Networks and Communications Demonstrations: Networks and

Networks and Communications

Demonstrations:


What is a "network" anyway? A network is a collection of computers,

communications channels, and appropriate software that allows the computers to communicate with each other in an organized way.


Network Architecture Network Protocols Network Applications Home Networks

Network Architecture

Local Area Networks (LAN) Wide Area Networks (WAN) Personal Area Networks (PAN) Wireless Personal Area Networks (WPAN)

Local Area Networks (LAN)

A local area network is a collection of computers located at a single location (possibly large, such as an entire campus) connected together in such a way that they can communicate with each other.

LAN's generally provide local file and printer sharing, and electronic mail.

Interconnection can be cables and/or wireless. Examples: the Engineering College, the UH

campus, a network in your home

Local Area Networks (LAN)

Wide Area Networks (WAN)

A wide area network consists of a diverse collection of computers communicating with each other over long distance, generally over common carrier lines.

WAN's generally provide access to remote files and electronic mail.

Example: There’s really only one: Internet!

Wide Area Networks (WAN)

Personal Area Networks (PAN)

A personal area network (PAN) is the interconnection of information technology devices within the range of an individual person, typically within a range of 10 meters.

For example, a person traveling with a laptop, a personal digital assistant (PDA), and a portable printer..

Wireless Personal Area Networks (WPAN)

A WPAN (wireless personal area network) is a personal area network - a network for interconnecting devices centered around an individual person's workspace - in which the connections are wireless .

Typically, a WPAN uses some technology that permits communication within about 10 meters - in other words, a very short range. One such technology is Bluetooth, which was used as the basis for a new standard, IEEE 802.15.1

WPAN (cont’d)

A key concept in WPAN technology is known as plugging in. In the ideal scenario, when any two WPAN-equipped devices come into close proximity (within several meters of each other) or within a few kilometers of a central server, they can communicate as if connected by a cable.

The technology for WPANs is in its infancy and is undergoing rapid development . Every device in a WPAN will be able to plug in to any other device in the same WPAN, provided they are within physical range of one another. In addition, WPANs worldwide will be interconnected.



Network Protocols

Network protocols provide the “language” for communications between devices.

The Internet network model had 5 protocol layers:

1. Physical – bits & hardware

2. Datalink – frames & physical addresses

3. Network – packets, path determination & logical addressing

4. Transport – end-to-end connections & reliability

5. Application – network process to application

Network Protocols SimplifiedWe’ll take a simplified view of these protocols: Low-level Protocols (Layers 1 & 2) - These relate

to the physical transmission of the data. (Ethernet, Wi-Fi, Bluetooth)

High-level Protocols (Layers 3 & 4) - These provide the general rules for communicating between two computers over the low-level protocol. (TCP/IP)

Client-server Protocols (Layer 5) - These specify how a specific client application will communicate with its server application over the high-level protocol. (Email, web, chat, instant messaging protocols)

Low-level Protocols (Layers 1 & 2)

Low-level protocols specify how basic packets of information are transmitted over a single physical network, and generally also include the hardware/electronics specification.

The computer must have a hardware interface corresponding to the low-level protocol!

Low-level Protocols (cont’d)

Analogy: Highway traffic laws describe rules for the

use of highways to carry vehicles with various contents from source to destination. Here we are including the physical specification of the highway and the vehicles, and the laws and procedures for using them.

Similarly, there are laws and procedures for air traffic, train traffic, ocean traffic, etc.

Low-level Protocols (cont’d)

Ethernet IEEE 802.3, 100 Mbits/s on twisted pair wire. Use an

Ethernet network interface card.

Wi-Fi – wireless LAN standard 802.11b (10 Mbps) and 802.11g (100 Mbps) All laptops, and newer PDA’s, have it built-in.

Bluetooth 802.15.1 (up to 2 Mbps) for short range networking

(usually up to 10 meters) Used for WPAN’s (PC to printer, PDA to PC, cell phone

to car audio system or earpiece)

Network Interface

There must be some sort of hardware connection between the computer and the network.

