Chapter 10: The Internet

INTRODUCTION

During the late 1960s, US government called the Advanced Research Projects Agency (ARPA) created one of the country's first wide area packet-switched networks, the ARPAET. Select research universities, military bases and government labs were allowed to access to the ARPANET. In 1983, the Department of Defense broke the ARPANET into two similar networks: the original ARPANET and MILNET, which was for military use only. During the time the ARPANET was phasing out and replacing with newer technology, the National Science Foundation funded the creation of a new high speed, cross-country network backbone called the NSFNET. The backbone is the main telecommunications line through the network, connecting major router sites across the country. In 1990s, the government essentially withdrew all direct support for the internet and turned it over to private industries and universities. There was no longer one single backbone but multiple supported by different businesses and organizations

INTERNET PROTOCOLS

The internet depends on many protocols, several commonly used are:

• Internet Protocol (IP)
• Transmission Control Protocol (TCP)
• Address Resolution Protocol (ARP)
• Dynamic Host Configuration Protocol (DHCP)
• Network Address Translation (NAT)

Recall that the Internet with all its protocols follows the TCP/IP protocol suite (Internet model)An application, such as e-mail, resides at the highest layerA transport protocol, such as TCP, resides at the transport layerThe Internet Protocol (IP) resides at the Internet or network layerA particular media and its framing resides at the network access (or data link) layer

1. The Internet Protocol

• Provides a connectionless data transfer service over heterogeneous networks by passing and routing IP datagrams
• IP datagrams or packets that are passed down from the transport layer to the network layer are encapsulated with an IP header that contains the information necessary to transmit the packet

• There are currently two versions of IP
• Version 4, which has been in existence for many years
• Version 6, which has been available for several years but is only now starting to see a substantial move towards replacing version 4

IPv4 Diagram and Format

• Makes routing decision based on the 32-bit destination address
• May have to fragment the datagram into smaller datagrams using Fragment Offset
• May determine that current datagram has been hopping the network too long and delete it (time to live)

IPv4 Addresses

• All devices connected to Internet have a 32-bit IP address
• Think of the IP address as a logical address (possibly temporary), while the 48-bit address on every NIC is the physical, or permanent address
• Computers, networks and routers use the 32-bit binary address, but a more readable form is the dotted decimal notation
• When IP addresses were originally created, they were called classful addresses
• That is, each IP address fell into particular class
• A particular class address has a unique network address size and a unique host address size
• There are basically five types of IP addresses: Classes A, B, C, D and E

• Each IP address can consist of three parts:
• A 1-, 2-, 3- or 4-bit identifier field (beginning bit pattern)
• A net ID, which indicates a particular network
• A host ID, which indicates a particular host, or computer on that network
• IP multicasting : the capability of a network server to transmit a data stream to more than one host at a time
• IP multicasting suffers from lack of security
• Subnet masking: take the host ID portion of an IP address and divide it into a subnet ID and host ID. each subnet can support a smaller umber of hosts
• Today, IP addresses are considered classless addresses
• With classless addressing, companies (users) do not apply for particular class of addresses 
• Instead, company will get its IP addresses from an Internet service provider (ISP)
• Most ISPs have already applied for a large number of IP addresses and are willing to lease those addresses to companies
• The addresses are not identified by any class – they are simply a contiguous block of IP addresses
• Classless addressing has led to a much more efficient allocation of the IP address space
• A company can lease only as many addresses as it needs 
• An IP address in slash notation has all the info we need about the block of addresses assigned to a user/company

Internet Protocol Version 6

• IPv6 was created as the demand o the internet began to grow
• Main feature include:
• Simple header
• 128-bit IP address
• Priority levels and quality of service parameters
• No fragmentation

