ch 15.txt

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  1. What are the two basic configurations of data communication networks?
    • 1) mesh configuration - direct, point-to-point. There is a direct connection between every pair of devices on the network.
    • 2) broadcast network - network lines are shared. There are various topologies for this type - ring and bus are two examples.
  2. What does LAN stand for?
    Local Area Network
  3. What is ethernet?
    A wired LAN technology, ethernet uses a bus configuration and a protocol for accessing the bus called CSMA/CD.
  4. What does CSMA/CD stand for?
    • CS - carrier sense - each machine constantly listens to traffic passing by on the bus
    • MA - multiple access - every machine has equal acces to the communication medium (i.e., bus)
    • CD - collision detection - each machine can determine whether its transmitted message collided with another message, corrupting it and necessitating a retransmission.
  5. What is a protocol (in the sense of an ethernet LAN)?
    A protocol is a set of rules that establish how communication will take place.
  6. How does a machine on an ethernet LAN sense when there is incoming traffic?
    It monitors the bus. As messages pass by, it examines the first few bits of the message (the address) and determines if it is the intended recipient.
  7. How does a machine on an ethernet LAN use its Carrier Sense when it wants to transmit a message?
    It monitors the bus. It waits a system-dependent amount of time, using its Carrier Sense to see if any traffic appears on the bus. If not, it transmits its message and 'hopes' no collision occurs.
  8. How does a machine on an ethernet LAN use its Carrier Sense when it has just transmitted a message?
    It monitors the bus. If there is a difference between the message it transmitted and the message on the bus, a collision is assumed to have occurred.
  9. Ethernet LANs work for small networks, but if your network is larger you may wish to employ a _______________.
    token-ring network topology
  10. How does a token-ring network topology work?
    A 'token' message perpetually traverses the ring. When a machine wishes to transmit, it waits for the token to arrive, removes it and transmits its message. When its message returns to the tramsmitting machine, it is removed and the token is placed back on the ring.
  11. What is a WAN?
    A WAN is a wide area network. They are large collections of smaller networks that use interconnection devices called routers to connect adjacent sub-networks.
  12. Name and describe two switching technologies.
    • 1) Circuit switching - a path from start to end is defined based on the current conditions of the network at the initial transmission time.
    • 2) Packet switching - each packet in the transmission is transmitted individually, not necessarily along the same path, and not necessarily arriving in order.
  13. What are some pros and cons of circuit switching?
    • Pros: fast transfers
    • Cons: reserving a path exclusively can be a huge waste of resources,
    • path may become less desireable than an alternate path during the transmission
    • because the entire path must be reserved before any communication is allowed, not great for many modern applications (web surfing, videoconferencing)
    • transmission errors propagated throughout the entire path
  14. What are some pros and cons of packet switching?
    • Pros: lines aren't dedicated so utilization is higher
    • messages are packetized so line sharing is reasonably fair
    • routing is dynamic - new path for each packet if need be
    • errors aren't propagated end-to-end
    • Cons: switches must be programmed to make sophisticated routing decisions
    • switches must manage memory for queued packets
    • packets must include headers, increasing overhead
    • endstations must deal with missing and out-of-order packets
    • since circuit not dedicated, transmission times unpredictable
  15. What is multiplexing?
    • It is the sharing of a transmission medium. Three types:
    • Frequency-Division - The spectrum of frequencies transmittable via the physical medium is divided into several channels (i.e., TV)
    • Time-Division - Each transmitter is allocated a periodic time interval in which to transmit.
    • Code-Division - Each transmitter is allowed to transmit all the time. All transmissions are coded uniquely (i.e., wireless communications)
  16. What do modems do?
    Modems modulate digital data into analog signals to be transmitted along an analog medium. After transmission, another modem demodulates the analog data to digital data.
  17. Networks are often organized into layered __________________ of network protocols. Hardware intensive protocols are on the ___________, while user applications are at the ______.
    hierarchies, bottom, top
  18. Consecutive network nodes only communicate directly at the _____________, or ____________ layer.
    lowest, hardware
  19. For communication to happen at higher layers, _______________________ inserts ___________________________.
    networking software, relevant data (like headers and footers)
  20. Name two different layered protocol models.
    • OSI - Open Systems Interconnection
    • TCP/IP - Transmission Control Protocol/ Internet Protocol
  21. Name and describe the top layer in the OSI protocol.
    Application layer - End user protocols - e.g., email, file transfer, Web browsing, network management, videoconferencing
  22. Name and describe the top layer in the TCP/IP protocol.
    Application layer - analagous to OSI's Application Layer
  23. The IP header on a packet of information contains many pieces of information (like source and destination IP address, identification, protocol, length of the packet in bytes, etc.) Name two other pieces of information contained in the IP header.
    • version - the version of IP used to create the packet, used by the nodes to process it correctly.
    • Time to live - length of time (in seconds) the packet can remain on the internet
  24. Describe how File Transfer Protocol works (in terms of connections).
    Before and after TCP connection there is one active TCP connection, just for control. During active FTP connections, there are two connections; one for control and one for data transfer.
  25. What is a firewall?
    A firewall is a program that helps to ensure the security of a private network. It filters incoming and outgoing packets based upon header information, but can also use an application gateway to inhibit application-specific traffic. (To attempt to send a bad packet it is to attempt to "finger" it, to attempt to send a bad packet out is to attempt to "http" it.
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ch 15.txt
diane cs111 ch 15
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