Digital Transmissions

* Line coding is the process of converting binary data to a digital signal.
* The number of different values allowed in a signal is the signal level. The number of symbols that represent data is the data level.
* Bit rate is a function of the pulse rate and data level.
* Line coding methods must eliminate the dc component and provide a means of synchronization between the sender and the receiver.
* Line coding methods can be classified as unipolar, polar, or bipolar.
* NRZ, RZ, Manchester, and differential Manchester encoding are the most popular polar encoding methods.
* AMI is a popular bipolar encoding method.
* Block coding can improve the performance of line coding through redundancy and error correction.
* Block coding involves grouping the bits, substitution, and line coding.
* 4B/5B, 8B/10B, and 8B/6T are common block coding methods.
* Analog-to-digital conversion relies on PCM (pulse code modulation).
* PCM involves sampling, quantizing, and line coding.
* The Nyquist theorem says that the sampling rate must be at least twice the highest-frequency component in the original signal.
* Digital transmission can be either parallel or serial in mode.
* In parallel transmission, a group of bits is sent simultaneously, with each bit on a separate line.
* In serial transmission, there is only one line and the bits are sent sequentially.
* Serial transmission can be either synchronous or asynchronous.
* In asynchronous serial transmission, each byte (group of 8 bits) is framed with a start bit and a stop bit. There may be a variable-length gap between each byte.
* In synchronous serial transmission, bits are sent in a continuous stream without start and stop bits and without gaps between bytes. Regrouping the bits into meaningful bytes is the responsibility of the receiver.

Signals

* Data must be transformed into electromagnetic signals prior to transmission across a network.
* Data and signals can be either analog or digital.
* A signal is periodic if it consists of a continuously repeating pattern.
* Each sine wave can be characterized by its amplitude, frequency, and phase.
* Frequency and period are inverses of each other.
* A time-domain graph plots amplitude as a function of time.
* A frequency-domain graph plots each sine wave’s peak amplitude against its frequency.
* By using Fourier analysis, any composite signal can be represented as a combination of simple sine waves.
* The spectrum of a signal consists of the sine waves that make up the signal.
* The bandwidth of a signal is the range of frequencies the signal occupies. Bandwidth is determined by finding the difference between the highest and lowest frequency components.
* Bit rate (number of bits per second) and bit interval (duration of 1 bit) are terms used to describe digital signals.
* A digital signal is a composite signal with an infinite bandwidth.
* Bit rate and bandwidth are proportional to each other.
* The Nyquist formula determines the theoretical data rate for a noiseless channel.
* The Shannon capacity determines the theoretical maximum data rate for a noisy channel.
* Attenuation, distortion, and noise can impair a signal.
* Attenuation is the loss of a signal’s energy due to the resistance of the medium.
* The decibel measures the relative strength of two signals or a signal at two different points.
* Distortion is the alteration of a signal due to the differing propagation speeds of each of the frequencies that make up a signal.
* Noise is the external energy that corrupts a signal.
* We can evaluate transmission media by throughput, propagation speed, and propagation time.
* The wavelength of a frequency is defined as the propagation speed divided by the frequency.

Network Models

* The five-layer model provides guidelines for the development of universally compatible networking protocols.
* The physical, data link, and network layers are the network support layers.
* The application layer is the user support layer.
* The transport layer links the network support layers and the user support layer.
* The physical layer coordinates the functions required to transmit a bit stream over a physical medium.
* The data link layer is responsible for delivering data units from one station to the next without errors.
* The network layer is responsible for the source-to-destination delivery of a packet across multiple network links.
* The transport layer is responsible for the process-to-process delivery of the entire message.
* The application layer enables the users to access the network.

Introduction

* Data communication is the transfer of data from one device to another via some form of transmission medium.
* A data communications system must transmit data to the correct destination in an accurate and timely manner.
* The five components that make up a data communications system are the message, sender, receiver, medium, and protocol.
* Text, numbers, images, audio, and video are different forms of information.
* Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex.
* A network is a set of communication devices connected by media links.
* In a point-to-point connection, two and only two devices are connected by a dedicated link. In a multipoint connection, three or more devices share a link.
* Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology.
* A network can be categorized as a local area network (LAN), a metropolitan-area network (MAN), or a wide area network (WAN).
* A LAN is a data communication system within a building, plant, or campus, or between nearby buildings.
* A MAN is a data communication system covering an area the size of a town or city.
* A WAN is a data communication system spanning states, countries, or the whole world.
* An internet is a network of networks.
* The Internet is a collection of many separate networks.
* TCP/IP is the protocol suite for the Internet.
* There are local, regional, national, and international Internet service providers (ISPs).
* A protocol is a set of rules that governs data communication; the key elements of a protocol are syntax, semantics, and timing.
* Standards are necessary to ensure that products from different manufacturers can work together as expected.
* The ISO, ITU-T, ANSI, IEEE, and EIA are some of the organizations involved in standards creation.
* Forums are special-interest groups that quickly evaluate and standardize new technologies.
* A Request for Comment (RFC) is an idea or concept that is a precursor to an Internet standard.