IPv6 Adresses

• IPv6 addresses are 128-bits in size (2128)
• They are also classless addresses, similar to IPv4 addresses
• Because of their size, a number of conventions have been adopted
• Binary addresses are written using the short-hand hexadecimal form:
• 0110 1010 0011 1110 1011 1010 ... 1110 1111
• 6A3E : BA91 : 7221 : 000 : 01FC : 922C : 877B : FFEF
• Four hex 0s in a row are truncated as:
• 6A3E : BA91 : 0 : 01FC : 922C : 877B : FFEF
• Longer strings of 0s can be abbreviated further:
•  6A3E : BA91 : 0 : 0 : 0 : 0 : 877B : FFEF
• Can be abbreviated as 
• 6A3E : BA91 : : 877B : FFEF

2. The Transmission Control Protocol (TCP)

• primary function is to turn an unreliable network (such as the one created in IP) into a reliable network that is free from lost and duplicate packets
• Performs 6 following functions:
• Create a connection
• release a connection
• Implement flow control
• Establish multiplexing
• Perform error recovery
• Establish priority

3. The Internet Control Message Protocol

• ICMP used by routers and nodes
• Perform error reporting for the Internet Protocol

4. User Datagram Protocol

• No frills transport protocol that does not establish connections, does not attempt to keep data packets in sequence and does not watch for datagrams that have existed for too long
• Its header contains 4 fields: Source port, Destination port, Length, and Checksum and is used by a small number of network services such as DNS that do not establish connection before sending data

5. The address Resolution Protocol

• Takes an IP address in an IP datagram and translates it into the appropriate medium access control layer address for delivery on a local area network
• When an IP packet has traversed the Internet and encounters the destination LAN, how does the packet find the destination workstation?
• Even though destination workstation may have an IP address, a LAN does not use IP addresses to deliver frames
• ARP translates IP address into MAC layer address so frame can be delivered to proper workstation

6. Dynamic Host Configuration Protocol

• An IP address can be assigned to a workstation permanently or dynamically
• Dynamic IP address assignment is a more efficient use of scare IP addresses
• When DHCP client issues an IP request, DHCP server looks in its static table
• Of no entry exists, server select an IP address from available pool
• The address assigned by DHCP server is temporary
• Part of agreement includes specific period of time
• DHCP clients may negotiate for a renewal before the time expires

7. Network Address Translation

• NAT lets router represent entire local area network to Internet as single IP address
• All traffic leaving LAN appears as originating from global IP address
• All traffic coming into this LAN uses this global IP address
• A level of security has been added because the outside world never sees any of the IP addresses used within the corporate network
• Company doesnt need to use purchased IP addresses
• A number of IP addresses have been designated as "phony" IP addresses
• NAT allows multiple workstations to access the internet with only one IP address

8. Tunneling Protocols and Virtual Private Networks 

• Virtual Private Network (VPN) is a data network connection that makes use of the public telecommunications infrastructure but maintains privacy through the use of a tunneling protocol and security procedures
• Tunneling protocol such as the Point-to-Point Tunneling Protocol (PPTP) is the command set that allows an organization to create secure connections using public resources such as the Internet
• Point-to-Point Protocol (PPTP) is used for communication between two computers using a serial connection such as DSL or cable modem connection between user's workstation and an internet service provider

THE WORLD WIDE WEB

• the world wide web (WWW) is a vast collection of electronic documents that are located on may different web servers, and contain text, images, videos and more that can be accessed by simple clicking links within a browser's web page
• Hypertext Markup Language (HTML) can be generated manually with a text-based editor such as Notepad, or through the use of a web page authoring tool
• Hypertext Transfer Protocol (HTTP) is an application layer protocol to transfer a web page

Locating A Document on The Internet

• Every document on the internet has a unique Uniform Resource Locator (URL)
• To find the document, part of the object's URL has to be translated into the IP address that identifies the web server where the object is stored
• This translation is performed by the domain name system (DNS)

Uniform Resource Locator:

