Just as in all other ethernet forms, FastEthernet (IEEE 802.3u) is based upon the ability for a node to start transmitting when it detect silence ... as well as determine when a different node talks at the same time (a collision).
Frame Format
The packet fields are identical in structure to those in 10BaseT
Bit Rates and Slot Times
A Helpful Table ...
Speed Bit Time Slot Time
10 Mbps 100 ns 51.2 µs
100 Mbps 10 ns 5.12 µs
1000 Mbps 1 ns 0.512 µs
A Digression: Quick Physical Layer Overview
Signal Characteristics:
* 100FX: 125 MHz (4B5B encoding of 100 Mbps input, converted to light pulses)
* 100TX: 100 MHz (scrambled 100 Mbps input, converted to three level signal: MLT3)
Both scrambling and 4B5B encoding assure adequate transitions in the bit stream to allow clock synchronization, as well as taking care of any DC bias issues.
Size of Collision Domain:
As discussed in the sections above, we know the Slot Time for Fast Ethernet to be 5120 ns. This information can now be used to determine configuration specifications in one of two ways ... bit time calculations and distance measures.
Network Interface Cards (NICs)
1000 ns set aside for processing (100 bit times)
Class I and II Repeaters
There are two types of repeaters, the distinguishing factor is the maximum amount of time allotted for signal processing:
* Class I: 1400 ns (140 bit times)
* Class II: 920 ns (92 bit times)
Class I repeaters typically use the additional time for translating signals from one media to another, e.g., 100BaseTX to 100BaseFX.
Category 5 UTP and MM Fiber
The balance of the time is available for transit through the connections. But just how long can these connections be? It depends on how fast the signals propagate through the media. This is expressed by the Nominal Velocity of Propagation (NVP) and is expressed as a percentage of the speed of light (3 x 10^8 m/s). If you work it all out, light in a vacuum takes 3.33 ns to travel a meter, or a round-trip time of 6.67 ns/m. Using a NVP of 66.67% yields a nifty conversion factor of 10ns/m (and SU's Cat5 specs are actually higher ... 72%) so that we can change over the bit time numbers to distances ... as shown in the following section.
Simplified Design Rules
Much simpler to do:
* 412 m maxmimum distance allowed
* 140 m deducted for each Class I repeater
* 92m deducted for each Class II repeater
Stretching the Rules?
Note that SU's NVP is higher ... so that you actually have a comfortable "cushion"!
Hublets and Shared 100: DANGER!
The above measures are for shared 100Base connections. Say you have Class I repeater in the nearby wiring closet ... if someone connects a hublet to a TSO being served by that closet, you immediately go out-of-spec! Arrrgh! There are only two ways around this problem: to ban such devices from your building, or to use a Fast switch instead of shared repeater. Since switched ports are bridged (and thus separate one collision domain from another) the hublet dangling off the TSO is the ONLY Class I repeater in it's domain.
Overcoming Distance Limitations: Switched Full-Duplex
Switched Full-Duplex connections (where sending and receiving is occurring simultaneously and independently) can extend the distance limitations, e.g., 100BaseFX connections can go over 2 km! This would most likely be used for runs to the gateway or to other buildings.
Switching Issues:
* Modes:
o Cut Through - forwarded as soon as source/destination received
o Store & Forward - forwarded only after entire packet received (default when converting between 10 and 100)
o In-between - forwarded after the 64 byte boundary is met
* Buffers: whether pooled or dedicated to specific ports, the switch stuffs the packets here until they can be sent. Once the buffers are full, incoming packets are ignored.
* Virtual LANS (VLANs): ports can be grouped together so as to see only the broadcast traffic destined for that group. VLANs from various switches can be interconnected via multiple links, or via a single link (or trunk) when using Cisco's Interswitch Link (ISL). ISL works across routers as well.
* Multicast traffic control: Most switches have methods to direct IP multicast to those specific ports which have subscribed via IGMP.
* Layer 3 Switching: can bypass router for installations with multiple subnets.
Carrier Sense Multiple Access/Collision Detect (CSMA/CD)
Just as in all other ethernet forms, FastEthernet (IEEE 802.3u) is based upon the ability for a node to start transmitting when it detect silence ... as well as determine when a different node talks at the same time (a collision).
