Frame Relay Voice-Adaptive Traffic Shaping(FR-VATS) (Bc=minCIR/100)

在QoS 2.2 Vol.2 P.7-72的slide中出現了一段"Bc to minCIR / 100"的內容,我翻遍了整本教材也完全沒有提到這個公式的由來,所以我特地去翻了一下Cisco官網的內容,後來在以下網址中找到一段相關的敘述,原來是Cisco IOS在PVC上為了因應Voice的特性因此選定Tc = 10ms,避免過大的延遲。

Configuring Frame Relay Traffic Shaping on 7200 Routers and Lower Platforms

Configurable Parameters

frame relay cir

The average rate you want to send traffic out on a given PVC in bps. This is generally higher than the guaranteed rate but less than the access rate (AR). It equals the guaranteed rate only if:

  1. The service provider does not allow you to send above guaranteed rate.

  2. The physical line rate on the interface is same as the guaranteed rate.

  3. There are Voice (voice over IP [VOIP] or voice over Frame Relay [VOFR]) packets on this PVC, therefore you cannot afford dropped packets for quality or service.

The value of the CIR is 56000 bps is by default.

frame relay mincir

The actual guaranteed rate obtained from service provider in bps. This value should be the minimum rate you should drop to in the event of congestion (dropping below this rate implies you are not getting the bandwidth you are paying for). In certain cases (listed above) the mincir and cir values must be the same. The value of mincir is half of the CIR value in bps by default.

frame relay bc

The amount of data to send per each Tc interval in bits. Ideally for data PVCs Bc = CIR/8 so that Tc = 125msec. Cisco IOS recalculates the FRTS parameter when Bc is greater than 10,000 bytes. If we are doing voice on the PVC, then Bc = CIR/100 is preferable, so that the interval Tc = 10msec (as voice packets cannot tolerate a longer delay). The value of Bc by default is shown as the CIR in bits in the output of the show traffic-shape command. However, internally, a different value is assigned to ensure optimal performance. This value is shown in the "Increment Bytes" column in the show traffic-shape output. A value of bc=CIR equates to a Tc of 1 second. Depending on how traffic arrives at the shaper, the router would have to stop transmission for close to 1 second if the burst was exhausted immediately at the start of the interval. Thus, the shaper assigns a different internal value which still allows for the configured Bc over the original Tc, only we will do it in a number of small bursts instead of one large burst.

frame relay be

The amount of excess data allowed to be sent during first Tc interval in bits once credit is built up. Configure Be only if the Frame Relay CIR value is less than the AR. For PVCs carrying voice packets, the Be must be set to zero to ensure best possible voice quality. The router only bursts (Be) when there are tokens in the token bucket. The token bucket does not accrue tokens unless the amount of traffic being sent out is less than the CIR. The router can only burst for the first Tc, after which the token bucket is empty. The value of Be by default is zero bits.

frame relay adaptive-shaping becn

Implies that the PVC adapts the rate of transmit in response to the BECNs received. The behaviour is as below:

  • If the PVC receives any BECNs during the current time interval (it doesn't matter if this is one or 1000) the transmit rate is decreased by 25 percent or to mincir and stops if mincir configured value is more than 75% of the cir value.

  • It continues to drop with each BECN (limit one drop per time interval) until the traffic rate gets to the mincir (guaranteed rate) where it stops.

  • Once the traffic rate has decreased, it must allow 16 time intervals of receiving no BECNs before starting to increase traffic again. The amount it increases by is the byte limit that shows up in the show frame pvc x output divided by 16. This increase occurs only if traffic shaping is active. Thus, it takes much longer to get back to the CIR than it did to drop to mincir.

Non-Configurable Parameters

interval (Tc)

The interval during which you send the Bc bits in order to maintain the average rate of the CIR in seconds.

Tc = Bc/CIR in seconds

The range for Tc is between 10 ms and 125 ms. The router internally calculates this value based on the CIR and Bc values in the map class. If Bc/CIR is more than or equal to 125 msec, it uses the internal Tc value. If Bc/CIR is less than 125 ms, it uses the Tc calculated from that equation.

byte increment

The actual number of committed bytes sent per Tc. We can calculate this using the following formula:

Cir * Tc / 8

byte limit

The actual number of bytes sent in the first Tc. We can calculate this using the following formula:

byte increment + Be/8 (measured in bytes)
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