Chapter 8: Local Area Network: Part 2

INTRODUCTION

Network operating systems and network support software are two of the most important. Network operating systems are essential if the network is going to allow multiple users to share resources. The network operating system provides users with password protection on their accounts and network administrators with services that help them control access to network resources as well as use and administer network


WIRELESS ETHERNET

• Wireless LAN or Wireless Ethernet: a LAN that is not based primarily on physical wiring but uses wireless transmissions between workstations
• Workstation can be anywhere as long as it is within transmitting distance to an access point
• The access point: is essentially the interface device between the wireless user device and the wired LAN
• Access point acts as a bridge between the wired and wireless networks and can perform basic routing functions
• Typically found in 3 basic configuration:
• Single-cell wireless LAN: at the center cell is the access point
• Multiple-cell layout: multiple cells are supported by multiple access points, as in cellular telephone work
• Ad hoc or peer to peer: the is no access point, each user device communicates directly with the other users devices

1. Wireless LAN Standards

• IEEE 802.11
• Original wireless standard, transmission rated for infrared wireless range from 1-2 Mbps
• IEEE 802.11b
• Second wireless standard, can transmit data at a theoretical rate of 11 Mbps using 2.4 GHz signals
• With directional antennae designed for point-to-point transmission (rare), 802.11b can transmit for more than 10 miles
• IEEE 802.11a
• Capable of supporting a theoretical transmitting rate at 54 Mbps  using the 5-GHz frequency range
•  IEEE 802.11g
• Capable of transmitting data at 54 Mbps (theoretical) but using the same frequencies as 802.11b (2.4-GHz)
• IEEE 802.11n (100 Mbps theoretical) is the latest standard to be approved
• 802.11n 
• has a theoretical maximum data rate of 600 mbps with actual rates of roughly 100-145 Mbps
• uses MIMO technology (multiple input multiple output)–Sender and receiver have multiple antennas for optimum reception
• 802.11ac uses advanced MIMO, wider channels in the 5 GHz band and advanced QAM techniques to achieve high data rates
• To provide security, most systems use either:
• Wired Equivalent Privacy (WEP): provides either 40- or 128-bit key protection (dated)
• WPA or WPA 2 (Wi-Fi Protected Access)
• WPA 2 uses the most advanced encryption techniques
• Wireless LANs may also be configured without an access point 
• These configurations are called “ad-hoc”

2. Wireless CSMA/CA

• Carrier sense multiple access with collision avoidance (CSMA/CD): supporting wireless LANs limits when workstation can transmit, in an attempt to reduce the number of collisions
• How does CSMA/CA do this?
• All devices, before they transmit, must wait an amount of time called an interframe space (IFS)
• Some applications have a short IFS, while others have a long IFS
• If two applications want to transmit at same time, the application with shorter IFS will go first

3. CSMA/CA Frame Format

• Frame format for wireless Ethernet CSMA/CA has four address fields

NETWORK OPERATING SYSTEM

• Operating system is the program initially loaded into computer memory when the computer is turned on; it manages all the other programs (applications) and resources
• Application programming interface (API): application makes use of the operating system by making service requests
• Multitasking operation system: allow multiple programs to run at the same time. The OS runs only one program at a time, but it jumps from one program to the next so fast that it appears as if multiple programs are running at the same time
• Network OS is large, complex program that manages the resources common on most local area network

NETWORK OPERATING SYSTEMS PAST AND PRESENT

1. Novell NetWare

• NetWare directory services (NDS): an intelligent system that authenticates users and includes a distributed database of information about every application, user, server, and resource on a network
• Version 3 
• User logs onto a particular server–Bindery maintains directory system that contains the usernames and passwords of network users and groups of users authorized to log in that server
• Version 4
• Bindery replaced by powerful NDS database–No longer supported by Novell (beginning of 2004)
• Novell NDS (NetWare Directory Services)
• A database that maintains information on, and access to, every resource on the network, including users, groups of users, printers, data sets and servers
• Network administrator creates a hierarchical tree structure that represents the layout of the organization
• Tree structure is composed of organizational units which are composed of further objects, and leaf objects which are usually entities such as users, peripherals, servers, printers, queues and other network sources

2. Microsoft Windows NT and Window Server

Windows NT Version 4

• User interface for single user personal computers
• NT had only Domain
• Container object that contained users, servers, and other resources that share account and security information
• Domains are not hierarchal and in many cases they increase the level of administration 

Window Server 2000

• Incorporated Active Directory: that stores information about all the objects and resources in a network and makes this information available to users, network administrators, and application program
• Active directory creates a hierarchical application structure of resources
• To construct an active directory hierarchy, a tree design is created
• Objects, such as users, groups of users, computers, applications, and network devices are the leaf items within the tree
• Leaf items are grouped in organizational units 

