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2011-06-12 15:53:06

345 Midterm
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  1. Define the components of a network:
    File Server
    Print Server
    Web Server
    • Server - stores data or software that can be accessed by the clients
    • Client - the input/output hardware device at the user's end of a communication circuit. it typically provides users with access to the network and the data software on the server.
    • Circuit- the pathway through which the messages travel. It is typically a copper wire, although fiber optic cable and wireless transmission are becomming common.

    Peer-to-Peer - a network that doesn't use a server, it connects a bunch of computers and shares data between them. Computers function as equals.

    Router - a device that connects two or more networks. It enables computers on this network to communicate with computers on other networks (ie. the internet)

    • File Server - stores data and software that can be used by a computer on the network
    • Print Server - so computers can share a printer
    • Web Server - stores documents and graphics that can be accessed from any web browser, such as internet explorer.
  2. What is a LAN? BN? MAN? What is a WAN?
    LAN stands for Local Area Network. A LAN is a group of microcomputers located in the same general area. It covers a clearly defined small area (such as a building or work area). They often share circuits where all computers must take turns using the same circuit.

    BN stands for Backbone Network. Most LANs are connected to BN, a larger, central network connecting several LANS, other BNs, MANs, and WANs.

    MAN stands for Metropolitan Area Network. MANs connect LANS and BNs located in different areas to each other to WANs. MANs typically span between three and 30 miles.

    WAN stands for Wide Area Network. WANs connect BNs to MANs. Most organizations do not build their own WANS by laying cable, building microwave towers or sending up satellites. Instead, most organizations lease circuits from (ie. AT &T) and use those to transmit data.
  3. What is the OSI model?
    OSI stands for Open Systems Interconnection Reference Model. it is the most commonly referenced network model. There are SEVEN layers to this model:

    1. Physical Layer - concerned with transmitting data bits over a communication circuit ( 0's and 1s)

    2. Data Link Layer - manages the physical transmission circuit in layer 1 and transforms it into a circuit that is free of transmission errors. Because layer 1 accepts and transmits only a raw stream of bits without uunderstanding their meaning, the data link layer creates and recognizes message boundaries (ie. where the message starts & ends)

    3. Network Layer - performs routing. It determines the next computer the message should be sent to

    4. Transport Layer - Deals with end-to-end issues, such as procedures for entering and departing from the network. It establishes, maintains and terminates logical connections for the transfer of data between the original sender and hte final destination of the message

    5. Session Layer - is responsible for managing and structuring all sessions. Session initiation must arrange for all the desired and required services between session particpants such as logging into circuit equipment, transferring files,

    6. Presentation Layer - formats the data for presentation for the user. Its job is to accomodate different interfaces on different terminals/computers so the application program need not worry about them

    7. Application Layer - the end user's access to the network. The primar purpose is to provide a set of utilities for application programs. Each user program determines the set of message and any action it might take on receipt of a message.
  4. Group the OSI layers: Application, Internetwork & Hardware
    • Application Layer - Application, Presentation, Session
    • Internetwork Layer - Transport, Network
    • Hardware Layer - Datalink, Physical
  5. Describe message transmission using layers
    Each layer in the network uses a protocol (a set of rules that define what the layer will do - ie HTTP). All layers except the physical layer add to a Protocol Data Unit.

    Application Layer - first the user creates a message at the application layer using a web browser by clicking a link. It uses a HTTP packet and places the users message in this packet.

    Transport Layer - The transport layer on the internet uses a protocol called TCP (transmission control protocol). This layer is responsible for breaking large files into smaller packets and for opening a connection to the server for the transsfer of a large set of packets. It places the HTTP packet inside a TCP PDU, fills the info needed, and passes the TCP segment to the network layer.

    Network Layer - The network layer uses IP. IP selects the next stop on the message's route through the network.

    Data Link Layer - If using a LAN, the data link layer may use a protocol called Ethernet. The data link layer formats the message with start and stop markers, adds error checking. It then places the packet inside an ethernet PDU and instructs the physical hardware to transmit the ethernet frame

    Physical Layer- the physical layer in this case is a network cable connecting your computer to the rest of hte network. The computer takes the ethernet frame (complete with the IP, TCP, HTTP and message), and sends it to a seris of electrical pulses through your cable to the server. When the server gets the message, this proces sis performed in reverse.
  6. What are some future trends in networking??
    Pervasive Networking - communication networks will one day b everywhere; virtually any device will be able to communicate with any other device in the world.