The low-level protocol determines the hardware interface, not the high-level protocol. That is, you would purchase an Ethernet or Wi-Fi or Bluetooth, not a TCP/IP, interface for your PC (more on TCP/IP later).

Examples: Network interface cards. Usually built-in on newer computers and PDAs.

MAC Addresses

Each device has a unique Layer 2 MAC address (Media Access Control).

Ethernet & Wi-Fi MAC addresses are twelve hex digits. E.g., 00-20-E0-6C-D8-2A . The address is built into the hardware and is not

changeable by the user.

Bluetooth MAC addresses are just 3 bits. A Bluetooth network has at most 1 master device and 7

slave devices (called a piconet).

High-level Protocols (Layers 3 & 4)

High-level protocols provide end-to-end addressing and delivery of high-level packets over any number of physical networks (internet) and low-level protocols.

The high-level packets are wrapped inside the low-level packets as they traverse each physical network.

High-level Protocols (cont’d)

Analogy: The postal service provides end-to-end

delivery of mail packages properly addressed.

It uses a variety of transportation means (highway, train, air).

The mail packages are carried inside the vehicles, trains, planes of the physical carrier.

High-level Protocols (cont’d)

Biggest Example:

TCP/IP (Unix , Windows, used on the Internet). TCP/IP runs over Ethernet, Wi-Fi, and many other

low-level protocols.

Application Protocols (Layer 5)

(Client-Server Protocols)

A network application is a program which transmits information over the LAN or WAN to another (similar?) application on a remote computer. Frequently we say that a client program communicates with a server program.

A server is an application (program) that offers a service to some other application.

A client is an application that requests the service from the server.

Application Protocols (cont’d)

The service is provided using some agreed-upon communication protocol that may be specific to that kind of service.

On the Internet, these protocols are transported on top of the TCP/IP protocol.

Application Protocols (cont’d)

Analogy: As a customer (client) in this country you may

send an order for goods to a company (server) in another country.

The filled out order form conforms to a specific procedure (protocol) for ordering from that company, and you mail the form to them in an envelope to be delivered by the postal system.

Common Client-Server Protocols

HTTP (Hypertext Transfer Protocol) - used on WWW for document transfer

SMTP (Simple Mail Transfer Protocol) - electronic mail POP (Post-Office Protocol) - email retrieval IMAP (Internet Message Access Protocol) – email

retrieval and management Telnet - remote terminal emulation FTP (File Transfer Protocol) - file transfer DHCP (Dynamic Host Configuration Protocol) –

dynamic assignment of IP addresses

Summary

A client application uses a particular language – a set of commands and responses -- to communicate with a remote server application.

These commands and responses are delivered to the destination machine (specified by its IP address) by the TCP/IP protocol.

The TCP/IP packets may be transported over multiple physical networks employing a variety of low-level protocols.

TCP/IP on the Internet

The TCP/IP suite has five protocol layers:

Application

Transport

Network

Datalink

Physical

5: e.g., FTP, telnet, HTTP

4: TCP

3: IP

2: e.g., Ethernet frames

1: e.g., voltages

} "client-server protocols"

"high-level protocols"

"low-level protocols"

TCP/IP (cont’d) From client to server:

Header information gets added to a packet at each protocol layer. For example:

"GET /"HTTP request:

TCP header "GET /"TCP segment:

TCP header "GET /"IP headerIP datagram:

TCP header "GET /"IP headerEthernet headerEthernet frame:

Voltages:

TCP/IP (cont’d)

The packet may traverse several devices on its path between client and server.

IP datagram

Ethernet frame

Voltage

Ethernet frame

Voltage

HTTP request

TCP segment

IP datagram

Ethernet frame

Voltage

HTTP request

TCP segment

IP datagram

Ethernet frame

Voltage

Client Server

Bridge

Router

TCP/IP (cont’d)

To send a message on the network, a computer only has to put its data in an envelope, called an Internet Protocol (IP) packet, and "address" the packets correctly.

