UNIT 2: Internet Technologies
Computer Engineering Section University Women’s Polytechnic Aligarh Muslim University, Aligarh
March 17, 2019
Outline Internet
Evolution of Internet The Internet Today Protocols & Standards
URL, Cookies, Types of Web Documents Wireless Networks
Elements of a Wireless Network Bluetooth
Introduction to Wi-Fi: 802.11 Wireless LANs
What is Internet? I
Internet is a global network of networks.
When large networks are built, interconnecting small networks, one should have as the main goal - interoperability.
By interoperability we mean the ability of Hardware and Software of multiple machines from multiple vendors to communicate and operate together meaningfully.
This is achieved to a great extent by a World Wide Computer Network known as Internet.
Internet is also defined as a collection of packet switched networks operating on a protocol (a term we shall discuss shortly) known as TCP/IP.
Internet is basically a shared network of government agencies, ed- ucational institutions, private organizations, and individuals from over a hundred nations.
What is Internet? II
No one owns the Internet and anyone can have access to it. Its user are increasing in number everyday and every hour.
The US has begun to put its weight behind the development of the National Information Infrastructure (better known as information superhighway).
The stated goals are to connect networks through ”superhighways”
with data rates higher than several Gbps.
Intranet and Intraware
A local area network (LAN) thatuses internet like servicesis called an Intranet.
In other words we can say that any LAN operating on TCP/IP and using services as email, chat, news groups, www etc., is an Intranet.
An Intranet is also called internet. (Notice the small case i).
Intraware was a Network Operating System from Novell to incor- porate Internet on Novells LAN.
Novell LANs used IPX/ SPX protocols but with the release of Intraware, Novell LANs had the support for TCP/IP.
Evolution of Internet I
The US Department of Defense (DOD) was an early supporter of the much of the research into advanced computing and networking.
In 1960s the US Military relied on Computer Technology. So, the advances in that area were of critical importance.
A network that could be shutdown by the malfunction of a single com- puter or node was highly undesirable and so the military wanted to develop a network that could remain in service even if one or many of computers on the network didnt work.
Around 1960 the ARPA (Advanced Research Project Agency) a part of the DOD, setup the first steps for what the world today knows as Internet.
The goal of agency was to design a computer network with following features:
Evolution of Internet II
1 The network should be able to run even if many of the computers or the connections between them fail.
2 To accommodate the many different types of computers coming into the market, the DOD wanted dissimilar computers to be able to exchange the information smoothly.
3 It had to be a Network of Networks.
4 Only one computer on the network had to be connected directly to this network and every other computer on that network would be on this network.
The above goals were implemented as ARPANET (Advanced Research Project Agency Network) and are still part of todays Internet.
Soon after the inception of ARPANET, military computers and educa- tional institutions were given access to this Network.
In the 1980s it was opened for commercial use.
Evolution of Internet III
In 1992, control of internet was transferred to non-profit organizations like:
ISOC (Internet Society)
IETF (Internet Engineering Task Force) IAB (Interactive Advertising Bureau)
IANA (Internet Assigned Numbers Authority) W3C (World Wide Web Consortium)
In 1993 Marc Andreesen (at NCSA) developed first graphical browser(MOSAIC) Marc Andreesen Left NCSA to found Netscape in 1994 and come up
with cheap/free browser
In 1995,Microsoft came out with Internet Explorer
Internet in India
The history of the internet in India began with the launch of the Educational Research Network (ERNET) in 1986.
ERNET was initiated by the Department of Electronics (DoE), with funding support from the Government of India and United Nations Development Program (UNDP).
The first publicly available internet service in India was launched by state-owned Videsh Sanchar Nigam Limited (VSNL) on 14 August 1995.
This internet service, known as the Gateway Internet Access Service (GIAS), provided a speed of 9.6 kbit/s speed and was priced at$160 for 250 hours for individuals,$500 for institutional usage and higher for leased line services.
In 2004, the government formulated its broadband policy which defined broadband as ”an always-on Internet connection with download speed of 256 kbit/s or above.
