All communication
protocols have a set of rules or specifications that govern their
operation. It is these rules that determine both the performance and the
limitations of each protocol. Both Ethernet and Fast Ethernet have, as a
part of their protocol, a collision sensing standard. This is known as
CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is the
protocol that allows multiple devices to access a shared Ethernet or Fast
Ethernet network.
A collision domain is a
group of Ethernet or Fast Ethernet devices that are directly connected by
repeaters. Only one device may transmit at any one time inside of this
collision domain. When a device is transmitting all other devices in the
collision domain listen. The method by which this is accomplished is a
simple means to guarantee that only one device is talking at a time. The
construction of a network also will include such products as switches and
routers. Collision domains are separated by these devices. These types of
products also allow the separate collision domains to communicate with each
other.
When communications
devices are sharing a network in any protocol, a MAC (Media Access Control)
protocol is used to establish access to that network. As mentioned above,
the CSMA/CD protocol is used by Ethernet and Fast Ethernet. The MAC resides
in all data terminal equipment (DTE's) in the network. PC's, servers,
bridges, and switches are typical DTE devices. Repeaters do not contain a
MAC and operate with a simplified set of rules that will be explained
below.
Illustration 1.1
To give an example of
the operation of the Ethernet MAC layer, consider the following: On a
network of Ethernet devices a DTE wishes to send a transmission to another
DTE that is in the same collision domain. In this example, DTE A wishes to
transmit to DTE C (see Illustration 1.1.) The MAC client will "listen" to
the network to determine if there is another DTE using the network. This is
done by the CSMA protocol. If another device is currently transmitting it
will not attempt to send it's packet. Instead it will back off and wait
until the network is clear. If no other signal is on the network, then the
DTE proceeds to transmit the packet onto the network. When the packet
completes transmission, the MAC client on the source DTE will consider the
packet sent understanding that since no other device was transmitting, it's
packet transmission was good. If there is a need to send more than the
single packet in this example, then the process in repeated.
The other half of this
MAC client will determine the actions to take if two devices do transmit
at the same time. Again, using the same example of a DTE wishing to send
a packet to another device in the same collision domain, we will explore
what happens to the network when collisions occur. The first DTE (DTE A)
will follow the same steps as before. This time after beginning the
transmission of a packet a different DTE (DTE B) also wishes to send a
packet. DTE B will also use the CSMA to determine if there is current
activity and make the decision to transmit or not. It is at this point
that the issue of timing becomes important. As you know from the earlier
example, DTE C will not transmit if it knows that DTE A is currently
using the network.
A collision occurs if
DTE B begins transmitting before the signal from DTE A is heard by the
CSMA/CD MAC of DTE B. If this is the case all DTE's in the collision
domain will "hear" the collision and they send a collision signal onto
the network. The two DTE's that are transmitting will then stop their
transmission and restart again later. Each MAC client has a unique timing
value that will determine when to restart.
Illustration
1.2
The key is the time it
takes for the first DTE's signal to reach the furthest DTE in the collision
domain and for this DTE to return the collision signal. Remember that the
first DTE will assume that it's packet was sent successfully if it does not
hear a collision before it has completed transmission. This packet will
then discarded by the Ethernet MAC client. If transmission was completed
before a collision was sensed, the packet is lost and requires a higher
software layer to realize the packet is lost and then retransmit the
packet. This is what is referred to as a late collision.
The collision domain's
diameter is limited primarily by this timing issue. The diameter of the
collision domain is the distance between the two furthest nodes. Because of
the potential for packet loss the dimensions are limited to reflect the
example above. The maximum diameter of a collision domain is the total time
it takes for the smallest packet to travel round trip between the two
furthest DTE's in that domain. Because Ethernet and Fast Ethernet allow for
variable packet sizes, the smallest packet size is used (64 Bytes). Another
important consideration is that collision domains do not cross over bridges
or switches. Every port on a switch represents a separate collision
domain.
The limitations posed
by the CSMA/CD protocol are important guidelines for designing a network.
