 1 1
receiver 1
Serial
Time to send: 4 clock pulses

8. So Why Is Serial The Current "Thing"?
Cost – serial takes fewer wires, so it's cheaper to develop
Wire Lengths – parallel systems require each wire to be exactly the same length (even a difference of millimeters can be problematic – especially at high speeds)
This basically means that the bits on different wires arrive at the receiver at slightly different times – a phenomenon called skew
Interference – signals on parallel wires can cause electromagnetic noise that interferes with signals on other wires

9. Transmission Order
Network transmission can be divided into two categories:
Little Endian – sends the least significant bit first
Big Endian – sends the most significant bit first
There is no real benefit of using one over the other, but the sender and receiver must agree on which to use.

10. Big vs Little Endian MSB = most significant bit
LSB = least significant bit
MSB
LSB 1
send 4th
Big Endian 1
send 3rd
send 2nd 1
send 1st
MSB
LSB 1
send 1st
Little Endian 1
send 2nd
send 3rd 1
send 4th

11. Example: Ethernet Networks
Ethernet networks are unique in that they use both orderings:
Big Endian for multiple bytes in a block of data
Little Endian for the individual bits in a byte
Example: Send the 32-bit integer 1,468,723,923
byte 1
byte 2
byte 3
byte 4
‭ ‬ 1 9 17 25 2 10 18 26 3 11 19 27 4 12 20 28 5 13 21 29 6 14 22 30 7 15 23 31 8 16 24 32
send ‬

12. Transmission Timing
Network transmission can be divided into three categories:
Asynchronous – data can be sent at any time, with unknown length of gaps between
Synchronous – data is sent continuously, with no gaps between two data items
Isochronous – data is sent at regular intervals, with a fixed gap between two data items

13. Asynchronous Transmission
The transmission medium can be idle for an arbitrary period of time between data items
Well suited for applications that generate data at random intervals (i.e. data typed on a keyboard, or mouse clicks)
Bits sent are on a timed interval, but idle periods are not timed
Disadvantages include:
Not knowing when the next block of data will arrive
Need to transmit extra bits before the data to indicate that a transmission is starting start bits
Some protocols require extra bits at the end of the transmission to indicate that it's ending stop bits

14. Ex: RS-232 Protocol (An asynchronous protocol)
Distance: must be less than 50 feet
Electrical: -15 volts to + 15 volts
Line coding: 1 = -15V / 0 = +15V / idle = -15V
Start bit: 0 (+15V) used to indicate start of transmission
Stop bit: 1 (-15V) used to indicate end of transmission
Note:
The "stop bit" here really just means that there must be at least one idle period between transmissions
Example: Transmit
-15
+15 1 1 1 1 1
start bit
stop bit
idle
idle

15. Example 2:
Transmit 2 bytes: with no idle period between
-15
+15
0 / start
1 / stop / idle
idle 1 1 1 1 1 1 1 1 1 1
idle
start
stop
start
stop
byte 1
byte 2

16. Example 2:
Transmit 2 bytes: with no idle period between
-15
+15
0 / start
1 / stop / idle
idle 1 1 1 1 1 1 1 1 1 1
idle
start
stop
start
stop
Note:
In order to send 8-bits of data, technically 10-bits have to be sent
byte 1
byte 2
Transmit 2 bytes: with idle period between
-15
+15
0 / start
1 / stop / idle
idle 1 1 1 1 1
idle 1 1 1 1 1
idle
start
stop
start
stop
byte 2
byte 1

17. Synchronous Transmission
Bits of data are sent continuously, with no idle time between bits
Start/stop bits are not needed, as a second byte of data can immediately follow
As a result of the fewer bits, transmission is faster than with asynchronous

18. Example Synchronous Transmission
Transmit 2 bytes:
-15
+15 1 1 1 1 1 1 1 1 1 1 1 1 1 1
previous byte
byte 1
byte 2
next byte
some data transmitted before our 2 bytes
some data transmitted after our 2 bytes

19. Synchronous Network Frames
Synchronous networks usually deliver blocks of bytes grouped into a frame
Frames start with a special sequence of bits to indicate the start of the frame (similar to a start bit in an asynchronous network)
These "frame start bits" help ensure the sender/receiver stay synchronized, since they will occur at a set interval
Synchronous networks also include a special idle sequence of bits that is transmitted when there is no real data to send

20. Example Framing For this example: a frame consists of 6 bits
frames start with the bit sequence 1 0
idle bits are 0's
we want to transmit:
8 bits of data:
An idle period
4 more bits of data:
frame
frame
frame
frame
frame
frame
-15
+15 1 1 1 1 1 1 1 1 1 1 1 1 1
previous frame
frame
start
data
frame
start
data
frame
start
idle bits
frame
start
data
frame
start
idle

21. Isochronous Transmission
This is actually a synchronous network which is guaranteed to deliver DATA BITS at a steady rate of X-bits per second
This means that:
The sender must generate data at a rate of X-bits per second to hand to the network
The underlying synchronous network must operate at a speed that's slightly faster than X-bits per second (because it may use framing, and could send additional bits that aren't data)
Mostly used for multimedia applications so the data is delivered at a steady rate

22. Example Isochronous Network
Consider our previously described synchronous network:
a frame consists of 6 bits
frames begin with a start sequence of 2 bits
frames contain 4 bits of data
Suppose we want an isochronous network to deliver data at a rate of 12 data bits per second
In order to guarantee data delivered at this rate, our underlying synchronous network must transmit 18-bits per second. This is because for every 4-data bits, an additional 2-bits of frame data must be sent.
synchronous speed
6 bits
12 bits
18 bits
isochronous speed
4 bits
8 bits
12 bits
frame
start
data
frame
start
data
frame
start
data
1 second

23. Direction of Transfer
Network transmission can be divided into three categories:
Simplex – data transfers only in one direction
Half-Duplex – data transfers in both directions, but only one way at a time
Full-Duplex – data transfers in both directions, simultaneously

24. Simplex Data transfers only in one direction Examples:
A temperature sensor
A radio or television broadcast
A single optical fiber
send
receive
single optical fiber
LED
Photodiode
One-Way Data Travel

25. Full-Duplex Data transfers in both directions simultaneously Examples:
A telephone conversation
A pair of optical fibers
send
receive
receive
send
Device 1
Device 2
two optical fibers

26. Half-Duplex Data transfers in both directions one way at a time
Examples:
A one-lane bridge
A pair of walkie-talkies
Additional equipment is needed on each end to coordinate and ensure that only one side transmits at at time
send
receive
receive
send