> 1. Give a possible explanation for transmission distortion that is based on > Fourier analysis. This one is pretty vague. The idea is that: * every signal can be represented as a sum of sine waves * distortion for sine waves is usually easier to analyze For example, in many cases sine waves aren't "distorted" at all, but different frequencies are attenuated by different amounts. The net effect for non-sine waves may indeed be distortion, as the overall shape is changed as the differently attenuated sine waves are reassembled. ======== > 2. Why is digital data often encoded via an analog signal? Having an analog "carrier" often improves reliability over long distances. ======== > 3. Give some advantages of microwaves versus land lines, for long-distance > transmission. * Microwaves may be cheaper. The cost of stringing wire is significant. * Any kind of cable is subject to exponential attenuation: if the signal is reduced by 1/2 in one distance unit, then it is reduced by 1/2^n in n distance units. Attenuation of wireless transmission is, in the long run, subject to much less attenuation due to the inverse-square law: if the signal is reduced by 1/2 in one distance unit then it is reduced by 1/2n^2 in n units, and as n gets large this means *much* less strength reduction. ======== > 4. List some disadvantages with NRZ encoding for long lines (>= 10 km). > Which, if any, are addressed by NRZI? > Which, if any, are not an issue for shorter lines? The problems with NRZ encoding are: * long runs of 1's or 0's are hard to keep count of * There is a significant DC component, and if that gets lost the signal is lost * the receiver must receive and recognize a specific voltage. NRZI can reliably send long strings of 0's (assuming 0 = transition), but still loses count sending 1's. By switching to transitions rather than absolute voltage levels, NRZI also fixes the third problem. For shorter lines, the last two issues are not significant; they become significant, however, for long lines. ======== > 5. Suppose we take a 4-kHz voice signal and digitize it to a 56kbps signal. > (a). What frequency bandwidth is needed to transmit this using binary signaling, > so the signaling rate is also 56kbps? > (b). What bandwidth is needed if we use 16-level signaling? How many bits can > be sent in a single signal element? 56kbps = 2B logM. For (a), logM = 1, and so B = 28 kHz. For (b), M=16 and logM = 4; each signal element encodes 4 bits. We have 28 kHz = B * 4, or B = 7 kHz. ======== > 6. Consider the following sequence of virtual-circuit routers, and tables. > > D > | > link3 > | > [1] > A------link1-------[0]-B-[1]------link2---------[0]-C-[2]-----link4------E > vc vc port vc vc port > in out out in out out > 3 5 1 5 4 2 > 4 2 1 2 3 1 > List the VC used on each link as A sends to D 4 2 3 A-------B-------C-------D > List the VC used on each link as A sends to E 3 5 4 A-------B-------C-------E ======== > 7. Why is TCP vulnerable to "spoofing" (forged connections appearing to come > from another source) but X.25 is not? TCP relies on the IP layer for packet exchange. This layer provides little authentication; it is subject to distributed rather than centralized control. In X.25, by comparison, packet delivery and authentication *is* under control of the core network, not subject to user interference. ======== > 8. Why are virtual circuits called that? That is, why are they like circuits, > and why is the adjective "virtual" there? Circuits can be defined as ways to send a stream of bits at a specified bandwidth. Virtual circuits provide this capability. However, they do entail some delay due to the need to wait for a packet to fill with data before it can be sent; while this delay is often small if the packet size is small, it is usually much larger than the corresponding circuit delay.