Hints for Exercises: Chapter 2

  1. The link layer in Ethernet adds 18 bytes of header and trailer. What does this mean for the maximum possible throughput of data for a 10Mb Ethernet? A 100Mb Ethernet? Does this align with real life throughput? Explain.

    Suppose, for the purposes of this exercise, that there are no other overheads (this is false). On top of a 1500 byte Ethernet frame 18 bytes are used in the encapsulation. This is 1.2%, so a 10Mb Ethernet has just 9.88Mb of available bandwidth, and a 100Mb Ethernet has just 98.8Mb available.

    In real life we never get those rates. There are many reasons:

  2. The network layer in TCP/IP adds a 20 byte header. What does this mean for the maximum possible throughput of TCP data for a 10Mb Ethernet? A 100Mb Ethernet? From your understanding of encapsulation, could this layer dispense with this header? Explain.

    This will reduce maximum throughput by another 1.3%, giving 9.86Mb (98.6Mb). But again, other factors are more important: we will see later that TCP has an elaborate technique for maintaining reliability: this costs in bandwidth.

    This header cannot be dispensed with, it is the basis of the TCP protocol's reliability. Besides trimming a few header bytes will not make a bit difference in available bandwidth: the other factors are much more important.

  3. A wireless network is described as being 11Mb, but when used can never seem to get more than half that. Explain why as (a) a network support officer, (b) a marketing officer.

    (a) This is all about overheads of various types, from encapsulation to protocol overheads. Wireless networks have to be particularly careful about the correct transmission of bits in the electrically noisy world we live in and this translates to bandwidth overhead.

    (b) I'd like to see any coherent answer to this!

  4. Find other examples of encapsulation in life.

    This is only limited by the imagination

  5. Compare and contrast the OSI model against the Internet model.

    We can match the various layers against each other, for example Presentation in ISO is subsumed by Application in the Internet model, but more interesting is to see how real-life networks (in particular the Internet, mobile phone, fixed line phone and others) align against the two models (or not, as the case may be).

  6. In real implementations of the Internet model (and others) the layers are sometimes blurred to aid efficiency. Discuss the pros and cons of doing this.

    Pros include

    Cons include

    And there are many more. Real life implementations tend to blur the layers when the cost in speed is too great to ignore, but otherwise we should keep the layers separate as much as possible to aid clear coding.

  7. Read about the ISO implementation of the seven layer model (ISODE, actually just layers 4 to 7) and make notes on its main features.

    The standards are available. Also see RFC2126

  8. Consider broadcast TV. Classify its parts according to (a) the OSI and (b) the Internet models. Which is a better match?


    1. Physical. Radio waves. Frequencies and powers.
    2. Data Link. Encoding of analogue or digital signal on the waves. Neither have flow control, only digital worries about error correction.
    3. Network. No routing needed: just blast out the signal. If you take the wider view of TV transmission you might want to think of how the signal is sent to the various transmitters dotted about the countryside, but again routing is all statically decided by the engineers designing the system.
    4. Transport. Analogue does nothing here, while you might regard the multiplexing of several digital channels on to one TV channel as part of the Transport. Whether it is connection oriented or connectionless is an interesting debate.
    5. Session. Nothing.
    6. Presentation. All decided and fixed at design time. In particular there is no variation allowed in digital TV transmission.
    7. Application. The TV programmes!


    1. Link. Radio waves. Frequencies and the encoding on those waves.
    2. Network. No routing.
    3. Transport. Digital multiplexing.
    4. Application. The TV programmes!

    Perhaps the Internet layering is slightly better as there are no real session or presentation issues, but it would be useful to have the physical and link layers separate to allow us to describe the same transmission system on different frequencies. This is just Tanenbaum's 5 layer classification again.

  9. The IEEE split the OSI Data Link layer into a logical link control (LLC) sublayer and a media access control (MAC) sublayer. Read up on this and discuss how it fits in with the OSI and Internet models.

    IEEE comparison

    The IEEE argue that a standard (such as 802.3 for Ethernet or 802.11 for Wireless) more naturally comprises the PHY (physical) part plus MAC (how to use the physical part) rather than the physical part then access plus other link layer stuff.

    In brief: OSI say PHY + (LLC + MAC); while IEEE say (PHY + MAC) + LLC, and Internet says (PHY + MAC + LLC).

  10. When layering goes wrong: find examples (e.g., on the Web) where insufficient attention has been paid to the presentation layer. Why do you think that people neglect the presentation layer?

    Examples abound, particularly if you use non-Microsoft products to view Microsoft-tool generated information. Also when looking at non-English Web pages.

    Presentation was another one of those things that were ignored in the fledgling Internet: as all the users spoke and wrote English (or variants, such as American) it never came into consideration that different people might want different things. A similar reason is why American websites are often the international .com rather than the more proper .us: when names were being devised, no thought was given to people outside of the USA.

Previous Index Next

Creative Commons License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.