Posts Tagged ‘bandwidth’



This publication discusses the spectrum of problems associated with transporting Constant Bit Rate (CBR) circuits over packet networks, specifically focusing VoIP services. It provides guidance on practical calculation for voice bandwidth allocation in IP networks, including the maximum bandwidth proportion allocation and LLQ queue settings. Lastly, the publication discusses the benefits and drawbacks of transporting CBR flows over packet switched networks and demonstrates some effectiveness criteria.


Historically, the main design goal of Packet Switched Networks (PSNs) was optimum bandwidth utilization for low-speed links. Compared to their counterpart, circuit-switched networks (CSNs such as SONET/SDH networks), PSNs use statistical as opposed to deterministic (synchronous) multiplexing. This feature allows PSNs to be very effective for bursty traffic sources, i.e. those that send traffic sporadically. Indeed, with many sources this allows the transmission channel to be optimally utilized by sending traffic only when necessary. Statistical multiplexing is only possible if every node in the network implements packet queueing, because PSNs introduce link contention. One good historical example is ARPANET: the network theoretical foundation has been developed in Kleinrock’s work on distributed queueing systems (see [1]).
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Computing voice bandwidth is usually required for scenarios where you provision LLQ queue based on the number of calls and VoIP codec used. You need to account for codec rate, Layer 3 overhead (IP, RTP and UDP headers) and Layer 2 overhead (Frame-Relay, Ethernet, HDLC etc. headers). Accounting for Layer 2 overhead is important, since the LLQ policer takes this overhead in account when enforcing maximum rate.

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Try assessing your understanding of Cisco’s CBWFQ by looking at the following example:

class-map match-all HTTP_R6
 match access-group name HTTP_R6
policy-map CBWFQ
 class HTTP_R6
  bandwidth remaining percent 5
interface Serial 0/1
  bandwidth 128
  clock rate 128000
  service-policy output CBWFQ

and answering a question on the imaginable scenario: Two TCP flows (think of them as HTTP file transfers) are going across Serial 0/1 interface. One of the flows matches the class HTTP_R6, and another flow, marked with IP Precedence of 7 (pretty high), does not match any class. The traffic flow overwhelms the interface, so the system engages CBWFQ. Now the question is: how CBWFQ will share the interface bandwidth among the flows.

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