Universidad de León

Dept IESA, José María Foces Morán, PTEU, IESA.


Arquitectura, Diseño y Gestión de Redes (ADG)

Computer Networks (CN)

Questions and Answers about Ex3 of CN2014

  1. Technical sheet of Ex3

    • Type: (Multianswer quiz, single-answer quiz, short questions, exercises)
      Quiz questions will be marked "single answer" or "multi-answer":
      In single-answer questions only one of the several options available must be ticked in order to obtain the full credit (1 point), if the answer is correct you get 1 point, otherwise you get 0 points.
      In multi-answer questions at least one of the options must be ticked in order to obtain the full grade, if you mark only the correct options you get 1 point, if you mark only some of them, you get a proportional grade; in these questions, if you mark incorrect options the grade is reduced proportionally to the number of non-to-be-marked options present in the question.
      Short questions and exercises count 1 point each.
    • Contents covered:
      - Lectures up to and including that of 24/April/2014
      - Textbook chapter sections 2.3, 2.3.1, 2.3.2;Error management: 2.4, 2.4.1, 2.4.3; Ethernet: 2.6 - 2.6.3 alongside additional slides provided on ch. 2 presentation at paloalto Pres ch2 2014 (Final version)
    • Language: English or Spanish. Those students willing to complete their exam in english must send me an e-mail to foces.informatica.unileon at gmail.com by 5/May/2014 with their request, then, in the exam I will hand them the exam in English and in Spanish.
    • Location, date and time: Classroom bulding, classroom no. 12. Thursday 8/May/2014 at 14:45. Please, be on time.
    • Duration of the examination: 35 min
    • In-class materials: Scientific calculator
      Use of laptops, tablets, mobile phones, etc. is not allowed
      You must show your National Identity Card (DNI) or University ID if required to do so
      You must write your DNI on all the exam sheets exclusively, in no case will you be allowed to write down your name or signature on them

  2. Which textbook exercises are recommended for properly preparing for this exam?

    • Recommended textbook exercises (Ch. 2): 5, 7, 8, 13, 19, 43. Obviously, it is recommended that you solve other similar exercises so that you gain more practice.
    • Use the presentations to guide your study of the textbook sections listed above (Contents section), it is essential that you study in a reflective and critical manner, always asking yourselves the why and the how.

  3. I don't understand why at the 3rd exercise of the exam of last year (Ex2), you divide the RTT by 4

    • Since the transmitter is located exactly in the middle position of the 2500m ethernet segment, the same signal bits will propagate to the left and to the right. Since the worst case RTT= 52.1µs (B is 2500m away from A), the one-way delay will be 25,6 µs in the worst case also. Now, 51.2 µs at 10Mpbs means 512 bits will fill up the RTT, consequently 256 bits will fill up the ONE-WAY delay A->B; now, locate A at exacty 1250 m from B, now the signal propagates to the left and to the right, it is exposed in the two directions and 128 bits will be enough to fill up the cable in this case. Please see the help diagram provided in the next section:
    • Help sketch for CN Ex2 exercise no. 3 (2013)

  4. Do we have to prepare for ethernet exercises that entail probability knowledge for this exam?

    • Yes, you have to recall the basic definition of a-priori probability and apply it as in the exercise mentioned in the following question

  5. Is exercise no. 1 of CN Ex2 (2013) correctly resolved?

    • The document version published before yesterday night was wrong (It incorrectly counted a number of favorable cases of 15 whereas it should have counted 16). The error had been corrected but I did not publish at the time of correction. I apologize for that.
    • The updated document containing the CN exams of 2013 can now be downloaded from its original URL:
      CN exam solutions (2013)

  6. Me gustaría saber si la comprobación de redundancia cíclica basta con que sepamos hacerla por el método que nos explicaste tu en la clase de práctica o también es necesario que sepamos hacerlo por divisiones como explica el libro.

    • Basta con que sepáis el método que explicamos en el laboratorio, el cual, está basado en un registro de desplazamiento con entradas condicionadas con puertas XOR. Este método está explicado detalladamente en la presentación ppt usada en clase.

  7. When there is a collision, all the nodes involved in it must send the 32-bits jamm. seq. and its purpose is to exaggerate the collision, is that right?

    • yes, you are right, nevertheless, please take a look at the included text from O'Reilly's "Ethernet: The Definitive Guide, 1st. ed by Charles Spurgeon". Jam Sequence

  8. I'd like to ask you about frame format, concrete about the Preamble.
    Isn't it considered part of the frame format? I'm asking you that because the min. data size of the body of the frame is 46 bytes longs so, adding all the bits in order to see the frames length we have: 48(addr) + 48(addr) + 16(type) +(46*8)(data) + 32(CRC) = 512 bits that is supposed to be the minimum Ethernets frame size. So what about the Preamble

    • Ethernet's Minimum Frame Size (512 bits) does not include the 64-bit preamble


  9. En el ejercicio nº 6 del examen nº 2 del año pasado, y según mi comprensión a la explicación de los ejercicios de "Bit Stuffing", ¿no habría 1 error en la trama del enunciado (1101011111010111110010111110110)?

    • A mi juicio, no hay ningún error: cada bloque de 5 1's va seguido de un 0 que fue incluido por el transmisor, el receptor simplemente lo suprime.


  10. ...duda en el ejercicio 5 del capitulo 2 del libro de texto. Dice que se recibe el siguiente bloque de datos: 110101111101011111101011111110 El bloque contiene un error, pero, el final de esa trama que es 011111110 se supone que es la secuencia de final del frame. Y aquí me surge la duda. ¿No se supone que la longitud de final de la trama es 8? Porque según lo visto, al recibir cinco 1s consecutivos, si el siguiente bit es un 1, puede distinguir si se trata de un error o es el final de la trama mirando otra vez el siguiente bit, si este es un 0, marca el final de la trama, si marca un 1, seria un error. En este caso es otro 1. Por lo que sería un error, pero según la solución del el libro de texto marca el final de le trama, y ese 1 sobrante ha desaparecido, y no entiendo el porqué. Espero que se entienda.

    • El enunciado del ejercicio pregunta por la secuencia de bits transmitida (transmitted), NO RECIBIDA, es decir, tienes que ponerte en el lugar del transmisor HDLC, por tanto, cuando detectes 11111 tienes que añadir (stuff) un bit 0, es decir: 111110