Window Server 2003

• Improvements to Active Directory, including new management tools
• Capability to interconnect up to 8 windows servers
• New and improved file and print support services
• Support for IPv6
• Better security features

Window Server 2008 and 2012

• The latest window of Window Network OS
• Expanded Active Directory, including new management tools
• New server core
• Self-healing server that can fix corrupted files and/or folders
• Increased processing speed
• Advancements in network security

3. Unix

• Well established and very popular multitasking OS capable of supporting network operations
• First OS written in the language C
• Very stable system capable of supporting very large operations

4. Linux

• OS based on the concept of Unix
• Many versions available for free or very small price
• Can receive the original source code along with the compiled code
• Very stable multitasking OS
• Is part of the growing family of open source software that is highly regarded within the business and educational industries

5. Mac OS X Server

• Apple created the Mac OS X server based on Unix concept, and shared some characteristics with both Unix and Linux operating systems such as fast, efficient, and stable code

SERVERS

• Server is the computer that stores software resources
• In order to support a network OS, you need one or more network servers
• Network servers are high-power workstations often with multiple processors, RAID, SCSI, and lots of memory and disk space–Various forms of servers include server appliances, and server blades
• Server virtualization allows you to create multiple servers in software all running on a single physical server
• To protect the server from catastrophic disk failure, disk drives on most network servers support one of the redundant array of independent disks (RAID) techniques
• RAID is a collection of techniques for interfacing multiple hard disk drives to a computer 
• Some of the more common RAID techniques include:
• RAID-0: Data is broken into pieces, and each piece is stored on different disk drives 
• This technique is known as striping–RAID-1
• Data is stored on at least two disk drives, in duplicate, to provide a level of redundancy (or fault tolerance), should one disk become corrupted
• This technique is known also as disk mirroring
• RAID-3: Data is redundantly stored across multiple disk drives (striping), and error-checking   information concerning the stored data is kept on a separate disk
• RAID-5
• Data is broken into pieces (stripes) and stored across three or more disks
• Parity information (error-checking code) is stored along with the striped data, not on a separate disk

2. Client/Server Networks versus Peer-to-Peer Networks

• A clear majority of LANs are client/server networks
• The client, or user workstation, requests something such as database from server. The server accepts the request, retrieves data and return a response
• Peer-to-peer networks also exist: may have servers, but the network relies less on the servers and more on the communications between workstations

NETWORK SUPPORT SOFTWARE

1. Utilities

• Utilities are software programs that operate int the background and support one or more functions to keep the network running at the optimal performance
• Some of the more common groups of network utility software:
• Antivirus software
• Anti-spam software
• Anti-spyware software
• Backup software
• Network-monitoring software
• Crash protection software
• Security software
• Remote software
• Uninstall software

2. Internet software

• the toolset to support internet-related services
• Web server software: the application or set of programs that stores web pages and allow users from anywhere in the world to access those web pages

SOFTWARE LICENSING AGREEMENTS

Licensing agreement: a legal contract and describes a number of conditions that must be upheld for proper use of the software package

Most licensing agreements specify conditions in the following areas:

• Software installation and use
• Network installation
• Backup copies
• Decompilation
• Rental Statement
• Upgrade availabilities
• Copyright restrictions
• Maintenance agreements

Most licensing agreements come in one of the following forms

• Single-user-single-station license: one station, one user at one time
• Single-user-multiple-station license: one user with multiple devices
• Interactive user license: operating system user licence and controlled number of concurrent users license
• Site license: allows software package to be installed on any and all workstations and servers at a given time
• Corporate License: allows software package to be install anywhere within a corporation, including multiple sites
• General public license: software that is free to share and change, however, the creator may still charge a fee

LAN SUPPORT DEVICES

Other devices necessary for the proper support of a LAN:

• Uninterruptible power supplies (UPS): a backup device that can maintain power to one or more pieces of equipment for short period of time
• Tape drives: backup device
• Printer
• Media converters: are necessary when connect one type of medium with another
• Workstation
• Thin client workstation: a computer with no disk drives of any kind, often with reduced memory and some kind of minimized operating system

Chapter 7: Local Area Network: Part 1

INTRODUCTION

A Local Area Network (LAN) is a communications network that interconnects a variety of data communications devices within a small geographic area and transmits data at high data transfer rates. The strongest advantage of a local area network is its capability of allowing users to connect their computers to the internet and share hardware and software resources. Since appeared in 1970, LAN 's use has become widespread in commercial and academic environments.

