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

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

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

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

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

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

Parallel vs Serial Speeds 5 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 1 2 3 4 1 0 1 1 sender 1 1 receiver 1 Serial Time to send: 4 clock pulses

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

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.

Big vs Little Endian MSB = most significant bit LSB = least significant bit MSB LSB 1 0 1 1 1 send 4th Big Endian 1 send 3rd send 2nd 1 send 1st MSB LSB 1 0 1 1 1 send 1st Little Endian 1 send 2nd send 3rd 1 send 4th

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 ‭0 1 0 1 0 1 1 1 1 0 0 0 1 0 1 0 1 1 1 1 0 0 1 0 1 1 0 1 0 0 1 1‬ 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 1 1 1 0 1 0 1 0 0 1 0 1 0 0 0 1 0 1 0 0 1 1 1 1 1 1 0 0 1 0 1 1‬

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

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

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 1 1 0 1 1 0 1 0 -15 +15 1 2 3 4 5 6 7 8 9 10 1 1 1 1 1 start bit stop bit idle idle

Example 2: Transmit 2 bytes: 1 1 0 1 1 0 1 0 1 1 1 0 0 1 0 1 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

Example 2: Transmit 2 bytes: 1 1 0 1 1 0 1 0 1 1 1 0 0 1 0 1 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: 1 1 0 1 1 0 1 0 1 1 1 0 0 1 0 1 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

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

Example Synchronous Transmission Transmit 2 bytes: 1 1 0 1 1 0 1 0 1 1 1 0 0 1 0 1 -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

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

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: 1 1 0 1 1 0 1 0 An idle period 4 more bits of data: 0 1 1 0 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

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

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

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

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

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

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