Design traffic video transmission systems around the constraints of bandwidth limitations and provide provisions for remote configuration of video compression hardware.
Statewide systems implementation experience from iFlorida Model Deployment
- Design traffic video transmission systems around the constraints of limited bandwidth availability. The FDOT SMS network consists of a set of hub sites with fiber connection and remote sites using microwave to communicate with the hub sites. The remote sites are daisy-chained, so that bandwidth usage at the remote site nearest the hub is the sum of the bandwidth required for that site and for all other remote sites directly or indirectly connected to it. This network is capable of transmitting up to 33 megabits per second (Mbps) between hub sites and up to 3 Mbps from remote sites to hub sites. One of the challenges faced in designing the Statewide Monitoring System to use the SMS was the limited bandwidth available and the relatively high bandwidth requirements of traffic video. The exhibit below lists the approximate bandwidth required for different types of video using different types of compression.
Approximate Bandwidth Requirements for Traffic VideoResolutionMPEG-2MPEG-4LowMediumHighLowMediumHighQCIF0.3126.96.36.199.120.07CIF1.010.620.380.500.310.194CIF2.881.921.101.440.960.55Mbps = Megabits per second. QCIF = Quarter Common Intermediate Format (176 pixels by 144 lines, 30 frames per second; 1.22:1).
CIF = Common Intermediate Format (352 pixels by 288 lines, 30 frames per second; 1.22:1).
4CIF = 4 times Common Intermediate Format (704x576 pixels).
Source: Table 13, Source Document.
- Provide provisions for remote configuration of the video compression hardware in designing traffic video transmission systems. FDOT also required that each site include video compression hardware that was remotely configurable. This allowed FDOT to easily vary the video type and compression after the system was deployed in order to find the combination that worked best with the limited bandwidth available through the SMS. At the time of this report, FDOT was using MPEG-4 with CIF resolution and low compression for an expected bandwidth usage of about 500 kilobits per second per camera. The resulting video appears slightly grainy on a full screen and slightly choppy. FDOT felt that the video was definitely of high enough quality to support traffic management decision making, though the quality was noticeably lower than most other video that FDOT D5 has available.
- Beware of trade off between cost savings from using an existing SMS network and the network’s capability to support essential functions. Florida’s Statewide Monitoring System demonstrated that using the pre-existing microwave communication network was a cost-effective approach for providing communications to remote traffic monitoring stations. FDOT noted that the network was reliable, except for some disruptions during bad weather. FDOT also noted that using the locations of microwave towers as the primary factor in selecting sites for the Statewide Monitoring System might have been a mistake. A better system may have resulted from selecting sites primarily on the basis of the usefulness of the sites for supporting transportation decision making, and only taking advantage of microwave tower locations when consistent with those site selections.
Author: Robert Haas (SAC); Mark Carter (SAIC); Eric Perry (SAIC); Jeff Trombly (SAIC); Elisabeth Bedsole (SAIC): Rich Margiotta (Cambridge Systematics)
Published By: United States Department of Transportation Federal Highway Administration 1200 New Jersey Avenue, SE Washington, DC 20590
Source Date: 01/30/2009
EDL Number: 14480URL: http://ntl.bts.gov/lib/31000/31000/31051/14480.htm
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CCTV, closed circuit television cameras, road monitoring, sensors, vehicle detector, traffic detection, traffic monitoring, congestion monitoring