From: Ron Royston (ccie6824@xxxxxxxxxxx)
Date: Thu Oct 18 2001 - 13:50:12 GMT-3
From: http://www.cisco.com/warp/customer/788/voip/fr_traffic.html
The smallest allowed Tc = 10ms, the largest = 125ms
Tc=Bc/CIR
*If Tc = 1 sec (It can't), then Bc = CIR
When you configure only CIR, it Cisco sets Bc equal to CIR, yielding Tc of 1
sec. The cisco box then rounds the Tc as close to that value as possible,
125ms. This tells the router to shape traffic by allowing only 64000 bits,
if CIR = 64000, to be transmitted every Tc interval. So if the port speed
is T1, 1.544 Mbps, Tc is 125ms, and CIR is 64Kbps, (my degree is in
Philosophy, don't laugh), then:
Port speed can transmit (1,544,000*.125) 193,000 bits per Tc, and traffic
shaped rate allowed is (64,000*.125) 8000 bits per Tc. Data will be
transmitted at port speed every Tc, then held until the next Tc interval,
then sent, and so on.
Another example:
If you set CIR = 64000 and Bc to 1000, you'll get:
Tc = Bc/CIR or .0156 = 1000/64000
So, Tc rounded is 15ms. These numbers are shown in the example on the link
above (if you have a CCO logon). In this example, if port speed is T1, then
port speed can transmit 23,160 bits per Tc, and traffic shaped rate
transmits at 960 bits per Tc. The same send/hold pattern occurs here, but
in smaller time intervals, every 15ms.
Remember though, we are talking about events on a micro scale. A layer 2
frame may be 1500 bytes (12000 bits). As far as the large gap between data
transmission when the Tc is too high and the port speed high/CIR low, it may
not be gaps between frame or packets, but gaps within frames or packets.
Hope that helps.
-Ron
>From: "Brian Lodwick" <xpranax@hotmail.com>
>Reply-To: "Brian Lodwick" <xpranax@hotmail.com>
>To: ccie6824@hotmail.com
>CC: ccielab@groupstudy.com
>Subject: Re: QOS configs
>Date: Thu, 18 Oct 2001 12:59:08 +0000
>
>Ron,
> It sounds like you have quite a grasp on this information. I'd like to
>run
>something by you and see if you can help clear something up I've been
>having
>difficulties with?
> In the best documentation I have found on VoIP over Frame-Relay QOS it
>notes the ideal Tc value is 10ms. It also says that 10ms is the lowest
>configurable value. I understand this is optimum because it will provide
>for
>very small time gaps between packets. The problem I am encountering is how
>to mathmatically calculate this formula.
>I found something saying something saying it was Tc = Bc/CIR
>But then it goes on to say In this example we have a CIR to 64000, and have
>set the Bc to 600 (forcing the Tc to it's minumum configurable value)
>Wouldn't that give you a Tc of 106.66....?
>
>>>>Brian
>
>>From: "Ron Royston" <ccie6824@hotmail.com>
>>Reply-To: "Ron Royston" <ccie6824@hotmail.com>
>>To: John.K.Feuerherd@WellsFargo.COM, ccielab@groupstudy.com
>>Subject: Re: QOS configs
>>Date: Wed, 17 Oct 2001 17:20:20 -0500
>>
>>No. I believe they are different. Scott Keagy talks about this in
>>"Integrating Voice and Data Networks". CIR is typically expressed in Bps
>>and is the AVERAGE rate of traffic guaranteed by the carrier; Bc and Be
>>are
>>expressed per unit of time, Tc, and are the ACTUAL number of bits that the
>>carrier commits to transport. Cisco routers take CIR as Bps, and Bc as
>>Bits.
>>
>>The only practical reason that you should use the latter of your examples
>>is
>>to malipulate the value of Tc, which plays an important role in
>>controlling
>>pausing time (which can manifest itself as jitter and delay in real-time
>>apps.) You don't explicitly configure Tc, you configure Bc and CIR and
>>IOS
>>implies the Tc from there.
>>
>>I believe that these are the same (note the Be, not Bc):
>>
>>map-class frame-relay sample1
>> frame-relay traffic-rate 9600 19200
>> no frame-relay adaptive-shaping
>>-and-
>>map-class frame-relay sample2
>> frame-relay cir 9600
>> frame-relay be 19200
>>
>>Having the bc set to 19200, as in your example, would double the Tc to 2
>>seconds, telling the router to push out 2xCIR bits every 2 seconds at the
>>port speed. If the port speed is high, and the CIR low, your data gets
>>sent
>>in short bursts followed by a long pauses and will likely exceed your CIR
>>from the carriers vantage point. The carrier's Tc is less than 2 seconds,
>>so they'll be getting 2 seconds worth of bits in less than 1 second if
>>your
>>port speed is high enough, and they'll De, discard eligible, that data.
>>Right?
>>
>>-Ron
>>
>>PS IMHO, CCO needs more understandable info on this subject, but you can
>>check out, http://www.cisco.com/warp/customer/125/21.shtml. That's all I
>>can find.
>>
>>
>>
>>
>>
>>>From: John.K.Feuerherd@WellsFargo.COM
>>>Reply-To: John.K.Feuerherd@WellsFargo.COM
>>>To: ccielab@groupstudy.com
>>>Subject: QOS configs
>>>Date: Tue, 16 Oct 2001 19:56:49 -0700
>>>
>>>Hello all,
>>> I have a quick question about traffic shaping. I think I understand
>>>this, but I just want to make sure:
>>>
>>>This two configs are basically the same right.....
>>>
>>>map-class frame-relay sample1
>>> frame-relay traffic-rate 9600 19200
>>> no frame-relay adaptive-shaping
>>>
>>>
>>>map-class frame-relay sample2
>>> frame-relay cir 9600
>>> frame-relay bc 19200
>>>
>>>Thanks in advance!
>>>
>>>JF
>>>John Feuerherd
>>>Wells Fargo Bank
>>>CCNP CCDP
>>>1-505-766-6118 <- Office
>>>1-505-301-1966 <- Cell
>>>John.K.Feuerherd@Wellsfargo.com
>>>MAC: Q2129-074
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