• Uniquely identifies files, web pages, images or any other types of electronic documents that reside on the internet
• All URL consists of 4 parts:
• Service type: identifies the protocol that is used to transport the requested document
• Domain Name: specifies a particular server at a particular site that contains the requested item
• Directory or Subdirectory Information: specifies that the requested item is located in the subdirectory
• Filename: the document title

Domain Name System:

• is a large, distributed database of internet addresses and domain names
• The first operation performed by DNS is to query a local database for URL/IP address information
• If local server does not recognize address, the server at next level will be queried
• Eventually root server for URL/IP addresses will be queried
• If root server has answer, results are returned
• If root server recognizes domain name but not extension in front of domain name, root server will query server at domain name’s location
• When domain’s server returns results, they are passed back through chain of servers (and their caches)

INTERNET SERVICES

Electronic Mail (email)

• Computerized version of writing a letter and mailing it at the local post office
• Most email programs offer the following services:
• Create an email message
• Sending an email message to one or multiple recipients
• Receiving, storing, replying and forwarding email messages
• Attaching a file
• Multipurpose Internet Mail Extension (MIME) is used to send e-mail attachments
• Simple Mail Transfer Protocol (SMTP) is used to transmit e-mail messages
• Post Office Protocol version 3 (POP3) and Internet Message Access Protocol (IMAP) are used to hold and later retrieve e-mail messages

The File Transfer Protocol

• The first services offered on the internet
• allow user to download a file from a remote site to the user's computer and to upload a file from the user's computer to a remote site
• The three most common ways to access an FTP site are:
• Through a browser
• Using a canned FTP program
• Issuing FTP commands at a text-based command prompt

Remote Login (Telnet)

• A terminal emulation program for TCP/IP networks such as the Internet that allows users to log in to a remote computer
• the Telnet program runs on your computer and connects the workstation to a remote server on the internet
• once connected to a server or host, you can enter command through Telnet program and those commands will be executed as if your were entering them directly at the terminal of the remote computer

Voice Over IP

• The transfer of voice signals using a packet-switched network and the IP protocol
• Voice over IP (VoIP) can be internal to a company (private VoIP) or can be external using the Internet
• VoIP consumes many resources and may not always work well, but can be cost-effective in certain situations
• Three basic ways to make a telephone call using VoIP:
• PC to PC using sound cards and headsets (or speakers and microphone)
• PC to telephone (need a gateway to convert IP addresses to telephone numbers)
• Telephone to telephone (need gateways)
• Three functions necessary to support VoIP:
• Voice must be digitized (PCM, 64 kbps, fairly standard)
• 64 kbps voice must be compressed
• Once the voice is compressed, the data must be transmitted 
• ENUM
• A protocol that supports VoIP
• Converts telephone numbers to fully qualified domain name addresses
• Example – telephone number (312) 555-1212 will be converted to 2.1.2.1.5.5.5.2.1.3.1.e164.arpa

Listservs

• A popular software program used to create and manage internet mailing lists
• When an individual sends an email to a listserv, the program sends a copy of the message to all listserv members
• Listservs can be useful business tools for individuals trying to follow a particular area of study 

Streaming Audio And Video

• The continuous download of a compressed audio or video file, which can be heard or viewed on the user’s workstation
• Streaming audio and video consume a large amount of network resources
• Real-Time Protocol (RTP) and Real-Time Streaming Protocol (RTSP) support streaming audio and video

Instant Messaging, Tweets, and Blogs

• IM allows a user to see if people are currently logged in on the network and to send short messages in real time
• Consumes less resources than e-mail, and faster
• Tweets occur when you Twitter.  Max 140 character messages
• Blogs are online web logs that people maintain

THE INTERNET AND BUSINESS

• E-Commerce – the buying and selling of goods and services via the Internet
• Many agree that e-commerce consists of four major areas:
• E-retailing:  the electronic selling and buying of merchandise using the web
• Electronic data interchange (EDI): electronic commercial transaction between two or more companies
• Micro-marketing: gathering and use of the browsing habits of potential and current customers
• Internet security: security systems that support all internet transactions