From 2005 onward, the growth of the broadband sector in the country accelerated, but remained below the growth estimates of the government and related agencies due to resource issues in last-mile access which were predominantly wired-line technologies.
This bottleneck was removed in 2010 when the government auctioned 3G spectrum followed by an equally high-profile auction of 4G spectrum that set the scene for a competitive and invigorated wireless broadband market.
Today, internet access in India is provided by both public and private companies using a variety of technologies and media including dial-up (PSTN), xDSL, coaxial cable, Ethernet, FTTH (Fiber To Home), ISDN, WiFi, WiMAX, etc. at a wide range of speeds and costs.
As of July 2016 374,328,160 people (29.5% of the country’s total population) were Internet
Protocols I
A protocol is synonymous with rule.
It consists of a set of rules that govern data communications.
It determines what is communicated,how it is communicatedand when it is communicated.
The key elements of a protocol are syntax,semantics and timing.
Syntax defineshow data is structured?
In other words,the order in which pieces of information are pack- aged by the sender and opened up by the receiver.
As an example, some protocol may use first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.
Semantics relates to the meaning of each section of bits.
Protocols II
How is a specified pattern to be interpreted and what action is to be taken based on that interpretation?
For instance does an address identify the route to be taken or the final destination of the message?
Timingrefers totwo characteristics: when data should be sent and how fast they can be sent.
For Example, if a sender produces data at 100 Mbps but the re- ceiver can process data at only 1 Mbps, the transmission will over- load the receiver and some data will be lost.
For communication to occur, both sender and receiver must agree on the exact way the packet should be formatted.
Protocols III
Clearly, to communicate smoothly the devices must have at least one protocol as common protocol.
Reference Model, Protocol Stack & Protocol Suite I
A reference model is a layered framework for design of network systems that allows communication across all types of devices.
For example OSI (Open Systems Interconnection) is a reference model for how applications can communicate over a network.
Two reference models have been instrumental in the communica- tion world: The OSI model and The Internet/DOD model.
The termsprotocol stackandprotocol suiteare used synonymously as an implementation of a reference model.
Protocol suite properly refers to a collection of all the protocols that can make up a layer in the reference model.
The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family.
Reference Model, Protocol Stack & Protocol Suite II
Suite is the definition of the communications protocols, stack is the software implementation of them.
The term stackalso refers to the actual software that implements the protocols.
For example, network administrators sometimes talk about loading a stack, which means to load the software required to use a specific set of protocols.
Another common phrase isbinding a stack, which refers tolinking a set of network protocols to a network interface card (NIC).
Every NIC must have at least one stack bound to it.
ISO-OSI Reference Model: A de jure standard I
International Standards Organization (Estd. 1947) - A multina- tional authority on International Standards.
Open System Interconnection (OSI) model: An ISO standard that covers all aspects of network communications.
Understanding the fundamentals of OSI model⇒ a solid basis for exploration of data communication.
The purpose of the OSI ref. model is to guide vendors and devel- opers to develop devices and software that will interoperate.
It isa layered frameworkfordesign of network systems that allows communication across all types of devices.
It consists ofseven separate but related layerseach of whichdefines a segment of the process of moving information across a network.
ISO-OSI Reference Model: A de jure standard II
S.No. Layer Duties (in brief)
1. Physical Moves bits between devices
Specifies voltage, wire speed and pin-out of cables.
2. Data Link Combines bits into bytes and bytes into frames (Framing) Physical Addressing (MAC Addresses).
Flow Control (synchronization between sender and receiver), Error Control (frame level) and Access Control (simultaneous access to media)
3. Ntework Logical Addressing (IP Addresses) Routing
Frames at this layer become packets.
4. Transport Service Point Addressing (Port Address - for source program to destination program delivery)
Segmentation (Breaking packets into segments) & Reassembly.
Connection control, flow control and error control (packet level).