The signals that packets consist of take time to travel over the network.
The time is doubled in the case of the collision signal returning to the
transmitting source from the furthest node. With the 64 byte packet (512
bits) being the smallest packet size we can use the time it takes this
size packet to travel the network serves as the measurement of a networks
collision domain diameter. When using the 512 bit packet in this way and
knowing that the signal travels over all media types in a finite fashion
we can measure the maximum diameter by the 512 bit times for a round
trip.
The goal of a network
designer is to guarantee that a collision will occur in the first 512 bit
times of transmission. If the collision occurs later than this, there is
no certainty that the packet transmission on the network is successful.
Limiting the collision domain diameter to 512 bit times ensures that all
collisions do occur in an acceptable time frame.
The table below
demonstrates the statistics for both Ethernet and Fast Ethernet:
Table 1.1
|
Description
|
10Mbps Ethernet
|
100Mbps Fast Ethernet
| |
Seconds
|
Bit times
|
Seconds
|
Bit times
| | Bit time |
100ns
|
1
|
10ns
|
1
| | Collision constraint |
51200ns
|
512
|
5120ns
|
512
| | Round trip cable delay |
27900ns
|
279
|
2337ns
|
234
| | Remaining budget |
23300ns
|
233
|
2783ns
|
278
| |
Maximum repeaters:
10Mbps Ethenet - 4; 100Mbps Fast Ethernet - 2
|
The limitations on
Fast Ethernet are the focus of this document. As you can see from the
table above, distances in Fast Ethernet are limited to 1/10 the size of
Ethernet. The design and layout of such a network must be carefully
planned. While the round trip cable delay is easy to understand, it is
helpful to see how these delay times affect the network's size.
Table 1.2
|
Item
|
Bit times
| |
Class I
repeater
|
140
| |
Class II
repeater
|
92
| |
DTE
|
50
| |
1 meter
UTP
|
1.11
| |
1 meter
fiber
|
1
|
The collision domain
limitations will make it impossible to upgrade many networks from 10Mbps
Ethernet to 100Mbps Fast Ethernet without making significant changes to
the topology of the network. Even fiber optics will only give a marginal
gain to total network size when used in a collision domain. Above and
beyond the collision restrictions are the media imposed limitations of
fiber optic cable and twisted pair wiring. Over multi mode fiber 2Km can
be achieved in a full-duplex environment and the 100 meter limitation
remains the same over copper wiring. The difference is attributed to the
lower signal loss of fiber optic cables.
The timing constraints
for Fast Ethernet will pose some interesting problems for network
administrators. While it is easy to draw out a network on a sheet of
paper, in real life there are numerous obstacles to overcome. In Fast
Ethernet there are two classes of repeaters. These are divided according
to the delay that they introduce into the network. The fastest of the
two, the Class II repeaters must have no more than 92 bit times of delay.
The second class, the Class I are restricted to 140 bit times or
less.
These two simple
illustrations demonstrate two repeater classes' largest Fast Ethernet
collision domains.
Illustration 1.3
This is the maximum network size for the Class II repeater. Class II
repeaters are the most common type of Fast Ethernet repeater.
Illustration 1.4
Class I repeaters do not allow for the use of more than one repeater
in a collision domain.
Collision domain size is
a major consideration for any Fast Ethernet network. While Fast Ethernet
shares the same CSMA/CD protocol as Ethernet, the distance limitations for
collision domain diameter are one tenth as large. In the 10mbps Ethernet
environment these distances are rarely pushed to the limit. However, as
speeds increase the distances and repeater limitations will make using Fast
Ethernet over existing cable plant a challenge. If you are planning to use
Fast Ethernet in the future keep in mind the following guidelines.
-
Fast Ethernet
collision domain diameters are limited to 412 meters over fiber and 205
meters over twisted pair
-
The collision domain
timing limit is a round trip figure of 512 bit times
-
To extend the
distance limitation the collision domain must be separated or
bridged
| 
