Questions for Chapter 6,9

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Questions for Chapter 6,9

• Ying Zhang

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Question 1

• Consider the two ways in which communication
  occurs between a managing entity and a managed
  device request-response mode and trapping. What
  are the pros and cons of these two approaches, in
  terms of (1) overhead, (2) notification time when
  exceptional events occur, and (3) robustness with
  respect to lost messages between the managing
  entity and the device?

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• request-response

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• Request response mode will generally have more
  overhead (measured in terms of the number of
  messages exchanged) for several reasons.
• First, each piece of information received by the
  manager requires two messages the poll and the
  response.
• Trapping generates only a single message to the
  sender.
• If the manager really only wants to be notified
  when a condition occurs,
• polling has more overhead, since many of the
  polling messages may indicate that the waited-for
  condition has not yet occurred.
• Trapping generates a message only when the
  condition occurs.

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• Trapping will also immediately notify the manager
  when an event occurs.
• With polling, the manager needs will need to wait
  for half a polling cycle (on average) between
  when the event occurs and the manager discovers
  (via its poll message) that the event has
  occurred.

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• If a trap message is lost, the managed device
  will not send another copy.
• If a poll message, or its response, is lost the
  manager would know there has been a lost message
  (since the reply never arrives). Hence the
  manager could repoll, if needed.

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Question 2

• In the book, we saw that it was preferable to
  transport SNMP messages in unreliable UDP
  datagrams. Why do you think the designers of SNMP
  choose UDP rather than TCP as the transport
  protocol of choice for SNMP?

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• Often, the time when network management is most
  needed is in times of stress, when the network
  may be severely congested and packets are being
  lost. With SNMP running over TCP, TCP's
  congestion control would cause SNMP to back-off
  and stop sending messages at precisely the time
  when the network manager needs to send SNMP
  messages.

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Question 3

• Consider Figure 9.9,in the text book, what would
  be the BER encoding of weight,271 lastname,
  Jackson

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• BER Basic Encoding Rules
• specify how ASN.1-defined data objects to be
  transmitted
• each transmitted object has Type, Length, Value
  (TLV) encoding

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TLV Encoding

• Idea transmitted data is self-identifying
• T data type, one of ASN.1-defined types
• L length of data in bytes
• V value of data, encoded according to ASN.1
  standard

Tag Value Type
Boolean Integer Bitstring Octet
string Null Object Identifier Real
1 2 3 4 5 6 9
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TLV encoding example
Value, 259 Length, 2 bytes Type2, integer
Value, 5 octets (chars) Length, 5 bytes Type4,
octet string
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• 4 7 'J' 'a' 'c' 'k' 's' 'o' 'n' 2 2 1 15

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Question 4

• Consider the single-sender CDMA example in Figure
  6.4 in the book. What could be the senders
  output ( for the 2 data bits shown) if the
  senders CDMA code were (1,-1,1,-1,1,-1,1,-1)?

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CDMA Encode/Decode
channel output Zi,m
Zi,m di.cm
data bits
sender
slot 0 channel output
slot 1 channel output
code
slot 1
slot 0
received input
slot 0 channel output
slot 1 channel output
code
receiver
slot 1
slot 0
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• Output corresponding to bit
• d1 -1,1,-1,1,-1,1,-1,1
• Output corresponding to bit
• d0 1,-1,1,-1,1,-1,1,-1

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• Consider sender 2 in Figure 6.5, what is the
  senders output to the channel ( before it is
  added to the signal from sender 1), Z2_ I,m

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CDMA two-sender interference
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• Sender 2 output 1,-1,1,1,1,-1,1,1
  1,-1,1,1,1,-1,1,1

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• Suppose that the receiver in Figure 6.5 wanted to
  receive the data being sent by sender 2. Show
  that the receiver is indeed able to recover
  sender 2s data from the aggregate channel signal
  by using sender 2s code

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CDMA two-sender interference