The communicating computers--not the network itself--are also given the responsibility to ensure that the communication is accomplished.

The philosophy is that every computer on the network can talk, as a peer, with any other computer.

Addresses, names, and domains

Each computer, or host, on the Internet has an IP address consisting of four numbers separated by periods, or dots. E.g., 129.7.4.131 is one of the computers in our network.

The domain name system provides an easier-to-remember name for an IP host on the net. E.g., violet.egr.uh.edu is the domain name for the IP address above.

IP Addresses

IP addresses are assigned two ways: Static addresses are sometimes assigned

by IT personnel to corporate servers. Dynamic addresses are assigned by a

DHCP server when a client is turned on and initiates the request over the network.

Dynamic addresses are used in all home networks, and especially in all wireless networks.

Addresses, names, and domains (cont’d)

Some top-level domain specifications are: edu educational institutions com commercial companies org organizations gov government entities net networks jp Japan us United States

Addresses, names, and domains - Examples

violet.egr.uh.edu a host in UH Engineering www.egr.uh.edu Engineering’s web server www.texas.gov Texas’ web server ftp.netscape.com Netscape’s FTP server



Network Applications

A network application is a program which transmits information over the LAN or WAN to a complementary program on a remote computer.

These generally work in a “client/server” configuration.

Examples: File and Print Services, World-Wide Web, Electronic Mail, Chat, Remote Terminal Emulation, File Transfer.

File Services

File service provides the ability to read and write files on a disk located on a remote computer, the "file server", as if the disk were connected directly to the local computer. Demonstration: Look at server directories in

Windows Explorer.

Print Services

Print service provides the ability to print to a printer located on a remote computer, the "print server", as if the printer were directly connected to the local computer. Demonstration: Look at printers on the various

servers.

Larger versions on next slides

Top half

Bottom half

World-Wide Web The WWW consists of web servers which

serve up documents that can be displayed in web client programs, which are usually called web browsers because of their use in “browsing” the web.

Web documents provide hyperlinks to other web documents on the same or other servers.

The web provides multimedia documents, including text, sound, graphics, 3D, and more.

URLs

Links to a document on the web are in the form of a Uniform Resource Locator (URL).

A URL expresses in a standard way both the protocol and the network path to any resource on the network.

Format (in its simplest form): protocol:host:port/path.../document

URLs (cont’d)

Some examples of URLs specifying various protocols:

http://www.egr.uh.edu:80/Welcome.html ftp://ftp.egr.uh.edu/pub/ file://h:/notes/textdocs/html/demo.html

Electronic Mail (email)

A mail client uses the SMTP protocol to send a mail message to an SMTP server (“post office”), which then takes the responsibility for sending the message to its destination SMTP server, retransmitting as necessary when errors are encountered.

Electronic Mail (cont’d)

A client mail program on the destination machine can then read the message. (PC’s are usually not SMTP servers.)

For example, if you receive mail on bayou.uh.edu, you can telnet to that machine and run the local mail program to read your mail.

POP Servers & Clients

If the destination SMTP server is also running a POP service, an alternative way to read mail is to have a mail client on some other machine, perhaps a PC, use the POP protocol to retrieve the mail from the POP server (which is the destination SMTP server).

POP is only used for picking up mail that has already arrived at the SMTP/POP server.

For example, Thunderbird and Outlook Express are POP clients you can use to retrieve and read mail from multiple accounts.

IMAP Servers & Clients

Some mail servers support IMAP instead of POP (or perhaps in addition to it).

IMAP first retrieves just message headers, and retrieves the message only when its header has been selected for reading.

Messages can be managed on the IMAP server (folders, etc.).

IMAP clients (e.g., Outlook Express, Thunderbird) can connect to multiple IMAP servers and POP servers at the same time.