PRIMARY FUNCTION OF LOCAL AREA NETWORKS

The majority of users expect a local area network to provide access to hardware and software resources that will allow them to perform one or more of the following:

• Access to the internet
• File serving: a large storage disk drive acts as a central storage repository
• Database and application serving
• Print serving: providing the authorization to access a particular printer, accept and queue print jobs, and providing a suer access to the print queue to perform administrative duties
• email serving
• Process control and monitoring
• Distributed processing
• Manufacturing support
• Academic support

ADVANTAGES AND DISADVANTAGES OF LOCAL AREA NETWORK

Advantages:

• Ability to share hardware and software resources
• Individual workstation might survive network failure
• Component and system evolution are possible
• Support for heterogenous forms of hardware and software
• Access to other LANs and WANs 
• Private ownership
• Secure transfer at high speed with low error rates

Disadvantages:

• Equipment and support can be costly
• Level of maintenance continues to grow
• Private ownership
• Some types of hardware may not interoperate
• Just because a LAN can support 2 different kinds of packages does not mean data can interchange easily
• LAN is only as strong as its weakest link

THE FIRST LOCAL AREA NETWORK: THE BUS/TREE

• Bus/tree local are network: simply called bus LAN was the first physical design when LANs became commercially available in the late 1970s that consisted of a simple coaxial cable, or bus, to which all devices attached.
• Connecting to the cable requires a simple device called Tap, a passive device
• Passive device: does not alter the signal and does not require electricity to operate
• Network interface card (NIC) is an electronic device, sometimes in the form of a computer circuit board or part of a larger circuit board that performs the necessary signal conversions and protocol operations that allow the workstation to send and receive data on the network
• Can be used with baseband signal and broadband signal
• Baseband signals are bidirectional and more outward in both directions from the workstation transmitting
• Broadband signals are usually uni-directional and transmit in only one direction, however, special wiring considerations are necessary
• It is also to split and join broadband cables and signals to create configurations called Tree

A MORE MODERN LAN

• Star-wired bus LAN: logically acts as a bus, but physically looks like a star
• Logical design: determines how the data moves around the network from workstation to workstation
• Physical design: refers to the pattern formed by the location of the elements of the network, as it would be drawn on s sheet of paper
• All workstations connect to a central device such as a hub
• Hub: is a relatively non-intelligent device that simply and immediate;y retransmits the data it receives from any workstation out to all other workstations connected to the hub

• Twisted pair cable has become the preferred medium 
• Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus
• Hubs can be interconnected with other cables
• Biggest disadvantage: when one station talk, everyone hears it, this is called Shared Network-all devices on the network are sharing the one bandwidth.
• Medium access control protocol: is the software that allows a device to place data onto a sub-based LAN and allows workstations to "take turn" at transmitting data
  a) Contention-based protocols, such as carrier sense multiple access with collision detection
  b) Round-robin protocols such as token passing

1. Contention-based protocols

• Essentially first-come, first-served: the first station to recognize that no other station is transmitting data and place its data onto the medium is the first station to transmit
• The most popular is Carrier sense multiple access with collision detection (CSMA/CD)
• If no workstation is transmitting, a workstation can transmit
• If another workstation is transmitting, the workstation wanting to transmit will wait and try again to transmit
• If two workstation transmit at the same time, collision occurs
  - when two workstation hear collision, they stop transmitting immediately
  - Each workstation backs off a random amount of time and tries again
• CSMA/CD is a nondeterministic protocol, at which cannot calculate the time at which a workstation will transmit

SWITCHES

• The hub is a simple device that transmits an incoming frame out to all connections
• the Switch: uses addresses and processing power to direct a frame out of a particular port, thus reducing the amount on traffic on the network
• A switch primary function is to direct the data frame only to the addressed receiver
• Switches have eliminated many hubs

• Most switches are transparent- which means they observe the addresses of the frames in transmission on the current network and creates an internal port table to be used for making future forwarding decisions.
• The switches create internal port by using a form of backward learning- they observe each frame that arrives at a port, extracts the source address from the frame, and places that address in the port's routing table

• Workstation that connect to a hub are on shared segment
• Workstations that connect to a switch are on a switched segment
• The backplane of a switch must be fast enough to support multiple data transfers at one time
• In a cut-through architecture, the data frame begins to exit the switch almost as soon as it begins to enter the switch
• In contrast, a store-and-forward device holds the entire frame for a small amount of time while various fields of the frame are examined, a procedure that diminishes the overall network throughput
• Shared segment network: a switch may be connected to a hub, which then connects multiple workstation
• Dedicated segment network: a switch may be directly connected to one or more workstations. Each workstation then has a private or dedicated connection that can increase the bandwidth , which is a very efficient way to isolate heavy users from the network

