02 Communication Technologies

Communication Tech

• Communication technologies like the Internet, radio, and LAN can be synchronous/asynchronous and broadcast/point-to-point
• Understanding the Internet requires knowledge of basic communication vocabulary

Basic Communication Vocab

• Synchronous communication involves active sender and receiver simultaneously, such as phone calls
• Asynchronous communication involves sending and receiving at different times, like emails and answering machines
• Broadcast communication involves a single sender and many receivers, like radio and TV
• Multicast involves specific groups of receivers and specialized topics
• Point-to-point communication involves one specific sender and receiver, such as a telephone call

Broadcast Example: ARP

• Address Resolution Protocol (ARP) requests occur when a device on a local network (LAN) needs the MAC address of another device
• The device sends an ARP request to the broadcast address (FF:FF:FF:FF:FF:FF) asking "Who has the IP address x.x.x.x?"
• All devices receive the broadcast, but only the device with the requested IP responds with its MAC address
• The requesting device uses the MAC address to communicate

Broadcast Example: Router Updates

• Routers send routing updates to all devices on directly connected networks
• Devices can learn about the network topology and update routing tables accordingly

Multicast Example: IGMP

• Internet Group Management Protocol (IGMP) is used by devices in local networks to join/leave multicast groups
• Routers learn which devices want to receive multicast traffic via IGMP
• Multicasting is popular for multimedia streaming applications, like video conferencing or live broadcasts
• Video streamer sends live video to a multicast group address, users can join by sending an IGMP message to the local router

Multicast Example: DNS

• The Domain Name System (DNS) typically sends queries to a unicast IP address
• DNS servers can respond with a multicast response, and multiple devices can receive the DNS response simultaneously, which reduces network traffic and boosts performance

Unicast Examples

• Web browsers send a unicast request when visiting a website to ask the web server for the web page
• The web server sends a unicast response back with the requested page
• Email clients send a unicast message containing the message and recipient's email to the email server
• The email server forwards it to the recipient's email server, which forwards it to the email client
• Voice over IP (VoIP) functions with devices sending a unicast stream of voice data to the recipient's device

Internet Properties

• The Internet uses point-to-point asynchronous communication
• The Internet is a universal communication "fabric" linking all computers, which makes computers and the network a single medium
• The Internet enables mimic synchronous communication due to its speed
• Multicasting is viable, which enables groups to communicate
• Videos can be posted for broad access

Client/Server Structure

• Client/server protocol is used for most interactions over the Internet
• Interaction begins when a web link is clicked, client computer gets a page from a web server
• Client computers receive services from the server, completing the client/server relationship
• This structure is key to Internet interactions, a response follows every request and relationships are brief
• Servers can handle many clients because of the brief interactions

Internet Connections

• The Internet is point-to-point asynchronous but fast enough to simulate synchronous communication
• Software is built to implement many forms of communication
• Video chat "slices up" its sound and video signals into chunks
• The content is transferred to the other party, whose client reassembles the sound and image for display.
• Fast, reliable transmission simulates a direct connection with the Internet Protocol

Appearing Connected

• Websites need multiple brief exchanges to give the appearance of a longer-lived connection
• Login information should be associated with online banking transactions
• Online purchases need to be associated with the shopping cart

Appearing Connected: Cookies

• Cookies are small files saved from the server to the client, returned with each request and contain enough information to associate the interactions.

Appearing Connected: URLs

• URL parameters pass information by adding it to the URL
• A google search demonstrates this

IP Addresses

• Each computer on the Internet has a unique IP address; this IP address consists of four numbers (one byte each) separated by dots
• IP addresses range from 0-255, which allows for billions of IP addresses
• There is, however, a shortage of IP addresses

IP Packets

• An IP packet is each separate message on the Internet
• IP packets will be sent to a particular IP address, however each packed may take different routes to reach an address
• The Traceroute tool can display the route

TCP/IP Postcard Analogy

• The Internet is similar to sending a novel to a publisher via postcards
• The novel gets broken into units that fit on a postcard
• The numbered postcards indicate where each belongs in the novel
• Each postcard is mailed individually; the publisher receives postcards in a random order using varying routes
• The receiver finally arranges in order and called IP packets
• IP packets contain the payload (unit of information) and header (necessary informations of the transmission)

IP Packet Header

• IP Packet headers allow you to identify payload, forward the sender to the receiver, allow the receiver to collect the payloads of different packets and rebuild the original file, and inform the sender of the success/failure during transmission

