Answers to Chapter 1 Review Questions

Answers to Chapter 1 Review Questions

1:

What is the pps rate for a 100BaseX network? Calculate it for the minimum and maximum frame sizes.

A:

Because all of the bit time values are one tenth that of 10 Mbps Ethernet, the pps for 100 Mbps Ethernet is 10 times the 10 Mbps pps values. So Fast Ethernet supports up to 148,800 pps for 64 byte frames and 8,120 pps for 1518 byte frames.

2:

What are the implications of mixing half-duplex and full-duplex devices? How do you do it?

A:

You need to ensure that your full-duplex devices attach to full-duplex hubs. Otherwise, the full-duplex devices need to run in half-duplex mode. Always attach full duplex together and half duplex together.

Another issue concerns bandwidth. The full-duplex devices effectively have 200 Mbps bandwidth, whereas the half-duplex run 100 Mbps. In fact, the half duplex might be even less than 100 Mbps if the device attaches to a shared hub. Therefore, communications between the full-duplex and half-duplex device is limited by the bandwidth of the half-duplex attachment.

3:

In the opening section on Fast Ethernet, we discussed the download time for a typical medical image over a shared legacy Ethernet system. What is an approximate download time for the image over a half-duplex 100BaseX system? Over a full-duplex 100BaseX system?

A:

The medical file size was 100 Megabytes. If the system is half duplex, it might be running on a shared hub. Assume that the station has an effective shared bandwidth, then, of 20 Mbps. In reality, it might be less or more. Then the transfer time is about 40 seconds. 100 MBx8/20 Mbps=40 seconds.

Full-duplex operations can provide 100 Mbps bandwidth in the receive direction. Therefore, the approximate transfer time is 100 MBx8/100 Mbps=8 seconds.

Both models assume that the servers can generate traffic at that rate.

4:

What disadvantages are there in having an entire network running in 100BaseX full-duplex mode?

A:

One disadvantage can be cost. Every full-duplex device needs to attach to its own dedicated switch port. You cannot have multiple full-duplex devices attached to the same port. If you have many devices, you need many ports, which equates to increased cost.

Another disadvantage can be congestion at the servers. As you increase the number of devices operating in full-duplex mode, higher amounts of bandwidth hit your servers. This amount can greatly exceed the bandwidth capacity of the server connection.

5:

Can a Class II repeater ever attach to a Class I repeater? Why or why not?

A:

You should not attach a Class I repeater to a Class II repeater because this violates the latency rules of Fast Ethernet. When you have a Class I repeater, there can be only one repeater .

6:

What is the smallest Gigabit Ethernet frame size that does not need carrier extension?

A:

The need for the carrier extension bytes is driven by the slotTime. Gigabit Ethernet uses a slot time for 4096 bits. This equates to 512 bytes. Therefore, any frames of 512 bytes or larger do not need carrier extension, whereas all frames less than 512 MUST have carrier extension.



Cisco(r) LAN Switching
Cisco Catalyst LAN Switching
ISBN: B00007FYCI
EAN: N/A
Year: 2005
Pages: 223

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