1. Isolating traffic patterns and providing multiple access

• Whether shared or dedicated segments are involved, the primary goal of a switch is to isolate a particular pattern of traffic from other patterns of traffic or from the remainder of the network
• Switches, because of their backplane, can also allow multiple paths of communications to simultaneously occur

2. Full-duplex switches

• Allow for simultaneous transmission and reception of data to and from a workstation
• This full-duplex connection helps to eliminate collisions
• To support a full-duplex connection 
•  NIC in the workstation must be capable of supporting, and then configured to support a full duplex connection
• A switch must be configured for a full duplex connection as well
•  The cable connecting must also be able to support full duplex connection

3. Virtual LANs

• Virtual LAN (VLAN) – logical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to another
• Even though employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network
• A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN
• Instead of sending technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator 

4. Link Aggregation

• Allow you to combine two or more links into one higher-speed link
• An IEEE protocol (802.3ad-2000) which typically runs in most LAN devices can support link aggregation
• Link aggregation can also be used in the event of a link failure
• Can be used to to allow multiple parallel links to a server

5. Spanning Tree algorithm

• The spanning tree algorithm (used in Spanning Tree Protocol and now Rapid Spanning Tree Protocol) runs in switches and can identify loops and remove them
• The way spanning tree algorithm works:
• Identify a switch as the root switch
• Visit each switch and identify the one port that has the shortest path back to the root switch.  Mark these ports with RP (root port)
• Visit each LAN and identify the port that provides the shortest path back to the root switch.  Mark these ports with a DP (designated port). 
• Are there any ports remaining that don’t have either an RP or DP designation?  Mark those ports as Removed.  (They aren’t physically removed, only removed in the forwarding tables)

6. Quality of service

• On a standard Ethernet LAN, all frames were created equal, or first come first served protocol
• Wireless Ethernet provides a level of priority
• The 802.1p standard adds a 3-bit field to the front of each Ethernet frameThis 3-bit field can be used to establish a priority

WIRED ETHERNET

• Ethernet: was the first commercially available local area network system and remains, the most popular LAN system today
• Primarily on the star-wired bus topology and uses essentially the CSMA/CD medium access protocol
• Comes in many forms depending upon the medium used and transmission speed and technology
• One additional improvement to Ethernet is Power over Ethernet (PoE), can place a NIC in a device, but dont have to connect the device to an electrical source

WIRED ETHERNET FRAME FORMAT

• To better support local area networks, the data link layer of the OSI model was broken into two sublayers:
• Logical link control sublayer
•  Medium access control sublayer
• Medium access control sublayer works more closely with the physical layer and contains a header, computer addresses, error detection codes, and control information
• Logical Link Control (LLC) sublayer: is primarily responsible for logical addressing and providing error control and flow control info.
• 
• EEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3) 
• The two frames do not have the same layout
  

  - If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another

D

Chapter 6: Errors, Error Detection, and Error Control

INTRODUCTION

Noise can creep in and disrupts data transmission even with the highest quality fiber-optic cable. When this occurs, error detection techniques become valuable tool. There are different form of noise that commonly occur during data transmission. So having a better understanding of different types of noise and what causes them will enable better application of noise-reduction techniques to communicate with the system. There are three error-control options when an error is detected: (1) toss the frame/packet (ignore the error), (2)return an error message to the transmitter, or (3)correct the error without help from the transmitter

NOISES AND ERRORS

1. White Noise

• White noise: also called thermal noise or Gaussian noise, is a relatively continuos noise and is much like the static we hear when a radio is being turned between stations.
• It is always present to some degree in transmission and depend on the temperature of the medium.
• It can be significantly reduce but never completely

2. Impulse noise:

• Impulse noise: or noise spike is a noncontinuous noise and one of the most difficult errors to detect because it can occur randomly
•  The noise is typically an analog burst of energy. If  impulse spike interferes with an analog signal, removing it without affecting the original signal  can be difficult
• If impulse noise interferes with a digital signal, often the original data can be recognized and recovered, but not recoverable if the noise is completely obliterates the digital signal

3. Crosstalk

• Crosstalk: is unwanted coupling between two different signal paths
• Telephone signal crosstalk is example. When crosstalk happens, you can hear another person talks back
• It can be reduced with proper precautions and hardware

4. Echo

• Echo: is the reflective feedback of a transmitted signal as the signal moves through a medium. 
• Occurs mot often at junctions where wires are connected or at the open end of a coaxial cable.
• Echo suppressor can be attached to a line to reduce echo