    Broadband communication- refers to the new highspeed communication circuits. Broadband is a technical term that refers to a specific type of data transmission that is used by one of these circuits.

    The integration of Voice, Video and Data - often times called convergence. Previouslly, these used to be transmitted seperately. SKYPE for example uses Voice Over Internet Protocol (VOIP)

    New Information Systems - the web has challenged the nature of computing so that now, anyone with a computer can be a publisher. Many companies today are beginning to use application service providers (ASPs), rather than developing their own systems. Companies purchase the service.
  7. What four functions are done by an application program?

    Describe Application Architectures:
    The four functions are: Data storage, data access logic (processsing required to access data), application logic, presentation logic

    Host-Based -
    Where the servers perform all four functions and hte clients enable users to send/receive messages to and from the host computer.

    Client-Based - the clients and microcomputers are on a LAN, and the server is usually another microcomputer on the same network. THe application software on the client computer is responsible for the presentation of the logic, the application logic and hte data access logic; the server simply stores the data.

    Client-Server - Attempt to balance the processing between the client and the server by having both do soe of the logic. In these networks, the client is responsible for the presentation logic, whereas the server is responsible for hte data access logic and data storage. The applicaiton logi may either reside on the client, reside on hte server or be split between both.

    Peer-to-Peer -all computers act as both a client and a server. All comoputers perform all functions.
  8. Describe client-server architectures - specifically Two-Tier, Three Tier, n-Tier....
    Client-Server architectures attempt to balance the processing between the client and server by having both do the sam elogic. In these networks the client is responsible for the presentation logic, whereas the server is responsible for the data access logic and data storage. The application logic may either reside on the client, on the server or be split between both.

    • Strength: enables hardware from different vendors to be used together
    • Weaknesss: Sometimes these don't work together
    • Solution: Using a middleware, a software that sits between the application software on the client and on the server.

    • Two-Tier, Three-Tier, N-Tier...
    • In two-tier, the server is responsible for the data and hte client, the application and hte presentation. It is called two tier because it uses only two sets of computers, one set of clients and one set of servers. Three tier uses three sets of computers. In three tier, the software on the client computer is responsible for presentation logic, an application server is responsible for application logic, and a seperate database server is responsible for data access logic and storage.

    An n-tier architecture uses more than three sets of computers. Int his case, the client is responsible for presentation logic, a atabase server is responsible for the data access logic and data storage, and the aplication logic is spread accross two or more different sets of servers
  9. Describe Thin Clients VS Thick Clients
    A thin client approach places little or no application logic on the client, whereas a tick client approach places all or almost all of the application logic on the client. There is no direct relationship between thin and fat client and two -, three-, n-tier architectures.
  10. How does the Web work?
    Each client computer needs an application layer software package called a Web browser. To get to a page from the web, the user must type the Internet Uniform Resource Locator (URL) for hte page they want or click on a link. THe URL specifices the internet address of hte web server and the directory name of the specific page wanted.

    The standard protocol for this is Hypertext Transfer Protocol (HTTP). To get to a page from a web server, hte browser issues a special packet called a HTTP request that contains the URL.

    Once the server receives the request, it processes it and sends back an HTTP response which will be the requeseted page or an error message.
  11. Describe a HTTP response
    • It has three parts, the first required and the last two optional:
    • 1. The response Status - which contains the HTTP version number the sever has used, the status code (ie. 404 means not found), and a reason phrase (text description of the status code)
    • 2. The Response Header - contains a variety of optional information, such as the Web server being used (eg. Apache), the date, and hte exact URL of the page in the response
    • 3. The Response Body - The web page itself
  12. How does E-mail work? Describe Two-Tier E-mail Architecture.
    Email works similarly to how the web works, but it is a bit more complex. Simple Mail Transfer Protocol (SMTP) e-mail is usually implemented as a two-tier thick client-server application, but not always.

    Two Tier E-mail Architecture - each client computer runs an application layer software package called a mail user agent, which is usually more commonly called an e-mail client. The user creates the email message, the program (ie outlook) formats the message into a SMTP packet that includes info sucha s the sender's address and the destination address.

