Networks and Communications by jizhen1947


									Networks and Communications


Networks and Communications
 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.

Networks and Communications

 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
 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.

Networks and Communications

 Network Architecture
 Network Protocols
 Network Applications
 Home Networks

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
    4.  Transport – end-to-end connections & reliability
    5.  Application – network process to application

Network Protocols Simplified
We’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
 The computer must have a hardware
  interface corresponding to the low-level

Low-level Protocols (cont’d)
 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
   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)
 The postal service provides end-to-end
  delivery of mail packages properly
 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
   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
 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)
 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

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

   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   5: e.g., FTP, telnet, HTTP   } "client-server protocols"

   Transport     4: TCP
                                                 "high-level protocols"
    Network      3: IP

    Datalink     2: e.g., Ethernet fram es
                                                 "low-level protocols"
    Physical     1: e.g., voltages

TCP/IP (cont’d)
From client to server:
 Header  information gets added to a packet at
   each protocol layer. For example:
HTTP request:      "GET /"

TCP segm ent:       TCP header        "GET /"

IP datagram :      IP header     TCP header         "GET /"

Ethernet fram e:    Ethernet header     IP header      TCP header   "GET /"


TCP/IP (cont’d)
 The  packet may traverse several devices on
  its path between client and server.
   Client                                               Server
 HTTP request                                          HTTP request

 TCP segm ent                                          TCP segm ent
  IP datagram                         IP datagram       IP datagram
 Ethernet fram e   Ethernet fram e   Ethernet fram e   Ethernet fram e

    Voltage           Voltage           Voltage           Voltage

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
   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
 Each  computer, or host, on the Internet has an
  IP address consisting of four numbers
  separated by periods, or dots. E.g., is one of the computers in our
 The domain name system provides an easier-
  to-remember name for an IP host on the net.
  E.g., 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

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.edua host in UH Engineering
   Engineering’s web server
    Texas’ web server
 Netscape’s FTP server

Networks and Communications
 Network Architecture
 Network Protocols
 Network Applications
 Home Networks

Network Applications
A   network application is a program which
  transmits information over the LAN or WAN to
  a complementary program on a remote
 These generally work in a “client/server”
 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.

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.

 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
 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, you can telnet to that machine
  and run the local mail program to read your

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,
    At home, whatever your ISP specifies, e.g., 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:
     for your UH IMAP mail
     At home, whatever your ISP specifies, e.g., for RoadRunner

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, 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:
 Example: .

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
 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
   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 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:

FTP Programs
 ftpis 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:

Networks and Communications

 Network Architecture
 Network Protocols
 Network Applications
 Home Networks

Home Networks
Let’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).
 You should then connect a broadband router to the
    The router provides multiple Ethernet and/or Wi-Fi
      connections to your home computers, printers, etc.

Local IP Addresses
 The   router uses DHCP to provide IP
  addresses to devices connecting to it.
 Home network IP addresses are usually: 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 these precautions:
      Do not broadcast the router’s SSID (name).
      Use WPA (Wi-Fi Protected Access) encryption.
   Most people do none of these, and their networks are
    wide open to nearby hackers!


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