1.4 Bandwidth Issues

This topic describes bandwidth issues in a multisite environment.

Individual sites of a multisite deployment are separated by an IP WAN:

  • All intersite traffic (voice, data, video, etc.) competes for available bandwidth.
  • Bandwidth on IP WAN links is usually limited and costly.
    1. Link bandwidth should be used as efficiently as possible.
    2. No unnecessary traffic should be sent over the IP WAN.
    3. Default codec is not efficient in noncircuit-based environments.
    4. Voice traffic causes much overhead.
      • Large headers, small payload
      • High packet rate

The individual sites of a multisite deployment are usually interconnected by an IP WAN. Bandwidth on WAN links is limited and relatively costly. Therefore, the goal is to use the available bandwidth as efficiently as possible. Discourage unnecessary traffic, and consider implementing methods for bandwidth optimization.

Voice streams can be wasteful, considering that voice is sent in small packets but at a very high packet rate. Voice streams are particularly wasteful when you use default codecs (G.711 requires 64 kbps for digitized voice.). The overhead of packetization—encapsulating digitized voice into RTP, UDP, IP, and a Layer 2 protocol—is extremely high compared to the size of the payload. The more of these packets that are sent, the more often the headers are added to the actual voice information (the RTP payload).

Example: Bandwidth Issues

A voice call with default settings uses the G.711 codec and a packetization period of 20 ms. The 160 bytes (equivalent to 20 ms of voice) are encapsulated into a 12-byte RTP header, an 8-byte UDP header, and a 20-byte IP header. The header-to-payload ratio is 1:4 and the bandwidth requirement is 80 kbps (instead of the 64-kbps nominal bandwidth of G.711).

Note

This calculation considers only encapsulation to IP packets. Layer 2 encapsulation (which varies, depending on the data link protocol being used) is not yet considered.

Bandwidth Issues Example: Voice and Data Traffic Competing for Bandwidth

The example illustrates the higher overhead of voice packets when comparing them with file transfer packets.

In multisite environments, media services such as MOH, conferences, annunciators, and so on, can cause considerable bandwidth consumption over the IP WAN.

  • Voice packets:
    • Small size
    • High packet rate
    • Large overhead
  • Data packets:
    • Large size
    • Lower packet rate
    • Small overhead

As shown in the figure, voice packets consume much bandwidth, which is caused by the overhead of IP, UDP, and RTP headers that are added to small packets and sent at a high packet rate. Data packets, such as file transfer, also add 40 bytes of overhead (20 bytes IP and 20 bytes TCP), but the payload is as large as possible (filling up the MTU)—typically about 1500 bytes. Because of the large payload, the packet rate is lower and overhead is not added, as is often the case with voice packets.

Because of the inefficiency of voice packets, all unnecessary voice streams should be kept away from the IP WAN. Media resources, in particular, can be optimized in such a way that they do not have to cross the IP WAN all the time, thus conserving valuable bandwidth. You can achieve this optimization by utilizing local media resources.

Bandwidth Issues Example: Load Caused by Centralized Media Services

The figure illustrates the bandwidth issue that is caused by a centralized conference bridge.

The use of a centralized conference bridge requires RTP streams of phones that are located at the remote site to cross the IP WAN, even when only remote phones are participating in the conference.

The same problem applies to MTPs, annunciators, and MOH.

In the example, a conference bridge has been deployed at the main site. There is no conference bridge at the remote site. If remote IP phones join a conference, their RTP streams are sent across the WAN to the conference bridge. The conference bridge mixes the received audio streams and then sends them back to the IP phones over the IP WAN.

There are three members in the conference in this example, and all of them are physically located at the remote site. In total, three RTP streams are flowing toward the conference bridge, and three RTP streams are flowing back to the remote site. Assuming default settings, each RTP stream requires 80 kbps (ignoring the Layer 2 overhead), resulting in 240 kbps of IP WAN bandwidth that is required by this voice conference. If the conference bridge was not located on the other side of the IP WAN, this traffic would have avoided the WAN link entirely, since all participants of the conference are local to the remote site.

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