5. Session Dialogue Control: allows the communication between two processes to take place.
Synchronization
6. Presentation Translation
Encryption Compression
6. Application File Transfer, Access & Management Mail Services
Directory Services
The Internet/ DOD Reference Model: Ade facto standard I
The Internet Protocols suite was developed along with its name sake the Internet.
The protocols that make up the Internet protocols suite are bet- ter known as TCP/IP (Transmission Control Protocol/ Internet Protocol) protocols.
They have become de facto standards because of the success of Internet.
The entire protocols suite is also referred to as TCP/IP.
The Internet Protocol suite is unique in that it is made up of nonproprietary protocols.
This means that they do not belong to any one company and the technology is available to anyone who wishes to use it.
The Internet/ DOD Reference Model: Ade facto standard II
The Internet/ DOD Reference Model: Ade facto standard III
As a result, the Internet Protocol suite is supported by the widest variety of vendors.
The suite was developed about ten years before the OSI model was defined and can therefore be roughly mapped to it.
It was defined according to its own model, known as the Internet or DOD model.
The DOD model layers and the OSI model layers correspond as follows:
Thenetwork access layercorresponds to thephysical and data link layers.
TheInternet Layercorresponds to the OSInetwork layer.
Protocols at this layer areconcerned with transporting packets through the internetwork. The main Internet Layer protocol isIP.
Thehost-to-host layercorresponds roughly to the OSItransport layer.
The Internet/ DOD Reference Model: Ade facto standard IV
Protocols at this layer communicate with the peer processes in other hosts or networked devices. An example of a host-to-host protocol is TCP.
Theprocess/ application layercorresponds to the OSIsession, presenta- tion and application layers.
Protocols at this layer provide application services on the network.
Examples of protocols at this layer are Telnet, FTP, DNS etc..
The Internet protocols do not cover the lower two layers of the OSI model.
This is because the designers of TCP/ IP used existing physical and data link standards to make TCP/ IP hardware independent.
As a result the protocols of Internet suite are widely used to con- nect heterogeneous systems and are classified as Internet Middle Layer Protocolsand Internet Upper Layer Protocols.
The Internet/ DOD Reference Model: Ade facto standard V
The implementation of different layers of Internet Protocols suite is shown in following figure.
Standards
Areessentialincreating and maintaining an open market for equip- ment manufacturers.
Guarantee national and international interoperability of data, s/w and devices.
Provide guidelines to manufacturers, vendors, government agen- cies and other service providers.
This way they ensure the kind of required interconnectivity.
Data communication standards fall intofollowingtwo categories:
De facto: Standards that have not been approved by an organized body but have been adopted as standards through widespread use arede facto standards.
De jure:Those standards that have been legislated by an officially recog- nized body arede jure standards.
Standards Organizations I
Standards are developedthrough the cooperation ofstandards cre- ation committees, forums, and government regulatory agencies.
Standards Creation Committees:
- International Organization for Standardization (ISO).
- International Telecommunication Union-Telecommunication Standards Sector (ITU-T).
- American National Standards Institute (ANSI).
- Institute of Electrical and Electronics Engineers (IEEE).
- Electronic Industries Association (EIA).
Forums:
- Telecommunicationstechnology development is moving faster.
- Standards committees areprocedural bodies and by nature slow-moving.
- To accommodate needs of working models and to facilitate the stan- dardization process, manyspecial-interest groups(SGIs)have developed forums.
- They have representatives from various interested corporations.
Standards Organizations II
- Theywork withuniversities and users totest, evaluate, and standardize new technologies.
Regulatory Agencies:
- All communications technology is subject to regulation by government agenciessuch as the Telecom Regulatory Authority (TRAI)in India.
- Their purpose is toprotect the public interest byregulatingradio, tele- vision, and wire/cable communications.
- TRAI has authority over interstate and international commerce related to communications.
Internet Standards
An Internet standard is a thoroughly tested specification that is useful for and to be followed byInternet users.
There is astrict procedureby which a specification attains Internet standard status.
It begins as an Internet draft, which is a working document with no official status and a 6-month lifetime.
Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC).