5. Jitter

• Jitter: is the result of small timing irregularities that become magnified during the transmission of digital signals as the signals are passed from one device to another
• Jitter can cause video devices to flicker, audio transmissions to click and break up, and transmitted computer data to arrive with errors
• If serious enough, jitter can cause the system to slow down transmission rates
• Causes can include electromagnetic interference, crosstalk, passing the signal through many repeaters and the use of lower quality equipment
• Possible solution involve installing proper shielding

6. Attenuation

• Attenuation: is the continuos loss of a signal;s strength as it travels through a medium
• Can be eliminated with the use of amplifiers for analog systems or repeaters for digital systems

ERROR PREVENTION

To prevent the occurrence of may types of transmission errors, several techniques can be applied:

• Install wiring with proper shielding to reduce electromagnetic interference and crosstalk
• Be aware that many different types of wireless applications share the same wireless frequencies
• Replace older equipment with more modern, digital equipment
• Use the proper number of digital repeaters and analog, amplifiers to increase signal strength
• Observe the stated capacities of a medium and to reduce the error, avoid pushing transmission speeds beyond their recommended limits

ERROR DETECTION

• Despite best attempts to prevent, errors still occur
• Error detection can be performed in several places within a communications model. The most common place is data link layer

1. Parity Check

• Simple parity: is the easiest error-detection method to incorporate into a transmission system.
• Even Parity: the 0 and 1 are added to the string produces an even number of binary 1s
• Odd Parity: the 0 and 1 added to the string produces an odd number of binary 1s

2. Longitudinal parity

• Longitudinal parity: tries to solve the main weakness of simple parity, that all even numbers of errors are not detected.
• Adds a parity bit to each character then adds a row of parity bits after a block of character. 
• The row of parity bits is actually a parity bit for each “column” of character. 
• The row of parity bits plus the column parity bits add a great amount of redundancy to a block of characters

• Both simple and longitudinal parities do not catch all errors
• Simple parity only catches odd numbers of bit errors
• Longitudinal parity is better at catching errors but requires too many check bits added to a block of data

3. Arithmetic checksum

• Used in TCP and IP on the Internet
• Characters to be transmitted are converted to numeric form and summed
• Sum is placed in some form at the end of the transmission
• Receiver performs same conversion and summing and compares new sum with sent sum

4. Cyclic Redundancy Checksum

• CRC or cyclic checksum: adds 8 to 32 check bits to potentially large data packets and yields an error-detection capability approaching 100%
• Transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given generating polynomial
• Generating polynomial: is an industry-approved bit string used to create the cyclic checksum remainder
• Quotient is discarded but the remainder is “attached” to the end of the message
• The message (with the remainder) is transmitted to the receiver
• The receiver divides the message and remainder by the same generating polynomial
• If a remainder not equal to zero results, there was an error during transmission
• If a remainder of zero results, there was no error during transmission

ERROR CONTROL

1. Toss the frame/packet

• Doesn't seem like an option, but has became a mode of operation for some newer wide area network transmission techniques
• If a data frame arrives at a frame relay switch and an error is detected, the frame is simply discarded
• Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission

2. Return a message

• Stop and Wait Error Control: the simplest of the error control protocols
  - A transmitter sends a frame then stops and waits for an acknowledgement
  a) If a positive acknowledgment (ACK) is received, the next frame is sent
  b) If a negative acknowledgement (ACK) is received, the same frame is transmitted again

• Siding Window Error Control: a flow control scheme that allows a station to transmit a number of data packets at one time before receiving some form of acknowledgement.
  - These techniques assume that multiple frames are in transmission at one time
  - When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next
  - Using TCP/IP, there are some basic rules concerning ACKs:
  

  a) Rule 1: If a receiver just received data and wants to send its own data, piggyback an ACK along with that data

  b) Rule 2: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms for another packet

  *If while waiting, another packet arrives, send the ACK immediately 

  c) Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms

  * No packet, send ACK

3. Correct the error

• For a receiver to correct the error with n further help from the transmitter requires a large amount of redundant information to accompany the original data
  - this redundant information allows the receiver to determine the error and make corrections
• This type of error control is often called forward error and involves codes called Hamming Codes
• Hamming Code is a specially designed code in which special check bits have been added to data bits such that, if an error occurs during transmission, the receiver might be able to correct the error using the included check and data bits

ERROR DETECTION IN ACTION

• FEC is used in transmission of radio signals, such as those used in transmission of digital television (Reed-Solomon and Trellis encoding) and 4D-PAM5 (Viterbi and Trellis encoding)
• Some FEC is based on Hamming Codes

4. 

Chapter 5: Making Connections Efficient: Multiplexing and Compression

Introduction

Under the simplest conditions, a medium can only carry one signal at a time. However, we want a medium that can carry multiple signals at the same time. The technique of transmitting multiple signals over a single medium is multiplexing. For multiple signals to share one medium, the medium must be divided to give each signal a portion of the total bandwidth. So a medium can be divided in three basic ways: a division of frequencies, a division of time, and a division of transmission code. Another way to make a connection between two devices more efficient is to compress the data that is transferred over the connection.

