Facing media traffic challenges
Feb 1, 2010 12:00 PM, By Luc Andries, MSC
The way the IT world characterizes and defines IP traffic must change to accommodate the demands of today's media.
Figure 4. The test environment is assessed for its buffer performance and overall functionality in a media environment.
Select figure to enlarge.
These results correspond to the specifications typically used by video engineers and media solution suppliers. Throughput is expressed on this macroscopic time scale, and network architectural designs are based solely on these values. However, zooming in on increasingly smaller time scales reveals a completely different story.
Now, examine the right side of Figure 3; the seemingly continuous throughput displayed on the left actually appears to consist of discrete blocks of five consecutive video frames. During the prefetch period, 37 groups of five video frames are sent over the network, almost back to back. In steady state, blocks of five video frames are interleaved with long periods of no traffic on the link. At the 10ms time scale, the network reaches an average bandwidth of around 600Mb/s during transmissions. This is slightly higher than the average throughput during the prefetch period, but more than four times higher than the average throughput during the steady-state playout. For this codec and frame rate, each video frame corresponds with 606,208 bytes of video data, so the five-frame burst corresponds to around 3MB, with a duration of 42.4ms.
Looking at an even smaller time scale, on the level of individual packets, each video frame is actually split into 47 smaller bursts. Each of these smaller bursts consists of 45 (14 for the smaller last burst) 1518-byte packets, transmitted back to back within each burst. Hence, on this µs time scale, a continuous burst of packets is measured at a throughput of 980Mb/s, reaching full line rate of the link for 555µs. This is 6.75 times higher than the average steady-state macroscopic bandwidth specified by the codec.
Implications for the IP infrastructure
Clearly, measuring data rates at a macroscopic average throughput is too limited to fully characterize media traffic. It is only by analyzing the traffic at smaller time scales that we can determine how media traffic will be processed by the IP switch and understand the requirements of the network.
As we have seen, media flows that share a common link can interfere with each other on a small time scale, generating a local oversubscription of the switch buffers and ultimately introducing packet loss. Similarly, a bandwidth mismatch in the networkwill put the internal switch buffers under pressure and can result in packet loss. The solution is to provide sufficient buffering.
The next examination used a Cisco Nexus 7000 IP switch to assess its buffer performance and overall functionality in a media environment. (See Figure 4.) Note that the conclusions drawn here are only valid for this particular setup, and have to be reconsidered for other protocols and applications.
The test analyzed the following links:
- 1Gb (server) to 1Gb (client)
- 10Gb (server) to 10Gb (client)
- 10Gb (server) to 1Gb (client)
In the first two cases, traffic passes unhindered through the switch with no oversubscription or bandwidth mismatch in the network path. Hence, the detailed structural description of the bursty media traffic given above is valid for both cases. The traffic is bursty, and the microscopic SMB bursts are concatenated in case of multiple streams per video. This leads to high throughputs compared with the average macroscopic video specifications. Under these tests, the Cisco Nexus 7000 switch was perfectly capable of transporting these high loads.
In the third case, the 10Gb-to-1Gb bandwidth mismatch creates an internal oversubscription in the switch. Hence, at the 1Gb egress ports, packets arrive at a much higher rate than they can be forwarded to the client, stressing the egress buffer. The maximum burst that the SMB traffic will produce before it requires a response from the client is 68,144 bytes (TCP/IP overhead included). Because the egress port sends out packets at a rate 10X slower than the incoming rate, the egress buffer must be able to store 90 percent of this burst to avoid packet loss. This leads to a buffer requirement of around 60Kb per single video stream, or around 300Kb for a test using five streams. This is well within the specifications of the 48 × 1Gb port blades of the Cisco Nexus 7000 switch (max 6.15MB/port).
Change must come
The way the IT world characterizes and defines IP traffic must change to accommodate the demands of today's media. Macroscopic quantities such as average bandwidth, oversubscription and available capacity are no longer the only relevant parameters and must be interpreted in a different way. Additional specifications on much smaller time scales are required, and a deeper understanding of the detailed traffic characteristics and network switch and buffer mechanisms should be modeled.
It is critical to understand that IP switches needed for media networks are not a commodity like in IT networking. Most classical IT switches are designed for environments where oversubscription is less likely. Such switches may lack the proper buffers or QoS capabilities to avoid transfer interference of large media files.
Be sure to carefully consider these factors when building a media network.
Luc Andries is ICT-architect for VRT-medialab.
VRT-medialab is the technological research department of the VRT, the public service broadcaster of Flanders, Belgium.
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