Data Comm + Networks
The successful transmission of data mainly depends on two factors:
(a) The characteristics of the transmission medium, and (b) The quality of the signal being transmitted
Properties of Radio Waves
*Frequency dependent*: (a) Low frequencies: radio waves pass through obstacles well, but the power falls off sharply with distance from the source. (b) Higher frequencies: radio waves tend to travel in straight lines and bounce off obstacles. Absorbed by rain. (c) At all frequencies: radio waves are subject to interference from motors and other electrical equipments. - Since radio waves travel long distances, interference between users is a problem. Government tightly licenses the users of radio transmitters.
Error Control
- Error detection - Error correction code
Connection-oriented Services
- Modeled after telephone system - Remote login - Web pages
Why Networking?
- Resource Sharing - High reliability - Saving money - Access to remote information - P2P communication - Communication medium: emails, newsgroup, video conference - Among many others
Modulo 2 Arithmetic
0 + 0 = 0 1 + 0 = 1 0 + 1 = 1 1 + 1 = 0 0 - 0 = 0 1 - 0 = 1 0 - 1 = 1 1 - 1 = 0
Parity Checking
A control that verifies that all data was transmitted correctly by counting the number of odd or even bits. - *Interleaving* of N parity bits detects burst errors up to N - Each parity sum is made over non-adjacent bits - If the remaining bits after the parity bit do not match its even or odd value, then there is an error. - First bit is *parity bit* (P) - Does not check if two bits change P = 0 --> even P = 1 --> odd
Multiplexing
A form of transmission that allows multiple signals to travel simultaneously over one medium. - Space division (FDMA) - Time division (TDMA) - Code division (CDMA) - Choose a right route among multiple paths between source and destination
Metropolitan Area Network (MAN)
A large computer network usually spanning a *city* - A scaled-up version of a LAN - Transmission technology: similar to that for LAN - Service area: a city - Privately owned
Cyclic Redundancy Check (CRC)
A mathematical algorithm that translates a file into a unique hexadecimal value.
Amplitude Modulation
A method of transmitting signals by changing the amplitude of a wave. - Multiplication of a baseband signal with a single-tone sinusoidal (called the *carrier*)
Amplitude Shift Keying
A modulation technique for encoding digital data using various amplitude levels of an analog signal - Changing the power level of the waveform
Frequency Shift Keying
A modulation technique for encoding digital data using various frequencies of an analog signal. - Changing the frequency of the waveform
Phase Shift Keying
A modulation technique for encoding digital data using various phases of an analog signal. Frequency stays constant but changes made are relative to a reference point - If the wave starts going up vs. starts going down
Wireless Network
A network where users can access the Internet without the use of fixed cables *Advantages:* - Keep in contact with home or office - Rescue workers - Military *Disadvantages:* - Slower than wired networks (limited bandwidth) - Error rates are higher - Security
Coaxial Cable
A single copper wire surrounded by layers of plastic insulation and sheathing; used mainly in cable television , LANs, and cable Internet service. - Long distance communication Contains: - Plastic cover - insulator - outer conductor (braided mesh) (1) Protecting the core from electromagnetic interference - inner conductor - Bandwidth: 500 Mhz, 500 Mbps - The repeater spacing is about the same as with twisted pairs (2.3 miles / 3 ~ 4 km) - Because: mode of transmission is still an electrical signal through a metal conductor - Baseband mode vs. Broadband mode
Quadrature Phase Shift Keying (QPSK)
A system of modulating digital signals onto a radio-frequency carrier signal using four phase 0, 90, 180, 270 degree states to code two digital bits - Two bits per modulation symbol - allows twice as much data to be carried as BPSK - Combines phase and amplitude modulation 1/f = period 1/4f = pi/2 2/4f = pi 3/4f = 3pi/2
Full-duplex transmission
A type of data transmission in which data can move in both directions at the same time.
Half-duplex transmission
A type of data transmission in which data can travel in either direction but only in one direction at a time.
Simplex transmission
A type of data transmission in which data travels in a single direction only.