Frequency Division Multiplexing

• Frequency division multiplexing (FDM) is the assignment of non-overlapping frequency ranges to each user of a medium. 
• To allow multiple users to share a single medium, FDM assigns each user a separate channel
• Channel: is an assigned set of frequencies that is used to transmit the user's signal
• Multiplexor: the device that accepts input from one or more users
• Demultiplexor: the device attached to the receiving end of the medium that splits off each signal to deliver it to the appropriate receiver
• Guard band: a set of unused frequencies inserted between the two signals to provide a form of insulation to keep one signal from interfering with another signal

Time Division Multiplexing

• Time division multiplexing (TDM) allows only one user at a time to transmit and the sharing of the medium is accomplished by dividing available transmission time among users.
• A time division multiplexor calls on one input device after another, giving each device a turn at transmitting its data over  a high-speed line

1. Synchronous time division multiplexing

• Synchronous time division multiplexing (Sync TDM) gives each incoming source signal a turn to be transmitted, proceeding through the sources in round-robin fashion

• the demultiplexor on the receiving end of the high speed link must disassemble the incoming byte stream and deliver each byte to the appropriate destination
• when a device has nothing to transmit, the multiplexor must still allocate a slot for that device in the high speed output stream, but the slot still be empty.
• to maintain synchronization between sending multiplexor and receiving demultiplexor, the data from the input sources is often packed into a simple frame and synchronization bits are added somewhere within the frame

• T-I Multiplexing
  - T-1 multiplexed stream is a continuos repetition of frames
  - The frame of the T-1 multiplexor's output stream are divided into 24 separate digitized voice/data channels of 64 kbps each.
  - Each frame consists of 1 byte from each of the 24 channels (users) plus 1 synchronization bit
  - The input data from a maximum of 24 devices is assigned to fixed intervals. Each device can transmit only during that fixed interval. and if a device has no data to transmit, the time slot is still assigned to it and data such as blanks or zero is transmitted

• SONET/SDH Multiplexing
  - Synchronous Optical Networks (SONET) and Synchronous Digital Hierarchy (SDH) are powerful standards for multiplexing data streams over a single medium.
  - Both are synchronous multiplexing techniques
  - A single clock controls the timing of all transmission and equipment across an entire SONET network. Using only a single clock to time all data transmissions yields a higher level of synchronization because the system does not have to deal with two or more clocks having slightly different times
  - SONET defines a hierarchy of signaling levels, or data transmission rates, called Synchronous transport signal (STS)
  - Two common users for SONET are the telephone company and companies that provide an Internet backbone services

2. Statistical time division multiplexing

• Statistical time division multiplexing (Stat TDM) transmit data only from active users and does not transmit empty time slots
• To transmit data only from active users, the multiplexor creates a more complex frame that contains data only from those input sources that have something to send.
• If a workstation is not active, no space is wasted in the multiplexed stream

• A statistical multiplexor accepts the incoming data stream and create a frame containing the data to be transmitted
• To identify each piece of data, an address is included

Wavelength Division Multiplexing

• Wavelength division multiplexing (WDM) multiplexes multiple data streams onto a single fiber-optic line.
• It uses different wavelength (frequency) lasers to transmit multiple signals at the same time over single medium
• The wavelength of each different colored laser is called Lambda
• Each signal can be carried on the fiber-optic line at a different rate, this means that a single fiber-optic can support simultaneous transmission speeds
• Dense wavelength division multiplexing (DWDM): when WDM can support a large umber of lambdas
• Coarse wavelength division multiplexing (CWDM): is designed for short distance connections and has only a few lambdas, with greater space between lambdas

Code Division Multiplexing

• Code division multiplexing (CDM) allows multiple users to share a common set of frequencies by assigning a unique digital code to each user
• Uses direct sequence spread spectrum technology that spreads the transmission of a signal over a wide range of frequencies using mathematical value
• Each binary 1 and 0 is replaced with a larger, unique bit sequence
• To send a binary 1, a mobile device transmits the unique code
• To send a binary 0, a mobile device transmits the inverse of the code

Discrete Multitone

• Discrete multitone (DMT) is a multiplexing technique commonly used in digital subscriber line (DSL) systems 
• DMT essentially combines hundreds of different signals or subchannels into one stream and are destined for a single user
• Each of the subchannel is quadrature amplitude modulated-256 subchannels, each transmits 60 kbps, yields 15.36 Mbps
• Unfortunately, because of noise, not all 256 subchannels can  transmit at full speed of 60-kbps rate

