Many Department of Transportation agencies are looking for cost-saving ways to improve the efficiencies of roadways without making actual surface improvements or roadway expansions. Wireless-IP-enabled traffic signal control can be used to improve traffic flow and enable new applications for higher efficiency and cost savings. Advanced transportation management systems (ATMSs) provide the ability to connect signal controllers to an intelligent network for enhanced data gathering and analysis and better timing of traffic signals. Connecting signal controllers to an intelligent network with a standards-based wireless IP infrastructure is cost-effective, provides flexibility in design, enables faster deployment, and provides a network infrastructure for future additional applications.
Many of today's traffic controllers are linked to a central network and are connected through low-speed serial connections. Often these serial links are leased lines that a city obtains from a service provider, which is expensive because of reoccurring costs. In addition to being limited in bandwidth, these serial links are often only 56-Kbps lines. The overall result is that the amount of data that can be sent and received to and from the controllers is restricted. In addition, with their limited bandwidth and speed, these connections cannot accommodate additional applications that would increase the efficiency and safety of roadways.
Cities that own their own wired infrastructure (whether fiber, Ethernet, or copper) can get around these reoccurring costs by connecting traffic signal controllers to their own existing IP networks. However, cities that do not have their wired backhaul to each controller and want the advantages of ATMS solutions must find other methods of connecting controllers to the main network. Using a broadband wireless link is a natural alternative to increase bandwidth and reduce costs compared to low-speed serial links. However, extending reliable wireless links from one coverage area to another can be challenging. Standard 802.11 wireless bridges can be used, but often their reach is limited to one hop away from where a wired backhaul point exists. Additionally, designs that lack link redundancy or failover are vulnerable when the sole wireless backhaul link fails.
® 3200 Series Rugged Integrated Services Routers provide a solution that can allow city transportation agencies to utilize scalable, standards-based wireless technologies to connect traffic signal controllers in locations where wireless backhaul is not available. The Cisco 3200 Series is a ruggedized, outdoor wireless solution with integrated 802.11 capabilities, as well as additional Ethernet and serial ports to connect traffic signal controllers and additional devices, such as surveillance cameras and chemical sensors. A unique product in Cisco's portfolio, the Cisco 3200 Series provides a ruggedized wireless solution that can be embedded in existing traffic control cabinets or external NEMA enclosures for outdoor deployments with the following:
• Multiple integrated 802.11b/g bridges or access points to connect traffic signal controllers at one intersection to controllers at another intersection and then relay the wireless network link on to additional controllers at multiple intersections. The wireless coverage can also be used to connect mobile workers along the streets such as police, fire, and other city agencies.
• Standards-based Layer 3 routing capabilities to scale coverage across a city, taking into account redundancy, quality of service (QoS), and network security.
• Multiple network ports available on the router to connect traffic signal controllers in the cabinets, as well as other fixed devices. such as video surveillance cameras, chemical or biological sensors, or weather instruments.
The Cisco 3200 Series provides IT managers with new capabilities and flexibility when deploying ATMS networks. First, the Cisco 3200 Series is meant to be an embedded networking solution that can either be integrated into existing traffic cabinets or use a wide array of off-the-shelf, NEMA-rated enclosures. An embedded solution can take advantage of existing assets for enclosures and power, as well as close proximity to traffic signal control units. The Cisco 3200 Series can operate at temperature extremes from -40C to 80C and meets ruggedization requirements for shock and vibration.
Many traffic controllers are constructed as modular systems, with the communications module and CPU often designed into separate and removable modules to simplify upgrades. To function as part of an ATMS solution, a traffic controller must be connected to an IP network. The basic requirements for the IP-enabling of a traffic controller are the availability of an Ethernet interface and a TCP/IP stack. The Naztec 2070-1B Traffic Controller, for example, can be upgraded to function as part of an ATMS solution and become IP-enabled with the addition of a CPU-1B processor board, which contains on-board Ethernet interfaces.
Figure 1 gives a detailed view of the wireless connectivity at an intersection. In order to provide network connectivity to the traffic signal controllers, the Cisco 3200 Series provides standards-based Ethernet and serial ports that provide the interface to the signal controller hardware. When a network exists at each intersection location where a traffic controller is located, additional devices can be connected through the same standards-based IP connection. These devices can include video surveillance cameras, call boxes, chemical sensors, or weather stations.