Each RFC is edited, assigned a number, and made available to all interested parties.
RFCs go throughmaturity levels and are categorized according to their requirement level.
Uniform Resource Locator (URL)
A URL is commonly termed as a web address.
It is a reference to a web resource.
It specifies resource’s location on a computer network (Internet) and a mechanism for retrieving it.
URLs most commonly refer to web pages (http).
Less commonly and sometimes rarely, are also used forfile transfer (ftp), email (mailto), database access (JDBC) etc..
Most web browsers display the URL of a web page in an address bar.
A simpleURL is: http://www.example.com/index.html, And, a complex one is:
http://www.google.co.uk/search?q=url&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:
en-GB:official&client=firefox-a.
URL Anatomy (Syntax) I
Every URL conforms to a particular syntax.
That syntax is termed asanatomy of a URL.
From simple to complex, a URL is of the form:
scheme://host:port/path?query#fragment
Scheme, identifies the protocol to be used to access the resource on the Internet.
It can beHTTP (without SSL)orHTTPS (with SSL), FTP, mailto etc.
Host name identifies the host that holds the resource.
Hosts on the Internet DNS infrastructure are organized into do- mains and subdomains.
URL Anatomy (Syntax) II
Port:Host names can also be followed by a port number.
Well-known port numbers for a service are normally not included in URL.
Serversuse thewell-known port numbersforHTTP (Port 80)and HTTPS (Port 443), so most HTTP URLs omit the port number.
Path: typically refers to afile or directory on the web server, e.g.
/directory/file.php.
Sometimes the file name won’t be specified: e.g. https://
doepud.co.uk/blog/.
In that case aweb browser will automatically look insidethe/blog/
folder for a file called index or default.
If none of these is found, a 404 Not Found error will usually be returned by the server.
URL Anatomy (Syntax) III
Aqueryis commonly found in the URL ofdynamic pages(database enabled websites).
It is represented by a question mark followed by one or more pa- rameters.
The query directly follows thedomain name, path or port number.
For example, have a look at this URL which was generated by Google when doing a search for the word URL:
http://www.google.co.uk/search?q=url&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:
en-GB:official&client=firefox-a.
The query part is:
?q=url&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:
official&client=firefox-a.
URL Anatomy (Syntax) IV
Parametersareparts of information found in the query string of a URL.
With reference to the Google query above, the parameters follow the question mark and are separated by an ampersand (&) char- acter so that they could be understood individually and used to display content on that page.
The parameters are:
- q=url - ie=utf-8 - oe=utf-8 - aq=t
- rls=org.mozilla:en-GB:official - client=firefox-a
URL Anatomy (Syntax) V
Fragmentis aninternal page reference, sometimes called anamed anchor.
It usuallyappears at the end of a URLand begins with a hash (#) character followed by an identifier.
It refers to a section within a web page.
Simple URL
Cookies I Origin:
- The termcookiewas coined by web browser programmerLou Mon- tulli.
- Derived from the term ”magic cookie”.
- Magic cookie is a packet of data a program receives and sends back unchanged.
- Magic cookies were used by old Unix programmers.
What?
- AnHTTP cookieis also calledweb cookie, Internet cookie, browser cookie, or simply cookie.
- is asmall piece of data sent from a website and stored on the user’s computer by the user’s web browser while the user is browsing.
- is simply a small text file stored by a web site on your computer.
Cookies II
- Perform essential functions in the modern web.
- Browsingmore websites means getting in more cookies.
- Are part of modern browsing and can’t be avoided.
Example
# Example Cookie
# Recorded 2015-10-15 username=John
password=treeline8
ItemReviewed=toys,shoes,watch
Why Cookies?
- A reliable mechanism for websites to remember stateful informa- tion: such as items added in the shopping cart in an online store - Reason?: HTTP is a stateless protocol.
Cookies III
- To record the user’s browsing activity: which buttons clicked?, which pages were visited in the past, username, password etc.
- To remember values that the user previously entered into form fields: such as names, addresses and credit card numbers.
