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Section 8: QoS (8 Points)

  • Achieve maximum quality of voice calls by ensuring the real-time packet interval of 10 ms is not exceeded. Do not use RSVP in your solution.

This question requires that fragmentation is used over the Frame Relay and ISDN networks that will transport voice traffic. You should remember that voice is still required should the Frame Relay network fail. By fragmenting the data, you can tailor the packet interval and ensure that voice quality is not compromised over low bandwidth links. Some basic math is required to calculate the current real-time packet interval over the Frame Relay and ISDN network to begin as detailed in Table 1-3. Note the Frame Relay speed is 256 kbps and the ISDN is 64 kbps using only one B channel.

Table 1-3 MTU Values According to Bandwidth

 

Frame Relay (256 kbps)

ISDN (64 kbps)

No. Bytes TX'd per second

32,000

8,000

No. of bytes TX'd in 10ms

320

80


As Table 1-3 shows, 32,000 are bytes transmitted every second over the Frame Relay circuit (256,000 divided by 8) if the real-time delay or serialization delay is to be 10 ms; 320 bytes can be transmitted in this period (32,000 * 10 ms). Similarly, 80 bytes can be transmitted for the ISDN with the circuit speed of 64 kbps.

After you have calculated that 320 bytes will be transmitted over the Frame Relay network and 80 bytes over the ISDN in the 10 ms interval, you can adjust the interface MTU to reflect this for the ISDN and then change the frame-relay fragmentation to 320 under the VoFR map-class on both R1 and R4. If you have configured this correctly as shown in Example 1-63 and Example 1-64, you have scored 4 points.

Example 1-63 R1 QOS MTU Configuration

interface BRI0/0
 ip mtu 80
!
map-class frame-relay ccie
 frame-relay fragment 320 

Example 1-64 R4 QOS MTU Configuration

interface BRI0/0
 ip mtu 80
!
map-class frame-relay ccie
 frame-relay fragment 320

  • To reduce the packet fragmentation in your network, allow R5 to determine appropriate fragmentation requirements when TCP sessions are originated from it to any part of the network.

R5 should be configured with the global command ip tcp path-mtu-discovery. If you have configured this correctly, you have scored 2 points.

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