Comparison of Multiplexing Techniques

Compression - Lossless vs. Lossy

• Compression: is the process of taking data and somehow packing more of it into the same space
• When data is compressed for transmission, it transfers more quickly, results in a more efficient connection
• It also allows more data to be stored in the same amount of disk space or memory

1. Lossless Compression

• Lossless compression: compresses data and then decompresses it back to original data, no data is lost due to compression
• Run-length encoding: replaces any repetition of the same bit or byte that occur in a sequence of data with a single occurrence of the bit/byte and run a count
• It compresses the 0s by counting the "runs" of 0s, that is it would start by counting the 0s until a binary 1s is encountered.
• The next step is to convert each of the decimal values into 4-bit binary values, or nibbles
• When encounter a decimal value of 15 or greater, we must convert it into multiple 4-bit nibbles

2. Lossy Compression

• Lossy compression: in the compression process, some of the data is lost
• Perceptual encoding or perceptual noise shaping: the compressed version of an audio stream sounds fairly close to the uncompressed version even though some of the original data as been removed
• MP3 is a common form of audio compression
• MP3 encoder produces a data stream that has a much slower rate than that of conventional CD music
• Video files can also be compressed by removing small details that the human eye will not notice
• JPEG: a technique that is very commonly used to compress video images that involves 3 phases:
  - Discrete Cosine transformation: the image is broken into multiple 8 by 8 blocks of pixels and then produce a new 8 by 8 blocks of values
  - Quantization phase: the object of this phase is to generate more zero entries in the 8  by 8 block
  Run-length coding: take the matrix of quantized values and perform run-length encoding on the zeros by going diagonal  to achieve longer runs of zeros
• MPEG (Motion Picture Expert Group) is a group of people that have created a set of standards that can use these small differences between frames to compress a moving video (and audio) to a fraction of its original size

Chapter 2: Fundamentals of Data and Signals

Introduction


Data and signals are two of the basic building blocks of any computer network. In order for a computer network to transmit data, the data must first be converted into appropriate signals. One thing data and signals have in common is that both can be either analog or digital form. The conversion is performed by modulation techniques and is found in systems such as telephones, AM and FM radio. Converting digital data to square-wave digital signals is relatively straightforward and involves numerous digital encoding techniques. Converting digital data to analog signals requires some form of a modem. Finally, converting analog data to digital signals is called digitalization; telephone system and music system are two common examples .


Data And Signals

• Data is entities that convey meaning within a computer or computer system.
• Signals are the electronic or electromagnetic impulses used to encode and transmit data

1. Analog Vs Digital

• Data and signals can exist in either analog or digital form
• Analog Data and Analog signals : are continuos waveforms that can be at an infinite number of points between some given minimum and maximum. The most common example is human voice or music and video in their natural states.

• It is difficult to remove noise from original waveform
• Noise: is unwanted electrical or electromagnetic energy that degrades the quality of signals and data

• Digital data and Digital signals: are composed of a discrete or fixed number of values, rather than a continuous or infinite number of values
• First type of digital signal is a "square wave" with simple patterns of high and low voltages

• It is fairly easy to separate original digital waveform from the Noise if the amount of noise is small enough

• If noise becomes so great that it is no longer possible to distinguish a high from low, then the noise has taken over the signals and can no longer understand the original waveform

2. Fundamentals of signals

There are three basic components: Amplitude, frequency, and phase

• Amplitude: is the height of the wave above or below a given reference point
• the height often denotes the voltage level of signal (measured in volts), but it also denote the current level of the signal (measured in amps) or the power level of the signal (measured in watts)

• Frequency: is the number of times a signal makes a complete cycle within a given time fram
  - Period: is the length, or time interval of one cycle
  -  Frequency is measured in hertz (Hz), or cycles per second (period = 1/frequency)
  - Spectrum: the range of frequencies that a signal spans from minimum to maximum
  - Bandwidth: the absolute value of the difference between the lowest and the highest frequencies
  - Extra noise degrades original signals, an electronic device usually has an effective bandwidth that is less than its bandwidth.
  

• Phase: is the position of the waveform relative to a given moment of time, or relative time to 0
  - The wave never makes an abrupt change but is a continuos sine wave
  - A phase change (or shift) involves jumping forward (or backward) in the waveform at a given moment of time.

• Attenuation is the loss of power of signal strength when signal travels through any type of medium.
• Decibel (dB) is a relative measure of signal loss or gain and is used to measure the logarithmic loss or gain of a signal
• Amplification: is the opposite of attenuation, the signal gains in decibels when a signal is amplified by an amplifier

• The formula to measure signal loss or gain is dB = 10 x log10(P2/P1) where P2 and P1 are the ending and beginning power levels, respectively and is expressed in watts. 