Multimode Graded Index
A type of fiber whose refractive index decreases gradually as the radial distance (distance to the core center) increases - The refractive index of the core is higher than that of the cladding to achieve total internal reflection, trapping the light inside the core. With a step-index core, the light only reflects at the core-cladding interface - However, when the refractive index drops gradually in a graded-index core, the light also slightly bends and follows a curved path
Microwave Transmissions
Above 100 MHz, the waves travel in *straight lines* and can therefore be narrowly focused. - Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal-to-noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. - Thus, microwave transmissions typically *occur between two ground stations*. - For 100-m towers, repeaters can be spaced 80 km apart. - Atmospheric conditions and solid objects interfere with microwaves ( higher frequency cannot penetrate walls etc.) - Before fiber optics, for many years microwaves and coaxial cables formed the heart of the long distance *telephone transmission system*. - microwaves have some other important uses, including cooking and Industrial/Scientific/Medical (ISM) bands
Hamming Code
An error detecting and error correcting code that greatly improves the reliability of data, named after mathematician Richard Hamming its inventor - Can be applied to data length - Used to detect and correct single-bit errors - All bit positions that are power of two are party bits (1, 2, 4, 8, 16, etc...) - P1 = check 1 bit, skip 1 bit - P2 = starting from 2 check 2, skip 2 - P3 = starting from 4 check 4, skip 4 - P4 = starting from 8, check 8, skip 8
Bending light
Angle < critical angle -> Refraction. Angle = Critical angle -> Refraction Angle > Critical angle -> Reflection
Internet Protocol Stack
Application, Transport, Network, Link, Physical - Physical layer is responsible for fast transfer - Physical and Link work together to make sure that that transfer is *reliable*
Broadcast networks (LANs)
Are designed to provide connectivity for devices within a limited area, typically within the premises of a *home, office building, business, or school* - Like in a conference or meeting - Transmission technology: broadcast - Multicasting - Local Area Networks - Topology: bus (IEE 802.3, Ethernet) IBM token ring (IEEE 802.5) - Traditional LANs run at the speed of 10 to 100 Mbps
Twisted-pair cable
Cables made two insulated copper wires that are twisted around each other and are surrounded by a plastic jacket (such as traditional home phone wire). - Twisted to *minimize the electromagnetic interference between adjacent pairs* - Bandwidth 250KHZ for analog signal - Digital (1) 10 Mbps = 300 ft (2) 2400 bps = 10 miles - Copper is used because of its electrical conduction properties (1) Electricity flows through copper with less resistance (2) More resistant to corrosion than metal and other conductors - Used in customer facilities + over a distance + data communication - Low frequency transmission medium
Modem
Communications hardware device that facilitates the transmission of data. - Does the work of modulation and demodulation of data
Frequency
Denoted by f : ________ of an electromagnetic wave is measured in terms of the number of its oscillations per second, called Hz.
Wavelength
Denoted by λ : the distance between two consecutive maxima.
TCP/IP (Transmission Control Protocol/Internet Protocol)
Developed prior to the OSI mode, therefore the laters do not match. - *Hierarchical protocol* made up of interactive modules, each of which provides a specific functionality
Quadrature Amplitude Modulation (QAM)
Digital modulation scheme that changes the phase and the amplitude of the carrier signal - An effective solution to increase the bandwidth efficiency with a lesser need for a signal power is to *combine amplitude and phase modulation*
Broadband transmission
Divides the bandwidth of a communication channel into many channels, enabling different types of data to be transmitted at one time.
Nyquist Theorem
For a finite bandwidth(H Hz) noiseless channel, if the signal consists of V discrete levels (different signal elements in different shapes), Nyquist's theorem states: Maximum data rate = 2H(log2(v)) bits/sec
Propagation of radio waves
Ground propagation (below 2Mhz). Sky propagation (2-30 Mhz) line of sight (above 30Mhz)
Light refraction and reflection
Interface between two media marks air vs. glass
Wide Area Networks (WANs)
Large area (*country, continent*). Combination of public, private and leased equipment. - Service area: country or continent - Transmission technology: point-to-point packet routing Topology: ...
Internetworks (Internet)
Many WANS,MANS,LANS interconnected by routers. Network of networks also called an internet. - Service area: worldwide
Layering
Means decomposing the problem into more manageable components - Provides more modular design - Easy to troubleshoot
Connectionless Services
Modeled after postal system - File transfer - Emails
Physical Layer
Move individual bits within the frame from one node to the next - Representation of bits. - Data rate. - Synchronization of bits.
Binary Phase Shift Keying (BPSK)
Only two signal elements 0 and 180 degrees. - One bit per waveform (modulation symbol) - Data is encoded as one of two phases of the carrier - Every bit period, you are changing the phase of the wave form to distinguish between 1's (cos(0 degrees)) and 0's.(cos(100degrees))
Network Hardware
Physical devices needed to create the network. Based on... - transmission technology - the service areas that the network covers
Flow control
Preserve the order of messages sent - Keep a faster sender from drowning a slower receiver with data.
Application Layer
Provides user interfaces and support for services such as e-mail, file transfer, etc. - Represents data to the user, plus encoding and dialog control - HTTP, DNS, DHCP, FTP
Radio Transmission
Radio waves: (a) Can travel long distance. (b) Penetrate building easily (therefore are widely used for communications, both indoors and outdoors). (c) *Omni* directional (travel in all directions). The transmitter and receiver do not have to be aligned physically. - In HF band, they bounce off the ionosphere
Modulation Types
Signal properties are varied according to the information w = wavelength RF signal: - *Amplitude, Frequency, Phase* A * cos (wt) - This variation could be continuous (analog modulation) or in discrete steps (digital modulation)
Rules for Data Transmission
Simplex transmission, Half-duplex transmission, Full-duplex transmission
What is a Network?
a communications, data exchange, and resource-sharing system created by linking two or more computers and establishing standards, or protocols, so that they can work together - Computer Networks - Communication Networks - Data Networks - Internet
Pulse Code Modulation (PCM)
converts analog to digital by: 1. Sampling the analog signal at regular intervals 2. Measuring the amplitude of each sample 3. Encoding (quantizing) the amplitude as binary data If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal. The function f(t) may be reconstructed from these samples by the use ofa low-pass filter.