    The user agent then sends the SMTp to a mail server that runs a special application software package called a mail transfer agent, the email server reads the SMTP to find the destination address and hten sends the packet on its way through the network from mail server to mail server until it reaches the server specified in the destination address. The message then sits in the mailbox assigned to the user who is to recieve the message until he/she checks their inbox.
  13. How many bits are in a byte? what usualy composes one character?
    There are 8 bits in a byte. One byte usually represents a character. For example, the character A is represented by a group of 8 bits (01000001)
  14. What are the three predominant coding schemes used today?
    ASCII - there are two types; one is a seven bit code that has 128 valid character combinations, and the other is an eight bit code that has 256 combinations

    ISO 8859 - uses an eight bit code that includes ASCII codes plus non english letters.

    Unicode - many different versions. UTF-8 is an eight bit version which is similar to ASCII which uses 16 bits per character.
  15. What is a circuit?
    The word circuit has two very different meanings in networking, and sometimes it is hard to understand which meaning is intended. Sometimes the word circuit referes to teh Physical Circuit - the actual wire used to connect two devices. In other cases, we refer to a logical circuit used to connect two devices, which refers to the transmission characteristics of the connection such as when we say a company has T1 connection to the internet. This refers to how fast the data is transmitted.
  16. What are point to point and multipoint
    Point to Point - it goes from one point to another (ie one computer to another). Each computer has its own circuit running from itself to the other computers.

    Multipoint circuit - many computers are connected on the same circuit. This means that each must share the circuit with others. THe disadvantage is that only one computer can use the circuit at a time.
  17. What is multiplexing? what is Frequency division multiplexing? Time Division Multiplexing? Statistical Time Division Multiplexing? Wavelength division Multiplexing? Inverse Multiplexing?
    Multiplexing means to break one high speed physical communication circuit into several lower speed logical circuits so that many different devices can simultaneously use it but still "think" that they have their own seperate circuits.

    Multiplexing is often done in multiples of 4 (ie 8, 16).

    Frequency Division Multiplexing - dividing the circuit horizontally so that many signals can travel to a single communication circuit simultaneouslyh. The circuit is divided into a series of sperate channels, each transmitting on a different frequency, much like a series of different radio or TV stations.

    Time Division Multiplexing- shares a communication circuit among two or more terminal sby having them take turns, dividng the circuit vertically, so to speak.

    Statistial Time Division Multiplexing - allows more terminals or computers to be connected to a circuit then does the others. It provides more efficient use of the circuit and saves money.

    Wavelength Division Multiplexing - is a version of FDM used in fiber optic cables.

    Inverse Multiplexing - uses several low speed circuits to make it appear as one highspeed circuit to the user.
  18. What is medium/media? What is Guided and wireless media?
    The Medium (media if there is more than one) is the physical matter or substance that carries the voice data transmission. There are two basic types f media: guided and wireless.

    • Guided Media - there are three basic types: twisted pair cable, coaxial cable, fiber optic.
    • - Twisted pair cable is insulated pairs of wires that can be packed close together. The wires usually are twisted to minimize interference.
    • - Coaxial cable has a copper core with an outer cylindrical shell for insulation. The outer shield is the second conductor.It costs three times as much as twisted pair.
    • -Fiber Optic Cable - uses high speed streams of light pulses from lasers or LEDs that carry information inside hair thin strands of glass called optical fibers

    • Wireless Media - when the message is braodcasted though the air such as infared, microwave or satellite.
    • - Radio - connecting a laptop to a network wirelessly (IE). Each device or computer on the network has a radio receiver/transmitter that uses specific frequency range that does not interfere with commercial radio stations.
    • - Infared - uses low frequency light waves to carry the data through the air on a direct light of sight path between two points. This technology is similar to technology used in infared TV remote controls.
    • - Microwave - High frequency radio communication beam that is transmitted over a direct line of sight path between any two points. As its name implies, a microwave signal is extremely short wavelength.
    • -Satellite - is similar to microwave transmission except instead of transmission involving another nearby microwave dish antenna, it involves a satelite many miles up in space.
  19. What is digital transmission?
    Digital transmission is the transmission of binary electrical or light pulses in that it only has two possible states, a 1 or a 0. All digital transmission techniques require a set of symbols (to define how to send a 1 or 0) and the symbol rate (how many per second)

    To successfully send and receive a message, both the sender and receiver have to agree on how often the sender can transmit data - that is, the symbol rate.

