Homework 1 Answers

Jeffrey Absher

xxx-xx-xxxx

 

1.       (5 pts) Ex. 16 in Chapter 2.

How are OSI and ISO related to each other?

      ISO is the International Standards organization. They seek to build models for international compatability and cooperation. ANSI is the ISO-aligned standards organization in the US. OSI (Open Systems Interconnection) is a model put forth by ISO for network communications. Though a valid and usable model, due to evolution, TCP/IP has outpaced the ISO model and is the de facto standard for network communications; ISO’s OSI is relegated to a few systems and its main use is as a teaching model.

 

2.       (20 pts) Ex. 18 to 20 in Chapter 2.

18) Match the following to one of the seven OSI layers:

    1. reliable end-to-end data transportation – Transport (4)
    2. network selection – Network (3)
    3. defines frames – Data Link (2)
    4. provides user services such as e-mail and file transfer – Application (7)
    5. transmission of bit stream across physical medium – Physical (1)

19) Match the following to one of the seven OSI layers:

a.      communicates directly with user’s application program – Presentation (6)

b.      error correction and retransmission – Transport (4)

c.      mechanical, electrical and functional interface – Physical (1)

d.      responsibility for information between adjacent nodes – DataLink (2)

e.      reassembly of data packets – Transport (4)

20) Match the following to one of the seven OSI layers:

a.      format and code conversion services.- Presentation (6)

b.      establishes, manages, and terminates sessions.- Session (5)

c.      ensures reliable transmission of data – Transport (4)

d.      log-in and log-out procedures – Application (7)

e.      provides independence from differences in data representation – Presentation (6)

f.       synchronization of users – Application (7)

 

 

3.       (10 pts) What is the principal difference between connectionless communication and connection-oriented communication?
Connection-oriented sets up a session between 2 endpoints much like a telephone call where both are communicating with each other and sending acknowledgements for receipt of data (and maybe requests to send), When the communication is complete, the connection is terminated. Usually packets arrive in order in a connection, and it may be possible to use abbreviations for addresses after the connection is setup. In connectionless communication, each messages is treated as an independent message and sent off similar to an envelope in the mail. Packets may arrive out of order, and each packet must contain the full address of the destination endpoint.

What is the difference between a confirmed service and an unconfirmed service?
In a confirmed service, the sending endpoint receives acknowledgements for data that is sent.
For each of the following, tell whether it might be a confirmed service, and unconfirmed service, both or neither:

a.      Connection establishment - Confirmed

b.      Data transmission - Both

c.      Connection release.- Both

 

4.        (10 pts) Refer to the figure on p.24 of Unit 1. Station M sends an IP packet to station E. Depict the frames (like those in the diagram) transmitted on each link. See attached sheet

 

5.       (10 pts) Ex. 55 and 57 in Chapter 3.

55/57) Explain CSMA/CD and its use. What part of the 802 project uses CSMA/CD

      CSMA/CD is used in the 802.2 and 802.3 sections of 802.CSMA/CD stands for Carrier Sense Multiple Access Collision Detection. What that means is that all devices are connected to a shared medium such as a coax cable (this is the “multiple access”). Prior to sending each device “listens” to see if the medium is free,(this is the “carrier sense”). If the medium is free, the device may commence its transmission. Due to temporal and distance issues it is still possible for 2 devices to transmit simultaneously; when 2 devices transmit simultaneously a collision occurs and data from both devices is corrupt. While sending, a device also must listen to the transmission medium, if it hears something other than what it is sending, a collision has occurred and that device must release the transmission medium for a random period of time, (this is the “collision detection”).

 

6.       (5 pts) Ex. 62 in Chapter 3

62) Suppose there is heavy traffic on both a CSMA/CD LAN and a TokenRing LAN. A station on which system is more likely to wait longer to send a frame and why?

      Under heavy traffic a station on the Ethernet-style LAN will most likely wait longer because CSMA/CD is non-deterministic. A single device may use the shared medium without relinquishing it in CSMA/CD. Also the “backoff” in CSMA/CD is exponential; in certain cases if the attempt to send fails, the wait time before retrying increases exponentially. Theoretically, a device could wait an infinite amount of time to send. In contrast, Token Ring is a deterministic system. There is a finite amount of time (increasing multiplicatively with the number of devices attached) that any station must wait to send data, as all stations have a maximum amount of time that they may possess the shared medium for transmission before they must relingquish it to the next station.