Index of refraction
the ratio of the speed of light in a vacuum to the speed of light in a medium - *Cos α/ Cos β* (if < 1 and a is less than a certain critical angle, there is no refracted light)
Transport Layer
The basic function of the transport layer is to accept data from above it, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. The transport layer also determines what type of service to provide to the ses- sion layer, and, ultimately, to the users of the network. - Responsible for delivery of a message from one process to another. - Oversees host to destination delivery of individual packets. - Treats packets as independent entities ensures packet arrives intact and in order. - Segmentation and reassembly - TCP, UDP
Modal dispersion
The broadening of a pulse due to different path lengths taken through the fiber by a) graded index multimode fiber b) single mode fiber λ(wavelength) = light speed / f 1014Hzà2 * 10-6(2 micron) - Index of refraction is a function of the wavelength - the smaller the wavelength, the greater the index of refraction - *induced in multimode applications where different light modes exit the fiber asynchronously even though they enter the fiber simultaneously* - happens because the velocities of all light modes are the same since the refractive index is uniform inside the core.
total internal reflection
The complete reflection that takes place within a substance when the angle of incidence of light striking the surface boundary is less than the critical angle - Light travels through fiber optic cables because of this
Light emitting diode (LED) vs semiconductor laser
The general difference between them as that *LEDS is the standard light source* which is short for light-emitting diodes. Laser light source like gas lasers may be mainly used in some special cases. *Lasers are more powerful and operate at faster speeds than LEDs*, and they can also *transmit light farther with fewer errors*. Laser are also much more expensive than LEDs.
Link Layer
The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors (error control). Move a pocket from one node (host or router) to the next node in the route -Framing. Physical addressing. Flow control. Error control. Access control. - Ethernet, WiFi
Shannon Theorem
The maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by Bandwidth xlog2 (1+SNR) Maximum data rate = H(log2(1+S/N)) bits/sec
Network Layer
The network layer controls the operation of the subnet. A key design issue is determining how packets are *routed from source to destination* - Finding the best path - Controls the hardware devices and media that make up the network - Logical addressing; physical addressing handled in data link layer. - Routing; route selection required if multiple networks interconnected, finds best path to the destination - IP, routing protocols
Modulation rate/Baud rate
The number of changes per second/the number of signal elements (S) per second N = Bit rate Baud rate <= Bit rate
Error Detection
The process of determining if transmitted data has been received correctly and completely.
Modulation
The process of varying or more properties of a periodic waveform, called the carrier signal, with a modulating signal that contains information to be transmitted. - turning information into electrical signals which are suitable for transmission - Digital to analog conversion: change characteristic of analog signal based on information in digital data. When characteristic of Sine wave is changed, a different version of wave is created. - 1's and 0's represented in wave form
Transmission Media
The radio waves or cable that transport data on a network. (a) Guided (wired) - Twisted pair cable - Coaxial cable - Fiber obit able (b) Unguided (wireless)
Quality of Service (QoS)
The result of specifications for guaranteeing data delivery within a certain period of time after their transmission. - File transfer - Banking transaction - Voice traffic - Video film
Fiber Optics
Uses a laser of light in very thin glass fibers that are about the diameter of a human hair, protected by an insulation layer. It is free from electromagnetic and radio interference is very secure and can transmit data for long distances at very high speeds without error. - Used by private companies in implementing local data 1) β<α, 1st medium has higher optical density 3) Light emitting diode (LED) vs semiconductor laser - With current fiber technology, the achievable bandwidth is > 50,000 Gbps (50 Tbps). - The current practical signaling limit is about 1 Gbps. - Because conversion between electrical and optical signals - In the laboratory, 100 Gbps is feasible.
Baseband transmission
Uses the full bandwidth for only one communication channel and has a low data transfer rate compared to broadband. Preserves shape of digital signal with a low pass channel (has infinite or very low bendwidth). Shape of digital signal is better preserved when channel bandwidth is greater. - LAN 50 ohm - 1 km cables : data rate 1 ~ 2 Gbps
Signal-to-noise ratio (SNR)
Usually the ratio itself is not quoted; instead, the quantity 10 * log10(S/N) is given These units are called decibels (dB) e.g. An S/N ratio of 10 is 10 dB, a ratio of 100 is 20 dB, a ratio of 1000 is 30 dB, and so on.
Wireless Transmission Media
When electrons move, they create electromagnetic waves that can propagate through free space (even in a vacuum). a. Predicted by British Physicist James Maxwell in 1865. b. Produced and observed by the German physicist Heinrich Hertz in 1887. e.In vacuum, all electromagnetic waves travel at the same speed, i.e.* λ×f = 3×108m/sec*