Figure 1. Wireless Connectivity at the Intersection
Figure 2 shows the wireless connections between intersections with redundant paths. Each intersection is classified as a primary or secondary intersection. The primary intersection is unique because it has a wired or wireless backhaul back to the central site. This design provides multiple backhaul points from various intersections for redundancy throughout the network. The primary intersection funnels all traffic from surrounding secondary intersections and acts as the communications path back to the core network using its backhaul link. These primary and secondary intersections are grouped into clusters. Within a cluster, there are two primary intersections and multiple secondary intersections. The primary intersections are the only intersections in each cluster with a wireless or wired backhaul back to the central site.
Figure 2. Connecting Intersections
There are two bridges at the secondary intersection used for point-to-point links to other secondary or primary intersections within line-of-sight limitations. In a similar configuration, the primary intersection also contains a Cisco 3200 Series Rugged Integrated Services Router where two 802.11 bridges provide point-to-point links to neighboring secondary intersections.
The cluster design of primary and secondary intersections shown in Figure 2 is used to connect all the intersections to the core network. The cluster begins with a primary intersection and daisy chains to other secondary intersections using the point-to-point bridge links. This setup can result in a chain of primary and many secondary intersections. The reason to have two primary intersections in a cluster, or chain, is for redundancy. The number of secondary intersections in between two primary intersections will depend on several factors, including line-of-sight limitations and amount of bandwidth needed.
Scaling Coverage on a Metropolitan Scale
Figure 3 depicts a standard grid of streets and intersections where the Cisco 3200 Series is being used to link intersections. The Cisco 3200 Series used at each intersection provides wireless network access with redundant paths. This critical benefit of Layer 3 functionality helps to ensure that there is no single point of failure in a network design. In this design, each intersection is equipped with a Cisco 3200 Series Rugged Integrated Services Router. Where wired backhaul is not available, the use of multiple 802.11 bridges on the Cisco 3200 Series Rugged Integrated Services Router allows the router to be linked to a backhaul point and in addition relay wireless links to neighboring intersections.
Figure 3. Cisco 3200 Series Wireless Network
The intent of this design is for all secondary and primary intersections to run some type of Interior Gateway Protocol (IGP), so that a path to every single secondary and primary intersection is advertised throughout the network. This is to provide accessibility of the traffic controllers present at the intersection and to be able to access applications such as video from anywhere in the network. When a packet arrives from a mobile unit, the Cisco 3200 Series has a choice of links to route the packet. The packet can be forwarded to one of the primary intersections in its cluster toward either end. Based on routing metrics, the Cisco 3200 Series will choose the shortest path.
There are various methods to protect traffic running between intersections. Added benefits of the Cisco 3200 Series Rugged Integrated Services Router's Layer 3 functionality include network security and manageability. To help ensure that wireless connectivity remains secure and reliable, layers of network security can be applied throughout the network. A VPN tunnel can be created from each intersection back to the VPN gateway in the core network. This will encrypt all data between the two endpoints with Triple Data Encryption Standard (3DES). Each intersection can also be configured with the Cisco IOS
® Firewall to prevent unauthorized access into the network. The filter on the firewall will allow only traffic coming from a mobile unit or from the intersection itself. Cisco LEAP can also be used to authenticate all wireless devices (root bridges, nonroot bridges, and mobile work group bridges). The keys derived during authentication will be used to provide Temporal Key Integrity Protocol Per-Packet Keying (TKIP PPK) encryption and TKIP Message Integrity Check (MIC) for message integrity for wireless links.
Using an intelligent network based on standard wireless IP to connect traffic signal controllers at intersections throughout a metropolitan area is a cost-effective solution and provides the security and flexibility that a city or county needs. The Cisco 3200 Series Rugged Integrated Services Routers are ideally suited to provide wireless connectivity and data security. Network Layer 3 routing capabilities and the cluster design of intersections provide for redundancy should a link fail.
In addition, the intelligent network infrastructure provided by the Cisco 3200 Series allows additional applications to be incorporated into the network. These applications include high-speed wireless access for city workers, such as public safety and public works employees. It can also incorporate fixed applications at the intersection such as IP video surveillance.