- Authentication cookiesare the most common method used by web servers to know whether the user is logged in.
- If logged insend sensitive information and advertisement otherwise dont.
Tracking cookies especially third-party tracking cookies, are used to compile individuals long term browsing history - a potential privacy threat.
Types of Cookies
Session Cookie:
Cookies IV
- Also known as an in-memory or transientcookie.
- Exists only in temporary memory (buffer)while the user navigates the website.
- Deleted by browsers when the user ends surfing.
- Have noexpiration date (assigned by web server)- This is how the browser knows to treat them as session cookies.
Persistent Cookie:
- Are sometimes referred to as tracking cookies.
- Expires at aspecific dateor after aspecific length of time (assigned by web server).
- During cookie’s entire lifetime its information will be transmitted to the web server every time the user visits the website.
Cookies V
- Good idea for inserting username/password automatically when- ever user visits website.
- ...and a great utility for advertisers.
Secure Cookie:
- A secure cookie can only be transmitted over an encrypted con- nection (i.e. HTTPS).
- Cannotbe transmitted overunencrypted connections (i.e. HTTP).
- This makes the cookie less likely to be exposedto cookietheft via eavesdropping.
- Secure flagis added to the cookie by web serverto make it secure.
Cookies VI
HttpOnly cookie:
- Cookies which cannot be accessed by client-side scripts, such as JavaScript.
- This eliminates the threat of cookie theft via cross-site scripting (XSS).
- Cross-site scripting (XSS) - Ascript which doesn’t belong to cookie’s parent website.
- HttpOnly flagis added by web server to make cookie HttpOnly.
SameSite cookie:
- Google Chrome version 51 introduced SameSite cookies.
- A cookie which can only be sent in requests originating from the same origin as the target domain.
Cookies VII
- This minimizes cross-site request forgery (XSRF).
Third-Party cookie:
- Normally, a cookie is a first-party cookie.
- Means, itserves to its parent website -i.e. the website which drops it to user’s system.
- Athird-party cookie, belongs to a domain (website) different from the parent domain (website)
- Typically appears when web pages feature content from external websites, such as banner or advertisements.
- Used for tracking the user’s browsing history, and is used by ad- vertisers.
Supercookie:
Cookies VIII
- A supercookie is a type of tracking cookie inserted into an HTTP header by an internet service provider (ISP).
- Collects data about a user’s internet browsing history and habits.
- Designed to be permanently stored on a users computer.
- Generally more difficult for users to detect and remove.
- Can not be deleted in the same fashionas regular cookies.
Zombie cookie:
- A cookie that is automatically recreated after being deleted.
- This is achieved by storing the cookie’s content in multiple loca- tions (both on client and server).
- When the cookie’s absence is detected, its restored from any of the locations.
Web Documents I Background:
- World Wide Web (WWW) is a repository of information (docu- ments) linked together from points all over the world.
- Has a unique combination of flexibility, portability, and user-friendly.
- Features that distinguish it from other services provided by the Internet.
Web Documents Types:
- Documents in the WWW can be classified as - static, dynamic &
active.
Static Documents
- Are fixed-content documents that are created and stored in a server.
- The client can get only a copy of the document.
Web Documents II
- The contents of the document are determined when it is created.
- Contents can’t be changed when document is in use.
Web Documents III
- The contents in the server can be changed, but user can’t change them.
- When a client accesses the document, a copy of the document is sent.
- A browser is generally required to display the document.
- Hypertext Markup Language (HTML) is used for creating Static Web pages.
Dynamic Documents
- A dynamic document is created by a Web server on receiving a request from a client (browser).
- To create the document, Web server runs a script or an application program.
Web Documents IV
- The output of the program or script is returned as a response to the browser that requested the document.
- Because a fresh document is created for each request, the contents of a dynamic document can vary from one request to another.
- Some technologies for creating dynamic documents are:
The Common Gateway Interface (CGI)- Allows use of C, C++, Unix Shell Programming, Tcl, Python, Perl etc.
