Describe DiffServ QoS implementations

Questions derived from the 642-845 – Optimizing Converged Cisco Networks Cisco Self-Test Software Practice Test.


Objective: Describe DiffServ QoS implementations
SubObjective: Describe WAN link efficiency mechanisms (e.g., Payload/Header Compression, MLP with interleaving, etc.)


Item Number: 642-845.3.6.3
Single Answer, Multiple Choice


What is the typical IP/UDP/RTP header size of voice-over Internet protocol (VoIP) packets without implementation of real-time protocol (RTP) header compression?



  1. 2 bytes
  2. 4 bytes
  3. 20 bytes
  4. 40 bytes

Answer:
D. 40 bytes


Tutorial:
Typically, VoIP packets are encapsulated into a 40 byte IP/UDP/RTP header. This header size is very large in comparison to 20 byte long payload. Therefore, RTP header compression is used to reduce the size of header to 2-4 bytes.


2 bytes is the VoIP packet header size after RTP compression and without UDP checksums. Therefore, this is an incorrect option.


4 bytes is the VoIP packet header size after RTP compression and with UDP checksums. Therefore, this is an incorrect option.


20 bytes is the typical payload size of voice packets. Therefore, this is also incorrect.


Reference:
http://www.cisco.com/en/US/tech/tk543/tk762/technologies_tech_note09186a0080108e2c.shtml#head

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