Using Common Applications over BridgesIn the service provider arena, bridges are often used to tie multiple companies back to a single point of presence. However, the traffic needs to be separated for security and privacy reasons. Here, you can use virtual LANs (VLANs).Because bridges are also used to replace T1 lines, fiber links, or other hard-wired systems, they are expected to carry any and all of the same type of network traffic between sites. In many cases, this can include time-sensitive traffic, for which quality-of-service (QoS) implementations should be used to improve performance. Bridges are increasingly replacing certain types of leased lines, and one common application used in such scenarios is VoIP. VLANsA VLAN is a group of end stations with a common set of requirements, independent of their physical location. A VLAN has the same attributes as a physical LAN, but enables you to group end stations even if they are not located physically on the same subnet.802.1Q VLAN support is provided to work in conjunction with the switch or router attached to the wireless bridge. Both the wired Ethernet and wireless radio interface should support VLAN trunking. Native Ethernet and 802.1Q tagging is supported on many of the available bridges today.Thus, the bridge participates in the 802.1d Spanning Tree Protocol process of bridging two networks together. VLANs can be extended into a WLAN by adding IEEE 802.1Q tag awareness to the bridge. The basic wireless components of a VLAN consist of two or more bridges communicating using wireless technology. The bridge is physically connected through a trunk port to the network VLAN switch on which the VLAN is configured. The physical connection to the VLAN switch is through the bridge's Ethernet port.Figure 14-4 shows a typical VLAN bridge topology. Figure 14-4. Bridge Topology Using VLANs[View full size image] ![]() QoSImplementing QoS in your wireless link makes applications run more predictably and bandwidth utilization more effective. By using QoS features, you can prioritize specific types of traffic instead of sending packets in a purely first in, first out (FIFO) manner. When subject to delays, voice and video traffic causes very unfavorable behavior in phone conversations (major lags or even dropped phone calls). The objectives of a QoS feature on a bridge would include providing 802.1p priority bits and 802.1Q VLAN tag-based QoS, and priority services for VoIP traffic, based upon IP TOS (type of service bits in the IP protocol header) / DSCP (differentiated services code point).Many bridges can only classify traffic based on IP TOS precedence and DSCP values and put it into the correct priority queues. Devices that follow the 802.11e priority values have eight priority queues. The class of service (CoS) values associated with the eight priority queues are the same as in 802.1p.User priority values are carried in an Ethernet frame, an 802.1Q priority tag, or an 802.1Q VLAN tag. The CoS value is used to select the appropriate 802.11 transmit queue.A bridge can use the priority tag to calculate back-off times for each packet, enabling high-priority packets to have short back-off times, setting them up for transmission before lower-priority packets. Voice over IPWhen using QoS, you should prioritize voice traffic by defining a policy and assigning that policy to the VLAN carrying the voice traffic. Many bridges support QoS only in point-to-point links. Voice in a point-to-multipoint configuration gets a bit tricky, because over-the-air QoS coordination between endpoints is difficult if they cannot hear each other. In point-to-multipoint systems, it is typical that one remote site can hear the central site but cannot hear other remote sites, because of distance or even antenna type and direction. This is commonly referred to as a hidden node. A protocol called Point Coordination Function (PCF) enables the central site to control when each endpoint has access. Voice in point-to-multipoint networks is possible if PCF mode is implemented, but not many products support this today.The typical number of voice lines for an 802.11a or 802.11g 54-Mbps bridge link operating in a point-to-point topology can vary based on the coding of voice traffic. For a typical 54-Mbps link, up to 15 calls can be handled when using G.711, and as many as 40 calls may be possible when using G.729 protocol. These numbers take into account some data traffic also running on the link. The number of voice lines has been calculated by using a maximum jitter of 25 ms. SecurityWEP, WPA, and 802.1x authentication schemes. One very common method is just to put a router behind each bridge that supports VPN tunneling, or to use a VPN hardware device behind each bridge (but be aware some VPN tunnels hide any 802.1Q tag). It is advisable to review some good security references before selecting the product to install. |