Independent Packets

• If a traffic is heavy the packet progress is slow, so the protocol allows the packet to be thrown away
• Killed packets result in recipient requests for a resend
• Packets take different routes, therefore can arrive out of order

WAN and LAN

• Internet comprises wide area networks (WAN) and point-to-point channels.
• WANs aren't nearby and packets must visit a number of computers
• Local area networks (LAN) connect computers nearby using cable, this is the main technology for local area networks

Ethernet Physical Setup

• The physical setup of an Ethernet network consists of a wire, wire pair, or optical fiber channel
• Engineers connecting a computer "tap" into the channel
• All computers (sender, and receivers) can detect the signal

Ethernet Transmission

• When a computer wants to transmit a message, it sends signals while listening to see if the arrived message matches the message it sent
• If the message is its own, the computer knows it's the only one sending and it completes the transmission
• The computer stops transmitting immediately if the arriving message is not its own, which is called a collision

Ethernet Transmission: Resolution

• The senders wait random amount of times and try to send again
• They wait different amounts of time; one will send before the others wait
• This process repeats, if there is another collision

Connecting to Internet: Basic Methods

• Connection via an Internet service provider (ISP), or a campus or enterprise network
• Many use both

Connecting to Internet: ISP

• Most home-users uses ISPs and ISP companies places a modem at the users house
• Modems convert bits a computer outputs into a form that is compatible with the carrier
• The signals are sent to the carrier's business where it is converted into form for the server to connect to the Internet
• Digital subscriber line (DSL or ADSL) and cable (TV) are two common providers

Connecting to Internet: Enterprise Networks

• An enterprise network connects network computer for schools, businesses, or government
• A LAN connects computers within the organization while LAN is connected to the Internet by a gateway

Wireless Networks

• Wireless Networks are a variation of LAN connection, referred to by protocol name of 802.11
• The router is physically connected to an ISP's modem, connected to the internet and capable of broadcasting and receiving frequency (rf) signals

Computer Address: Domains

• Domain names are easier to remember than a numeric IP address
• The Internet uses a hierarchy of domains of symbolic names

Computer Address: Domain Examples

• "spiff.cs.washington.edu" breaks down to 'spiff' being computer's name
• 'cs' being Computer Science and Engineering Department domain
• 'washington' being the University of Washington domain
• 'edu' being the educational domain

DNS: Domain Name System

• The Domain Name System (DNS) translates the hierarchical and readable names of a computer into it's numerical IP address
• Every Internet host knows it's nearest DNS name server and is used to send information to the intended server by that address after translation
• The server asked may not know the answer even if there are root name servers

DNS Lookup

• The nearest DNS server translates the name, the local server asks the root name server, the edu server, and the si server to find the IP address of the www
• Some root name servers include are there are 13 root name servers can crash

Top-Level Domains: TLDS

• TLD: .edu is for educational groups
• .com is for commercial enterprises
• .org is for organizations
• .net is for networks
• .mil is for the military
• .gov is for government agencies

Expanded TLDs

• Includes biz, info, name and travel
• ICANN is Internet Corporation for Assigned Names and Numbers
• The original apply to the USA

More Top-Level Domains

• There is a set of two-letter country designators, Canada's is ca
• UK's is united Kingdom and Germany is de
• These allows codes grouped of their country

World Wide Web: Web Servers

• The World Wide Web hosts the Web servers programed for Browsers
• The Web are comprised of these servers and files

World Wide Web: Server Content

• Server content are Web Pages are often used for images and animation used to enhance other web services

What Web Requests Use

• Web requests use client/server interaction for browser is looking for a file from web server
• URL (Universal Resource Locator) helps with looking
• Web browsers and Web servers both “speak” the HTTP

URLs

• Each the URL shows the (Protocol that pulls Fetch, Server Name used in Hierarchy, and Pathname is what and where the request file to be)
• An incorrect path or hostname produces an error however URL can be arrange to redirect others

Descriptions of Web Page

• Webpages are stored in the form on screens, a description of how they should appear
• The browser produces a description of the file and creates the web page
• Description of those is less than image file for adaption to computer easier

File Structure

• Folder: Names of files that's connected
• Hierarchy: file structure of the computer. it's called for the directory
• Hieareichy: the tree in hierarchy and file are the leaves

More File Structure

• All hierarchies have branches and leaves with folders, also standard is to have: "Down" and, "Up" to move.
• A slash can tell you about directory hierarchy for going forward and back

Index.html

• Last one in sequence or if one in the folder
• If not built (blank page will show up)

Hierarchies

• Used to organize information, they usually cost anything