    Three types: Unipolar, Bipolar, Manchester
  20. Discuss the analog transmission of digital data. What are the three important characteristics?
    Analog transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wavelike pattern much like the human voice. Modems translate the digital binary data produced by computers into the analog signals required by voice transmission circuits. One modem is used by the transmitter to produce the analog signals and a second by the receiver to translate the analog signalsback into the digital signals.

    The three important characteristics are Aplitutde (high of the wave), Frequency (number of waves per min), and Phase (the direction in which the wave begins)
  21. What is modulation? How much information is stored in a modulation scheme?
    When we transmit dat through telephone lines, we use the shape of the sound waves we transmit (amplitude, frequency, phase), to represent different data values. We do this by transmitting a simple sound wave through the circuit and then changing the shape in different ways to represent a 1 or a 0. Modulation is the technical term used ot refer to the "shape changes".

    Each modulation techniue can send more than 1 bit at a time. For example, basic AM sends 1 bit per wave by defining two different aplitudes, onf or a 1 and one for a 0. it is possible to send 2 bits on one wave symbol by defining four different amplitudes.
  22. What is the capacity of a circuit?
    The data capacity of a circuit is the fastest rate at which you can send your data over the circuit in terms of hte number of bits per second. The data rate (or bit rate) is calculated by multiplying the number of bits send on each symbol by the maximum symbol rate.

    The maximum symbol rate in any circuit depends on the bandwidth available and the signal to noise ratio. The bandwidth is the difference between the highest and hte lowest freuencies in a band or set of frequencies.

    The range of human hearing is between 20 HZ and 14,000 HZ, so the bandwidth is 13,880 HZ

    The maximum symbol rate for analog transmission is usually the same as the bandwith measured in HZ
  23. What is Media Access Control?
    Media access control refers to the need to control when computers transmit. It is important when several computers share the same communication circuit, when computers have to take turns. It is critical to ensure that no two computers attempt to transmit data at the same time - but if they do, there must be a way to recover from the problem.

    Controlled Access - Polling is the process of sending a singal to a client that gives it permission to transmit and asks it to receive

    Contention - the opposite of controlled access. Computers wait until the circuit is free and then transmits whenever they have data to send.

    Relative Performance - whatever gives the best throughput (controlled access VS contention) gives the best relative performance
  24. Discuss error control in networks
    There are human errors (typing the wrong number) and network errors (those occuring during transmission). The two categories of network errors are:

    • Corrupted data - data that has been changed
    • Lost data

    • Sources of errors include:
    • - Line Noise (distortion can cause errors)
    • -Line outages
    • -White noise
    • -Impulse Noise
    • -Cross Talk
    • -Echos
    • -Attenuation (loss of power signal)
  25. What are some ways to prevent network errors?
    Shielding (protecting wires by covering them with insulating coating)

    Moving cables ( away from sourcs of noise) - would reduce cross talk, etc

    Tune transmission equipment - this would reduce echos,white noise, jitter, harmonic distortion, etc

    Repeaters/Amplifiers - to prevent attenuation
  26. What is parity checking?
    One additional bit is added to each byte in the message. The value of this additional parity bit is based on the number of 1s in each byte transmitted. This makes the total number of 1s in the byte either an even or odd number. This is an error detection method that has a 50% success rate.
  27. Discuss error correction via retransmission
    With retransmission, a receiver that detects an error simply asks the sender to retransmit the message until it is received without error. This is often called Automatic Repeat reQuest (ARQ). There are two types:

    Stop and Wait ARQ - The sender stops and waits for a response from the receiver after each data packet. After receiving the packet, the receiver sends either an acknoweldgement or a negative acknowledgement.

    Continuous ARQ - The sender does not wait for an acknowledgement after sending a message; it immediately sends the next one. The sender then examines the stream of returning acknowledgements. If it receives a negative acknoweledgement, the sender retransmits the needed messages.
  28. What is asynchronous and synchronous transmission?
    Asynchronous Transmission - often refered to as "start and stop" transmission because the transmitting computer can transmit a character whenever it is convenient and the receiving computer will accept that character. With this type of transmission, each character is transmitted independantly of all other characters. A start bit and a stop bit are put on the front and back of each individual character.