Server Side Scripting Technologies: Hypertext Processor (pHP), Active Server Pages (ASP), Java Server Pages (JSP) etc.
Active Documents
- For many applications, programs or scripts which run at the client site are required.
- For example: A program/ script to create animation on the screen.
Web Documents V
- The program definitely needs to be run at the client site where the animation takes place.
- When a browser requests an active document, the server sends a copy of the document with associated script.
- The document is then run at the client (browser) site.
- Some technologies for designing active documents: Java Applets, JavaScript, VBScript and JQuery etc.
Assignment 3
Questions
Write a basic CGI script using C language.
Write at least three small pHP scripts of your choice. Test them on a web server.
Write at least three small client side scripts of
your choice and test them.
Wireless Networks I
Background:
- Wireless communication is one of the fastest-growing technologies.
- Make use of a wireless transmission medium.
- Typically restricted in their diameter: buildings, campus, single room etc..
- The global goal is to replace office cabling and to introduce high flexibility for ad hoc communicaton.
- Here, we mainly focus on two promising wireless technologies for LANs:
1 IEEE 802.11 wireless LANs - sometimes called wireless Ethernet, and
2 Bluetooth - technology for small wireless LANs.
- Both protocols need several layers to operate, we concentrate mostly on the physical and data link layers.
Wireless Networks II
Classification of Wireless Networks:
- Wireless networks can be classified into four main types.
- This is based on the geographical scope where a signal and service is available.
Wireless Personal Area Network (WPAN)
- Uses lowpowered transmitters for networking in a very short range.
- Usually 20 to 30 feet (7 to 10 meters).
- Are based on the IEEE 802.15 standard
- Include technologies like Bluetooth and ZigBee.
- Unlicensed ISM frequencies are used, including the 2.4-GHz band.
- ISM: Industrial Scientific and Medical.
Wireless Local Area Network (WLAN)
Wireless Networks III
- Connects multiple devices over a medium-sized range.
- Usually up to 300 feet (100 meters).
- Uses the IEEE 802.11 standard.
- Uses unlicensed frequencies in the 2.4-GHz and 5-GHz.
Wireless metropolitan-area network (WMAN) - A wireless service over a large geographic area.
- Such as all or a portion of a city.
- One common example is WiMAX.
- Governed by IEEE 802.16 standard.
- Licensed frequencies are commonly used.
Wireless wide-area network (WWAN) - A wireless data service for mobile phones.
Wireless Networks IV
- Offered over a very large geographic area.
- Offered only by telecommunications carriers.
- Regional, national, and even global - Licensed frequencies are used.
IEEE Project 802 I
Local Area Networks:
- The LAN market has seen several technologies but the most dominant today is Ethernet.
- Here we shall concentrate on Ethernet and its various implementations.
IEEE Project 802:
- In 1985, the Computer Society of the IEEE started a project, called Project 802.
- Goal was to set standards to enable intercommunication among equip- ment from a variety of manufacturers.
- Doesn’t seek to replace any part of OSI model.
- Instead, is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.
- Also specifies functions of the network layer to a lesser extent.
IEEE Project 802 II
- Contains a section (802.1) governing internetworking.
- This section ensures the compatability of different LANs and MANs across protocols and allows data to be exchanged across networks.
- The strength of Project 802 is modularity (Look at 802 subdivisions).
- By subdividing the functions necessary for LAN management the de- signers were able to standardized them.
- Each subdivision is identified by a number e.g. 802.1.
IEEE 802.1:
- Is the section of Project 802, devoted to internetworking.
IEEE 802.2:
- Understanding the importance of data link layer, Project 802 splits its functionality into two sublayers.
IEEE Project 802 III
IEEE Project 802 IV
- One set of functions is performed by IEEE 802.2 logical link layer control (LLC) protocol.
- LLC is considered the upper layer of Project 802 data link layer.
- It is common to all LANs.
- After 802.2 each higher subdivision of Project 802, i.e. 802.x, where x≥3, defines a particular set of MAC protocols along with the physical layer standards.