Configuring a Mail client

“Outgoing SMTP server” – the server that transmits messages you write.

This server depends on where the mail client accesses the Internet, for example, at UH or at home.

Examples: At UH, mail.uh.edu. At home, whatever your ISP specifies, e.g.,

smtp-server.houston.rr.com for RoadRunner.

Configuring a Mail client (cont’d)

“Incoming POP or IMAP server” – the server where your incoming mail arrives.

This server depends on where you receive the mail, not where the client accesses the Internet.

Examples: mail.uh.edu for your UH IMAP mail

([email protected]). At home, whatever your ISP specifies, e.g.,

pop-server.houston.rr.com for RoadRunner ([email protected]).

Web-based Mail

Most popular, however, is web-based mail: You simply connect to a web page and login to read

and send mail. Your mail and folders are stored on the server. Mail is accessed anywhere from a browser.

You don’t need to configure incoming and outgoing mail servers (for POP, SMTP, IMAP) – probably the biggest advantage!

Mail is not dependent on your ISP. At UH, mail.uh.edu provides a web-based interface to

the IMAP folders. Hotmail and Gmail are other popular examples.

Electronic Mail Addresses

The standard Internet "domain-style" addressing is common.

Address format: [email protected]

Example: [email protected] .

Remote Terminal Emulation

Remote terminal emulation is the basic function of providing a command line interface to a remote computer as if the user were connected locally to it via a terminal or console.

telnet: A popular program used on all Unix systems and the Internet for remote terminal emulation over TCP/IP.

File Transfer

ftp: This is the standard Internet "file transfer protocol“ used for file transfer long before the web arrived.

File Transfer Modes: Binary - byte-for-byte file image transfer. Text (or ASCII) - In this mode, the assumption is that the

file is a standard ASCII text file. If the text file format is different on the source and destination file systems, then ftp performs the appropriate conversion during the transfer.

Anonymous FTP Sites

Hundreds of sites around the country where anyone can login via FTP and pick up files (recently most have been replace by web sites).

Sites usually have a "theme": Windows software, educational applications, music, images, etc.

Public files are in the pub directory and below. Some have a submissions directory where you can submit files.

Anonymous FTP Sites (cont’d)

Login username: anonymous Password: your return email address One list of sites:

http://www.ftp-sites.org/

FTP Programs

ftp is a command-line program available on both Windows and UNIX. Demo: Enter ftp at the DOS Prompt

There are also many programs that provide a GUI interface to ftp. Demo: WS_FTP LE, available free at UH

Most web browsers support the ftp protocol, so you can just browse the site. For example:

ftp://ftp.egr.uh.edu



Home NetworksLet’s review and apply these ideas to a home

network. You’ll get to the Internet through an ISP (Internet

Service Provider). For high-speed/broadband service, you’ll use

either cable (cable company) or DSL (phone company).

You will be provided a cable modem or DSL modem. On one side it connects to the provider. On the other side it has Ethernet (twisted pair).

Your should then connect a broadband router to the Ethernet. The router provides multiple Ethernet and/or Wi-Fi

connections to your home computers, printers, etc.

Twisted pair

Ethernet

Local IP Addresses

The router uses DHCP to provide IP addresses to devices connecting to it.

Home network IP addresses are usually: 192.168.xxx.yyy and are local to the home network, not seen on the Internet. The router uses NAT (Network Address

Translation) to allow multiple home computers to share the router’s single IP address (the only one assigned/allowed to it by the ISP).

Wireless Router Security

The router provides a form of “hardware firewall”, hiding your computers from Internet probes.

However, the wireless link presents security risks, and you should take one or more of these precautions: Do not broadcast the router’s SSID (name). Use WPA (Wi-Fi Protected Access) encryption. Filter MAC addresses, allowing connections only by those

computers you know. Most people do none of these, and their networks

are wide open to nearby hackers!

Documents

Networks and Communications Demonstrations: Networks and