    Synchronous Transmission -
    all the letters or data in one group of data is transmitted at one time as a block of data. This block of data is called a frame. The start and end of the frame must be marked.
  29. What is transmission efficiency and throughput?
    Transmission Efficiency - the total number of information bits divided by the total bits in transmission. ie. asynchronous transmission has only 70% efficiency because 30% is used for the transmission protocol.

    Throughput - the total number of information bits received per second, after taking into account the overhead bits and the need to retransmit frames containing errors. Generally speaking, smaller frames provide better throughput for circuits with more errors.
  30. What is ethernet?
    it is a very popular LAN protocol. It uses a contention media access protocol. The fram estarts with a 7 byte preamble which is a repeating pattern of 1s and 0s. This is followed by a start of fram edelimiter, which marks the start of the frame. The destination addres specifies the receiver, the source address specifies the sender. The length indicates the length in 8 bit bytes of the message. the VLAN tag field is an optional four byte address field used by virtual LANS.

    See page 134 for diagrams. Review.
  31. What is the general format of any packet or frame?
    • IP packet:
    • Version # (4bits)
    • Header Length (4 bits)
    • Type of Service (8 bits)
    • Total Length (16 bits)
    • Identifiers (16 bits)
    • .....
    • Source Address (32 bits)
    • Destination Address (32 bits)
    • .....
  32. What are subnet masks?
    Subnet masks tell computers what part of an IP adddress is to be used to determine whetehr a destination is on the same subnet or on a different subnet. A subnet mask is a four byte binary number that has the same format as an IP address: ie 255
  33. What are the tree types of addresses? How do they work together?
    Addresses are used on the three layers: Application, Network and Data link.

    On the application layer, the address looks like "www.google.ca"

    The network layer, the address gets translated into IP which looks like this, ""

    The data link layer, the address is translated into an ethernet address looking like this "00-0C-00-F5-03-5A"
  34. What is address resolution?

    What is Server Name resolution?

    What is Data Link Layer Address Resolution?
    When the application layer address of the destination is translated into a network layer address, and in turn translated into a data link address. It can be centralized (one computer knows all addresses) or decentralized (each computer responsible for knowing addresses).

    Server Name Resolution - the translation of application layer addresses into network layer addresses (ie from google.ca to IP). This is done using the Domain Name Service (DNS). DNS holds all IP addresses for their domains (ie hinf.uvic.ca - uvic.ca is responsible for knowing the IP for hinf.uvic.ca)

    Data Link Layer Address Resolution - Routing the message to the destination address "hopping routers". Uses Address Resolution Protocol (ARP) that says, "whosever IP is xxx please send me your datalink address"
  35. What is routing?
    The process of determining the route or path though the network that a message will travel from the sending computer to the receiving computer. Every computer that performs routing has a routing table developed by the network manager that specifies how messages will travel though the network.
  36. What are some types of routing?
    Centralized Routing- all routing decisions re made by one central computer or router. Centralized routing is commonly used in host-based networks.

    Static Routing - decentralized, which means that all computers or routhers in the network make their own routing decisions following a formal routing protocl. In MANs and WANs, the routing table for each computer is developed by its individual network manager. in LANs or backbones, the routing tables used y all computers on the network are usually developed by one individual committee

    Dynamic Routing - routing decisions are made in a decentralized manner by individual computers. This approach is used when there are multiple routes though a network and it is important to select the best route. With distance vector dynamic routing, computers or routers count the number of hops along a route. A hop is one circuit, so from one computer to another. With link state dynamic routing, computers or routers track the number of hops in the route, the speed of the circuits in each route, and how busy each route is.
  37. What are Routing Protocols?
    Routing protocols are protocols that are used to exchange information among computers to enable them to build and maintain their routing tables.

    Routing protocols used inside an autonomous system ( a network operated by one organization, such as IBM) are called interior routing protocols.

    Protocls used between autonomous systems are called exterior routing protocols.

    Boarder Gateway Protocol (BGP)- used on the internet to exchange routing information between autonomous systems - large sections of the internet

    Internet Control Message Protocol (ICMP) - the simplest interior routing protocol on the internet. It's an ERROR reporting protocl

    Routing Information Protocol (RIP) - distance vector interior protocol that is commonly used in smaller networks such as those operated by one organization.