Converting Data into Signals

There are four main combinations of data and signals:

- Analog data transmitted using analog signals
- Digital data transmitted using square-wave digital signals
- Digital data transmitted using discrete analog signals
- Analog data transmitted using digital signals

1. Transmitting analog data with analog signals

• Data is analog waveform is simply being transformed to another analog waveform, the signal. The basic operation performed is modulation
• Modulation: is the process of sending data over a signal by varying its amplitude, frequency or phase 
• Landline telephones, AM radio, FM radio and older broadcast television are the common examples

2. Transmitting digital data with square-wave digital signals: digital encoding schemes

• Nonreturn to zero digital encoding schemes:
  - Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltage and 0s as positive voltages
  - Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0
  - The difference between these two schemes is that with NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or 1 and with NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1
• Manchester digital encoding schemes
  - The manchester class of digital encoding schemes ensures that each bit has some type of signal change.
  - To transmit a 1, the signal changes from low to high in the middle of the interval
  - To transmit a 0, the signal changes from high to low in the middle of the interval
  - The manchester schemes have an advantage over the NRZ schemes is that there is always a transition in the middle of a bit. So, the receiver can expect a signal change at regular intervals and can synchronize itself with the incoming bit stream
  - One big disadvantage of Manchester schemes is that roughly haft of the time there will be 2 transitions during each bit.
  - Baud Rate: is the number of times a signal changes value per second.
  - Data rate is measured in bits per second (bps)
• Bipolar-AMI encoding scheme
  - Bipolar-AMI is unique because it uses three voltage levels
  - When a device transmits a binary 0, a zero voltage is transmitted
  - when the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted
• 4B/5B digital encoding scheme
  - 4B/5B: convert the 4 bits into a unique 5-bit sequence and encode the 5-bits using NRZI

3. Transmitting digital data with discrete analog signals

• Amplitude shift keying
  - A data value of 1 and data value of 0 are represented by two different amplitudes of a signal
  - During each bit period, the amplitude of the signal is constant
  - Can have two or more possible amplitude levels
  - Weakness:  it is susceptible to sudden noise impulses such as the static charges by lighting storm
  - The least efficient encoding techniques
• Frequency Shift keying
  - Uses two different frequency ranges to represent data values of 0 and 1
  - Frequency shift keying doesnt have problem with sudden noise spikes that can cause data loss
  - However, it is subject to Intermodulation distortion- a phenomenon that occurs when the frequencies of two or more signals mix together and create new frequencies
• Phase Sift Keying
  - One phase change encodes a 0 while another phase change encodes a 1
  - Less susceptible to noise and can be used at higher frequencies
  - It is so accurate that the signal transmitter can increase efficiency by introducing multiple phase shift angles
  - Quadrature phase shift keying: incorporates four different phase angles, each of which represent 2 bits
  - Quadrature amplitude modulation: a signaling method in which a combination of 12 different phase shift angles with two different amplitudes

4. Transmitting analog data with digital signals

• Pulse Mode Modulation (PCM)
  - Hardware, specially a codec converts the analog data to a digital by tracking the analog waveform and taking "snapshots" of the analog data at fixed intervals
  - An analog value is converted ti ab equivalent fixed-sized binary value
  - Binary value can then be transmitted by means' of a digital encoding format
  - The closer the snapshots are taken to one another, the more accurate the reconstructed waveform will be
  - Sampling rate: the frequency at which the snapshots are taken
• Delta Modulation
  - A codec tracks the incoming analog data by assessing up and down "steps"
  - Uses a binary 1 to represent a rise in voltage and a 0 to represent a drop
  - Two problems with delta modulation are (1) if the analog waveform rises or drops too quickly, the codec may not be able to keep up with the change and slop overload noise results (2) when analog waveform doesnt change at all, the waveform generates a pattern of 1010101010... because the codec outputs a 1 or 0 only for a rise or fall, thus generating quantizing noise.

Data Code

• Data Code: is the set of all textual characters or symbols and their correspoding binary patterns 
• Three important data codes are
  1. EBCDIC
  2. ASCII
  3. Unicode

1. EBCDIC or the extended binary coded decimal interchange code

• An 8 bit code allowing 256 possible combinations of textual symbols

2. ASCII - the American Standard Code ode Information Interchange

• Is government standard in the US and one of the most widely used codes in the world.
• Character set exists in a few different forms including a 7-bit version that allows for 128 possible combinations of textual symbols

3. Unicode

• Is an encoding technique that provides a unique coding value for every character in every language, no matter what platform
• Currently supports more than 110 different code charts