Cisco Cisco IOS Software Release 12.4(4)T Datos agregados

Product Bulletin

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track topology changes, but MANETs reliance on these mechanisms can result in unacceptably

slow convergence.

The Cisco solution provides faster network convergence by using link status signals generated by

the radio. The radio notifies the router each time a link to another neighbor is established or

terminated. In the router, the routing protocols (OSPFv3 or EIGRP) respond immediately to these

signals by expediting formation of a new adjacency (for a new neighbor) or tearing down an

existing adjacency (if a neighbor is lost). If, for example, a vehicle drives behind a building and

loses its connection, the router will immediately sense the loss and establish a new route to the

vehicle through neighbors that are not blocked. This high speed network convergence is essential

for minimizing dropped voice calls and disruptions to video sessions.

Link Quality Metric Reporting for OSPFv3 and EIGRP

The quality of a radio link has a direct impact on the throughput that can be achieved by router-

router traffic. The PPPoE protocol has been extended to provide a process by which a router can

request, or a radio can report, link quality metric information. Cisco OSFPv3 and EIGRP

implementations have been enhanced so that the route cost to a neighbor is dynamically updated

based on metrics reported by the radio. This capability allows the best route to be chosen within a

given set of radio links.

The routing protocols receive raw radio link data, and compute a composite quality metric for each

link. In computing these metrics, the following factors may be considered:

●

Maximum Data Rate: the theoretical maximum data rate of the radio link, in bytes per

second

●

Current Data Rate: the current data rate achieved on the link, in bytes per second

●

Latency: the transmission delay packets encounter, in milliseconds

●

Resources: a percentage (0-100) that can represent the remaining amount of a resource

(such as battery power)

●

Relative Link Quality: a numeric value (0-100) representing relative quality, with 100 being

the highest quality

Metrics can be weighted during the configuration process to emphasize or deemphasize particular

characteristics. For example, if throughput is a particular concern, the current data rate metric

could be weighted so that it is factored more heavily into the composite metric. Similarly, a metric

that is of no concern can be omitted from the composite calculation.

Link metrics can change rapidly, often by very small degrees, which could result in a flood of

meaningless routing updates. In a worst case scenario, the network would be churning almost

continuously as it struggled to react to minor variations in link quality. To alleviate this concern,

Cisco provides a tunable dampening mechanism that allows the user to configure threshold

values. Any metric change that falls below the threshold is ignored.

PPPoE Credit-based Flow Control

Each radio initiates a PPPoE session with its local router as soon as the radio establishes a link to

another radio. Once the PPPoE sessions are active for each node, a PPP session is then

established end-to-end (router-to-router). This process is duplicated each time a radio establishes

a new link.

The carrying capacity of each radio link may vary due to location changes or environmental

conditions, and many radio transmission systems have limited buffering capabilities. To minimize