Cookies and State Information

• A cookie is data created by a Web server that is stored on the hard drive of a user’s workstation
• This state information is used to track a user’s activity and to predict future needs
• Information on previous viewing habits stored in a cookie can also be used by other Web sites to provide customized content
• Many consider cookies to be an invasion of privacy

Intranets and Extranets

• An intranet is a TCP/IP network inside a company that allow employees to access the company’s information resources through an Internet-like interface
• When an intranet is extended outside the corporate walls to include suppliers, customers, or other external agents, the intranet becomes an extranet

THE FUTURE OF THE INTERNET

• Various internet committees are constantly working on new and improved protocols
• Some committees and groups include:
• The internet society (ISOC)
• The internet architecture Board (IAB)
• The internet engineering task force (IETF)
• the internet research task force (IRTF)
• the world wide web consortium (W3C)
• Internet Corporation for assigned names and numbers (ICANN)
• A new form of the Internet is being developed by a number of businesses and universities
• Internet2 will support very high-speed data streams
• Applications might include:
• Digital library serviceS
• Tele-immersion
• Virtual laboratories

Chapter 9: Introduction to Metropolitan Area Networks and Wide Area Networks

INTRODUCTION

Metropolitan area network (MAN) is a network that expands into a metropolitan area such as a city or a region and exhibits high data rates, high reliability and low data loss. Wide area network is a network that expands beyond a metropolitan area. They interconnect with huge numbers of workstations and can cover large geographic distances, including the entire earth.

METROPOLITAN AREA NETWORK BASICS

• MANs has many of the same technologies and communications protocols found in LANs and WANs
• Support high-speed disaster recovery systems, real-time transaction backup systems, interconnections between corporate data centers and internet service providers, support high speed connections among government, business, medical, and educational facilities
• almost exclusively fiber-optic networks and capable of supporting data rates into the tens of millions and hundreds of millions bits per second
• cover greater distance than LANS
• can recover very quickly from a link or switch/router failure
• MAN topologies are based on a ring
• the ability of a user to dynamically allocate more bandwidth on demand

1. SONET versus Ethernet

• Most MANs are SONET network built of multiple rings (for failover purposes)
• SONET is well-proven but complex, fairly expensive, and cannot be provisioned dynamically.
• Ethernet MANs generally have high failover times
• SONET is based upon T-1 rates and does not fit nicely into 1 Mbps, 10 Mbps, 100 Mbps, 1000 Mbps chunks, like Ethernet systems do.

• Metro Ethernet: is a data transfer service that can connect your business to another business using a standard Ethernet connection

• You may also connect your business with multiple businesses using a connection similar to a large local area network (Figure 9-4b)
• Thus, by simply sending out one packet, multiple companies may receive the data
• Neat thing about metro Ethernet is the way it seamlessly connects with a company’s internal Ethernet network(s)

WIDE AREA NETWORK BASICS

• WANs is a collection of computers and computer-related equipment interconnected to perform a given function typically using local and long-distance telecommunications systems
• Typically used to transfer bulk data between two endpoints and provide users with electronic mail services, access to database systems, and access to the internet
• WANs are very high speed with low error rates
• Usually follow a mesh topology

• A station is a device that interfaces a user to a network.
• A node is a device that allows one or more stations to access the physical network and is a transfer point for passing information through a network.
• A node is often a computer, a router, or a telephone switch.
• The sub-network or physical network is the underlying connection of nodes and telecommunication links.

Types of Network Structures

1. Circuit switched network:

• A network cloud in which a dedicated circuit is established between the sender and receiver, and all data passes over this circuit
• The telephone system is a common example.
• The connection is dedicated until one party or another terminates the connection.

