Chapter 6 Data Transmission
Data Transmission
Data is transmitted via an electrical signal. For every message that is sent, the signal must be changed somehow to reflect the bits that make up the message. The bits can flow one way or both ways. Hardware and software determines how the data flows on the network.
There may be error in the data as it is transmitted. Internal network problems or external conditions. Use hardware and software to detect these errors.
Network software also distinguished on group of bits from another as they travel over the network.
Digital and analog transmission.
Modulation
All data sent on a network is sent by means of an electrical signal called a carrier signal. Signal oscillates back and forth across a 0 line. Sometimes called a carrier wave or sine wave. Carrier wave is an analog signal. Have to distinguish between the individual bits that make up the transmission. The change to the signal is called signal modulation or modulation. Modems are used in many networks to perform modulation. 3 types:
1. Amplitude Modulation (AM) - change in the height of
the carrier wave; radio transmission especially AM; interference from
thunderstorm
2. Frequency Modulation (FM) - number of waves used to
represent a single cycle; also called frequency shift keying (FSK);
generally used by FM stations
3. Phase Modulation (PM) - change in direction to indicate
a bit; also called phase shift keying (PSK) and
Differential phase
shift keying (DPSK) - the 180 change in the wave occurs only when a
1 bit is detected.
Transmission Direction
The way in which data travels or flows is determined by the type of medium used and the devices attached to the network. Must be controlled so that the sending and receiving stations know when data will arrive and when it should be sent. 3 types:
1. Simplex Transmission - one direction; example:
water faucet
2. Half-Duplex Transmission - data can go in either direction,
but only one way at a time; example: CB radio
3. Full-Duplex Transmission - data can travel in either
direction simultaneously; example: 4 lane highway
Transmission Modes
Data can be sent as entire characters or as individual bits. Transmission mode determines which of these ways data will be sent. 2 modes:
1. Serial Transmission - transmitting data as individual
bits on a single communication line. Modem
2. Parallel Transmission - transmitting an entire character
at one time on mutliple lines. Printer
Synchronization
Refers to how the data bits are grouped for transmission. 2 types:
1. Asynchronous Transmission - each character is transmissited
individually. Uses start and stop bits therefore also called
start-stop transmission.
Start bit - space(0) Stop bit - mark (1); Untimed
2. Synchronous Transmission - blocks of data separated
by special characters called SYN characters.
Errors
Sources of Errors
External sources or internal sources, such as signal strength or type
of medium used.
1. Impulse Noise - external electrical interference primarily
electric storms; copper wires placed near equipment that generates
fluctuations in electrical
output
2. White Noise (Gaussian noise) - in a radio, any static
or constant hiss; caused by the movements of electrons in the wire,
and the amt of noise is
directly proportional to the temp of the medium (thermal noise)
3. Attentuation - single loses some of its strength;
To overcome attentuation, amplifiers (analog) and repeaters (digital) are
used to boost the signal.
4. Crosstalk - occurs when the signals from two transmission
lines interfere with one another; Intermodulation noise -
two
signals combine to produce
a frequency outside the range of frequencies for a type of transmission
5. Delay Distortion - depending on frequency used, data
will arrive at its destination at different times. A device called
an
equalizer can correct
delay distortion and make sure that data arrives at the same time.
6. Line Failure - entire communication line can be out
of service
Error Detection
To help with error detection and correction, datais divided into smaller groups called packets and frames. They are basic units of data transmission which consists of data to be transmitted along with error-correction bits and control information required by the network for transmission.
1. Parity Checking (vertical redundancy check(VRC)) - determines
if bits have been changed as they were transmitted; Used
with 7-bit codes with an
extra bit, parity bit, to provide the parity checking
a. even parity - an even number of 1 bits is used as the measurement
valid data
b. odd parity - an odd number of 1 bits are used
2. Cyclic Redundancy Check (CRC) - mathematical method
of detecting errors; 6-14
Error Correction
Error detected, must be corrected. Depends on the types of errors encountered and the effect that sending extra data for error detection and correction has on the transmission line. 2 methods:
1. Forward Error Correction - used only in cases where
single-bit error may occur; Errors are corrected w/o retransmitting.
Most common code is Hamming Code - uses some ot the techniques used
in parity checking, but also appends
additional bits to the data to ensure accuracy of the received data.
2. Error Detection with Retransmission - if an error has
occured, the data is retransmitted. Automatic Repeat Request
(ARQ) 3 types:
a. stop-and-wait ARQ - packets of data are numbered and sent
along the network. Each packet is checked for
errors as it is received. If data is correct, and acknowledgement
(ACK) is sent to the receiver and the next
packet is sent. If data is incorrect, an negative acknowledgement
(NAK) is sent and the packet is resent.
b. go-back-N ARQ - sender starts transmitting numbered packets,
one after another. Receiver checks the
packets for errors and sends ACKs or NAKs back to the sender. If
an error occurs in #4, the sender resends
#4 through the end.
c. Continuous ARQ – numbered packets of data are sent, Only
the packet with a NAK is resent. The packets
may be out of order so the receiver has to put them back in order.
Sliding window - Sender transmits a
number of packets before an acknowledgement must be sent.
Error Prevention
Ultimate goal is to ensure accurate data transmission. Can not
eliminate noise completely therefore steps must be taked to reduce the
amt of noise and resulting interfernce. A simple way to do these
is to improve the shielding around cables. Speed also has an effect
on how many error will occur. Lower speed, less errors.
Digital Transmission
Refers to sending the data in its original form without the conversion to an analog signal.
Intergerated Services Digital Network (ISDN) - network designed to handle only digital transmission. Reduced errors, elimination of conversion to analog and back to digital, and higher transmission rates. Designed to allow many types of signals such as voice, data, video, graphics, text, and network control to be transmitted on the same communication line.
Asymmetric Digital Subscriber Line (ADSL) - allow the user to send and reciver data while talking on the telephone at the same time over regular copper telephone line.
Coder/decoder (CODEC) - used to convert analog to digital and back. The conversion of analog to digital is usually accomplished by using pulse code modulation (PCM).
Digital communication is regenerated as it travels therefore has a stronger signal therefore peformance is improved and fewer error are in the data.
T-1 lines - leased digial circuits provided by common carriers
for high-speed tranmission and digital transmission.
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