- Each of these subdivisions is corresponding to a different LAN tech- nology (e.g. 802.3 - Ethernet(CSMA/CD), 802.4 - Token Bus, 802.5 - Token Ring, 802.11 WLAN(CSMA/CA). . . 802.16 - WiMax).
- In essence a LAN Technology means:
LAN Technology = Physical Layer Standards + MAC Standards of Data Link Layer
IEEE Project 802 V
Improtant Note
Most of today’s LANs don’t use LLC.
All LLC functions are also covered under MAC in such cases.
- Clearly, Modern LANs differ only in Physical and MAC layer standards.
- It should now be obvious that why Ethernet, Token Ring etc. are called physical and data link layers protocols.
IEEE Project 802 VI
Wireless Networks
IEEE 802.11:
- IEEE has defined the specifications for a wireless LAN (WLAN).
- It is termed as IEEE 802.11.
- It covers the physical and data link layers.
IEEE 802.11 WLAN Architecture
WLAN Configurations/ Topologies
Independent Basic Service Set (Ad hoc/Peer-to-Peer) Infrastructure Basic Service Set (IBSS)
Distribution System(DS) Extended Service Set (ESS) Others:
Workgroup Bridges Repeaters APs
Outdoor Bridge (point-to-point and point-to-multipoint) Mesh
Independent Basic Service Set (Ad hoc or Peer-to-Peer) - Easiest way to install wireless 802.11 devices.
- The 802.11 standard allows two or more wireless clients to communicate directly with each other, with no other means of network connectivity.
- This is known as an ad hoc wireless network, or an independent basic service set (IBSS).
- Wireless devices see each other after sharing some basic configuration parameters.
- Do not scale well beyond 8 to 10 devices and 20-30 feet.
Infrastructure Basic Service Set(IBSS)
- Identified by the presence of an Access Point (AP).
- Basic service set (BSS) is the area, or cell, defined by the wireless signal served by the Access Point(AP).
- Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS.
- A service set identifier (SSID) is a tag name given to the BSSID.
- Using SSID, the AP advertises the existence of the BSS so that devices could find it and try to join.
- Unlike the ad hoc network, this network scales better - supports more hosts.
- This is because the AP manages all network traffic.
- Two devices can’t communicate without AP.
Distribution System
- Notice that a BSS involves a single AP and no explicit connection into a regular Ethernet network.
- In that setting, the AP and its associated clients make up a stan- dalone network.
- Wireless clients may need to communicate with other devices that are not members of the BSS.
- Fortunately, an AP can also uplink into an Ethernet network be- cause it has both wireless and wired capabilities.
- The 802.11 standard refers to the upstream wired Ethernet as the distribution system (DS) for the wireless BSS
Extended Service Set
- Normally, one AP cannot cover the entire area where clients might be located.
- For example, we might need wireless coverage throughout an entire floor of a business, hotel, hospital, or other large building.
- To cover more area than a single APs cell, we simply need to add more APs and spread them out geographically.
- When APs are placed at different geographic locations, they can all be interconnected by a switched infrastructure.
- The 802.11 standard calls this an extended service set (ESS),
- The idea is to make multiple APs cooperate so that the wireless service is consistent and seamless from the clients perspective.
- SSIDs that are defined on one AP should be defined on all the APs in an ESS.
- Notice that each cell in following figure has a unique BSSID, but both cells share one common SSID.
- Regardless of a clients location within the ESS, the SSID will remain the same but the client can always distinguish one AP from another.
- In an ESS, a wireless client can associate with one AP while it is physically located near that AP.
- If the client later moves to a different location, it can associate with a different nearby AP automatically.
- Passing from one AP to another is called roaming.
Others I
Workgroup Bridges
- Suppose we have a device that supports a wired Ethernet link but is not capable of having a wireless connection.
- We can use a workgroup bridge (WGB) to connect the device’s wired network adapter to a wireless network.
Others II
- Rather than providing a BSS for wireless service, a WGB becomes a wireless client of a BSS.