2. Packet switched network:

• All data messages are transmitted using fixed-sized packages, called packets
• More efficient use of a telecommunications lie since packets from multiple sources can share the medium
• Datagram packet-switched network, each data packet can follow its own possible unique course through the cloud
• Virtual Circuit packet-switched network create a logical path through the subnet and all packets from one connection follow this path

3. Broadcast network

• typically found in LANs, but occasionally found in WANs
• A workstation transmits its data and all other workstations "connected to the network hear the data. Only the workstations with the proper address will accept the data

Connection-oriented Versus Connectionless Network Application

• Connection oriented network application: provides some guarantee that information traveling through the network will not be lost and that the information packets will be delivered to the intended receiver, which is called Reliable Service
• A connection-oriented application requires both sender and receiver to create a connection before any data is transferred.
• Applications such as large file transfers and sensitive transactions such as banking and business are typically connection-oriented.
• A connectionless application does not create a connection first but simply sends the data.

• A connection oriented application can operate over both a circuit switched network or a packet switched network
• A connectionless application can also operate over both a circuit switched network or a packet switched network but a packet switched network may be more efficient

ROUTING

• Each node in a WAN is a router that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line.
• A router must select the one transmission line that will best provide a path to the destination and in an optimal manner.
• Often many possible routes exist between sender and receiver.

• The communications network with its nodes and telecommunication links is essentially a weighted network graph.
• The edges, or telecommunication links, between nodes, have a cost associated with them.
• The cost could be a delay cost, a queue size cost, a limiting speed, or simply a dollar amount for using that link.

• The routing method, or algorithm, chosen to move packets through a network should be:
• Optimal, so the least cost can be found
•  Fair, so all packets are treated equally
• Robust, in case link or node failures occur and the network has to reroute traffic.
• Not too robust so that the chosen paths do not oscillate too quickly between troubled spots.

1. Dijkstra's Least-cost Algorithm

• Is executed by each node and the results are stored at the node and sometimes shared with other nodes
• Calculation is time consuming so it is done on a periodic basis or when something is in the network changes
• By identifying all possible paths, it also identifies the lest cost path

2. Flooding Routing

• When a packet arrives at a node, the node sends a copy of the packet out every link except the link the packet arrived on.
• Traffic grows very quickly when every node floods the packet.
• To limit uncontrolled growth, each packet has a hop count.  Every time a packet hops, its hop count is incremented.  When a packet’s hop count equals a global hop limit, the packet is discarded

3. Centralized Routing

• One routing table is kept at a “central” node.
• Whenever a node needs a routing decision, the central node is consulted.
• To survive central node failure, the routing table should be kept at a backup location.
• The central node should be designed to support a high amount of traffic consisting of routing requests.

4. Distributed Routing

• Each node maintains its own routing table.
• No central site holds a global table.
• Somehow each node has to share information with other nodes so that the individual routing tables can be created.
• Possible problem with individual routing tables holding inaccurate information.

5. Adaptive Versus Fixed Routing

• With adaptive routing, routing tables can change to reflect changes in the network
• Static routing does not allow the routing tables to change.
• Static routing is simpler but does not adapt to network congestion or failures.

NETWORK CONGESTION

• When a network or a part of a network becomes so saturated with data packets that packet transfer is noticeably impeded, network congestion occurs.
• What can cause network congestion?  Node and link failures; high amounts of traffic; improper network planning.
• When serious congestion occurs buffers overflow and packets are lost.
• An application can observe its own traffic and notice if packets are disappearing.  If so, there may be congestion.  This is called implicit congestion control
• The network can inform its applications that congestion has occurred and the applications can take action.  This is called explicit congestion control
• Before making a connection, user requests how much bandwidth is needed, or if connection needs to be real-time
• Network checks to see if it can satisfy user request
• If user request can be satisfied, connection is established
• If a user does not need a high bandwidth or real-time, a simpler, cheaper connection is created
• This is often called connection admission control

3. 

3. 

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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.