Frame Format
The packet fields are identical in structure to those in 10BaseT
Bit Rates and Slot Times
A Helpful Table ...
Speed Bit Time Slot Time
10 Mbps 100 ns 51.2 µs
100 Mbps 10 ns 5.12 µs
1000 Mbps 1 ns 0.512 µs
Signal Characteristics:
* 100FX: 125 MHz (4B5B encoding of 100 Mbps input, converted to light pulses)
* 100TX: 100 MHz (scrambled 100 Mbps input, converted to three level signal: MLT3)
Both scrambling and 4B5B encoding assure adequate transitions in the bit stream to allow clock synchronization, as well as taking care of any DC bias issues.
Media: 100TX; 100FX; 100T4; MII
Note that both copper and fiber media have Distance-Bandwidth specifications:
Frequency MM Fiber Cat 5
250 MHz 2 km 100m
500 MHz 1 km 50m
1000 MHz 500m 25m
Size of Collision Domain:
As discussed in the sections above, we know the Slot Time for Fast Ethernet to be 5120 ns. This information can now be used to determine configuration specifications in one of two ways ... bit time calculations and distance measures.
Network Interface Cards (NICs)
1000 ns set aside for processing (100 bit times)
Class I and II Repeaters
There are two types of repeaters, the distinguishing factor is the maximum amount of time allotted for signal processing:
* Class I: 1400 ns (140 bit times)
* Class II: 920 ns (92 bit times)
Class I repeaters typically use the additional time for translating signals from one media to another, e.g., 100BaseTX to 100BaseFX.
Category 5 UTP and MM Fiber
The balance of the time is available for transit through the connections. But just how long can these connections be? It depends on how fast the signals propagate through the media. This is expressed by the Nominal Velocity of Propagation (NVP) and is expressed as a percentage of the speed of light (3 x 10^8 m/s). If you work it all out, light in a vacuum takes 3.33 ns to travel a meter, or a round-trip time of 6.67 ns/m. Using a NVP of 66.67% yields a nifty conversion factor of 10ns/m (and SU's Cat5 specs are actually higher ... 72%) so that we can change over the bit time numbers to distances ... as shown in the following section.
Simplified Design Rules
Much simpler to do:
* 412 m maxmimum distance allowed
* 140 m deducted for each Class I repeater
* 92m deducted for each Class II repeater
Stretching the Rules?
Note that SU's NVP is higher ... so that you actually have a comfortable "cushion"!
Hublets and Shared 100: DANGER!
The above measures are for shared 100Base connections. Say you have Class I repeater in the nearby wiring closet ... if someone connects a hublet to a TSO being served by that closet, you immediately go out-of-spec! Arrrgh! There are only two ways around this problem: to ban such devices from your building, or to use a Fast switch instead of shared repeater. Since switched ports are bridged (and thus separate one collision domain from another) the hublet dangling off the TSO is the ONLY Class I repeater in it's domain.
Overcoming Distance Limitations: Switched Full-Duplex
Switched Full-Duplex connections (where sending and receiving is occurring simultaneously and independently) can extend the distance limitations, e.g., 100BaseFX connections can go over 2 km! This would most likely be used for runs to the gateway or to other buildings.
Switching Issues:
* Modes:
o Cut Through - forwarded as soon as source/destination received
o Store & Forward - forwarded only after entire packet received (default when converting between 10 and 100)
o In-between - forwarded after the 64 byte boundary is met
* Buffers: whether pooled or dedicated to specific ports, the switch stuffs the packets here until they can be sent. Once the buffers are full, incoming packets are ignored.
* Virtual LANS (VLANs): ports can be grouped together so as to see only the broadcast traffic destined for that group. VLANs from various switches can be interconnected via multiple links, or via a single link (or trunk) when using Cisco's Interswitch Link (ISL). ISL works across routers as well.
* Multicast traffic control: Most switches have methods to direct IP multicast to those specific ports which have subscribed via IGMP.
* Layer 3 Switching: can bypass router for installations with multiple subnets.
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