Knowledge Resources

Florida’s Statewide Microwave System (SMS) was originally deployed in the 1980s to support emergency call boxes available at regular intervals on Florida Interstate Highways. Subsequently over the years, the combination of Road Rangers (motorist assistance patrols), cameras monitoring roads, and the availability of E911 service, and cell phones have made the call boxes less important in helping travelers report incidents that do occur. Consequently, FDOT is in the process of removing call boxes from many Florida Intrastate Highway System (FIHS) roads. Starting in 2002, the SMS was upgraded from an analog, non-integrated system to a digital backbone to create a seamless and homogenous statewide monitoring system, which was intended to primarily support traveler information and hurricane evacuation services. For iFlorida project, FDOT decided to use the SMS to support the transmission of multiple streams of IP-based traffic information from remote field devices to Regional Traffic Management Centers (RTMC) that were connected to the microwave system data network. FDOT’s experience with the use of SMS network for statewide monitoring offers a number of lessons learned as presented below.

• 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 Video
  Resolution	MPEG-2			MPEG-4
  	Low	Medium	High	Low	Medium	High
  QCIF	0.34	0.24	0.14	0.17	0.12	0.07
  CIF	1.01	0.62	0.38	0.50	0.31	0.19
  4CIF	2.88	1.92	1.10	1.44	0.96	0.55
  Mbps = 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.

  Given that the bandwidth available from remote sites to hub sites was limited to 3 Mbps and part of this bandwidth was reserved for other applications, FDOT determined that it could deploy, at most, three monitoring sites along a single "spoke" of remote microwave towers. The first step taken by FDOT in designing this system was to review the network topology for the microwave network to identify the remote towers that shared a single spoke. This allowed FDOT to select Statewide Monitoring System sites so that no more than three sites lay along the same spoke before reaching a hub site.
• 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.

It is prudent to consider cost savings while also achieving the optimum functionality in ITS deployment. Lessons from Florida’s experience with using an SMS network offers guidance on how to design around external constraints, such as bandwidth capacity, so that a desired statewide monitoring system performs essential functions of transmitting traffic video to traffic management center to help improve operational efficiency and mobility.