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Showing posts from November 30, 2008

How to calculate fragment size or fragment delay (FRF.12 or MLPPP)?

Serialization Delay = frame size (bits) / link bandwidth (bits per second [bps])

在QoS中我們可以利用LLQ(Low Latency Queueing)來提供VoIP封包低延遲(delay)及減少抖動(jitter)發生的情況。雖然VoIP封包總是傳送到software queue的前端,serialization delay(Layer 2 Frame encoded into Layer 1 Bits)的問題仍無可避免。一個大型封包可能正在hardware queue中使用FIFO。當VoIP封包被傳送至software queue的前端,在hardware transmit queue中的大型封包進行serialization時會導致VoIP封包必須等待一段較長的時間之後才能被傳送出去。

這時我們就可以使用fragment將大型封包切割成許多的小型封包,同時搭配interleaving的方式,將VoIP封包穿插在這些被切割之後的小型封包之間,藉此減少抖動(jitter)情況的發生。

當你在某鏈結上要設置適合的fragment size(切割尺寸)時,比較常見的目標是使得最大serialization delay維持在10~15ms之間。

假設實體連接埠線路速度為512Kpbs,所需要的serialization delay不應該超過10ms(記住,fragment size是根據實體連接埠線路速度而計算出來的!),fragment size(切割尺寸)必須設定為:
512000(bps)/8*0.01(sec)=640 bytes

我們可以使用下列指令來進行設置:
Router(config-if)# ppp multilink fragment 640
or
Router(config-map-class)# frame-relay fragment 640



如果在Cisco IOS CLI上如果今天要使用的是fragment delay數值(milliseconds),那就必須再乘上所使用的interface頻寬(假設MLPPP virtual-template上的頻寬為384Kbps)。

因此,我們使用virtual-template interface上的頻寬(384Kpbs)並且調整del…

IP RTP Priority

在還沒有發明LLQ(Low Latency Queue)之前,我們要在interface上調整Voice RTP的保留頻寬,只能使用

Router(config-if)# ip rtp priority starting-rtp-port-numberport-number-rangebandwidth

設定完成之後,我們可以使用以下的指令來檢查我們的配置:

Router# show queue interface-type interface-number

以下是Cisco官網上的說明:

Feature Overview
The IP RTP Priority feature provides a strict priority queueing scheme for delay-sensitive data such as voice. Voice traffic can be identified by its Real-Time Transport Protocol (RTP) port numbers and classified into a priority queue configured by the ip rtp priority command. The result is that voice is serviced as strict priority in preference to other nonvoice traffic.

The IP RTP Priority feature extends and improves on the functionality offered by the ip rtp reserve command by allowing you to specify a range of User Datagram Protocol (UDP)/RTP ports whose traffic is guaranteed strict priority service over any other queues or classes using the same output interface. Strict priority means that if packets exist in the priority que…

Cisco Introduces New CCIE Wireless Certification, CCIE Voice Lab Exam Enhancements

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[轉載]Understanding 4-Byte Autonomous System Numbers

一般BGP網管大都知道BGP AS號碼範圍從0~65535,Public AS Range: 0~64511,Priavte AS Range: 64512~65535,所以很多人會誤解以為AS Number的位元只有16 bits。

事實上AS Number總共有32 bits(4 bytes),前面16 bits以往都是用0,所以會造成這樣的錯覺。根據Jeff Doyle blog上面提到的內容,在不久的將來所有新申請的AS Number format將會是0.XX。以下是完整4-bytes ASN的介紹請參考!

Understanding 4-Byte Autonomous System Numbers
By jdoyle on Fri, 11/28/2008 - 4:40pm.

For all the harping I do on this blog about IPv4 address depletion and the need to prepare yourselves for IPv6, there is another number resource that is also being quickly depleted, and that I haven’t written about before: the 2-byte autonomous system numbers (ASNs).

A 16-bit number space gives you 65,536 possible numbers (AS numbers 0 – 65535). Out of these, the IANA reserves 1,026 of them: 64512 – 65534 for private, reusable ASNs (similar to private RFC1918 IPv4 addresses) and a few others such as 0 and 65535 and one that is important to this article, 23456. Presently 49,150 ASNs have been allocated out of the public pool, so there are 15,360 ava…