- In effect, the WGB acts as an external wireless network adapter for a device that has none.
Repeater APs
- Normally, each AP in a wireless network has a wired connection back to the DS or switched infrastructure.
Others III
- To extend wireless coverage, additional APs and their wired con- nections are added.
- Sometimes, it is not possible to run a wired connection to a new AP because the cable distance is too large to support Ethernet communication.
- In that case, we can add an additional AP that is configured for repeater mode.
- A wireless repeater takes the signal it receives and repeats or re- transmits it.
- The idea is to move the repeater out away from the AP so that it is still within range of both the AP and the distant client, as shown in figure.
Others IV
Outdoor Bridges
- An AP can be configured to act as a bridge to form a single wireless link from one LAN to another over a long distance.
- Outdoor bridged links are commonly used for connectivity between buildings or between cities.
- If the LANs at two locations need to be bridged, a point-to-point bridged link can be used.
- One bridge mode AP is needed on each end of the wireless link.
- Directional antennas are normally used with the bridges to maxi- mize the link distance, as shown in following figure (a).
- Sometimes the LANs at multiple sites need to be bridged.
- A point-to-multipoint bridged link allows a central site to be bridged to several other sites.
Others V
Others VI
- The central site bridge is connected to an omnidirectional antenna so that its signal can reach the other sites simultaneously.
- The bridges at each of the other sites can be connected to a directional antenna aimed at the central site.
- Figure (b) shows point-to-multipoint bridging.
Mesh
- To provide wireless coverage over a large area, it is not always practical to run Ethernet cabling to every AP that is needed.
- Instead, we could use multiple APs configured in mesh mode.
- In a mesh topology, traffic is bridged from AP to AP, in a daisy- chain fashion as shown in figure
Others VII
IEEE 802.11 WLAN Stations
- All components that canconnect into a wireless medium in a net- work are referred to as stations (STA).
- All stations are equipped withwireless network interface controllers (cards) (WNICs).
- Wireless stations fall into two categories: wireless access points and clients.
- Access points (APs), normally wireless routers, are base stations for the wireless network.
- They transmit and receive radio frequencies for wireless enabled devices to communicate with.
- Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones and other smartphones, or non-portable devices such as desktop computers and workstations etc.
IEEE 802.11 WLAN Station Types
- IEEE 802.11 defines three types of stations based on their mobility in a wireless LAN - no-transition, BSS- transition, and ESS-transition mobility.
- A station with no-transition mobility is either stationary (not moving) or moving only inside a BSS.
- A station with BSS-transition mobility can move from one BSS to another, but the movement is confined in- side one ESS.
- A station with ESS-transition mobility can move from one ESS to another.
- However, IEEE 802.11 does not guarantee that com-
munication is continuous during the move.
IEEE 802.11 MAC Sublayer I
- Wireless network devices are half-duplex devices, just like hosts connected to an Ethernet hub.
- It means that if two hosts transmit at the same time a collision will occur and both frames will be discarded.
- It requires that the transmission of frames must be controlled.
- Distributed Coordination Function (DCF) - a common media ac- cess method is a tool devised to do that.
- It is called ”distributed” because each device on network handles this job.
- An AP can performit as well. If the AP performs this function, it is calledPoint Coordination Function (PCF), althoughno vendors actually use PCF.
- DCF is based on CSMA/CA (Carrier Sense Multiple Access/ Col- lision Avoidance) while PCF uses polling.
IEEE 802.11 MAC Sublayer II
IEEE 802.11 MAC Sublayer III
Contention Based vs. Contention Free
You wish to transmit some data but you know there is only one medium and that is shared by many hosts.
Now there are two options: First, just transmit your data without thinking. Second, transmit only if it is your turn to transmit.
First approach is contention-less in which there are high chances some collision will occur with some other host transmitting before your transmission finished and you might need to retransmit your data.
In second (contention-based) approach you wait for your turn and are allowed to transmit only when you are up.
There are various MAC (Medium Access Control) protocols based on both the approaches.
