Collisions and Collision Domains in Shared Layer Environments

Some networks are directly-connected; all hosts share Layer 1. Examples are:

·         shared media environment - occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire, optical fiber, or share the same airspace, they all share the same media environment. Occasionally you may hear someone say "all the computers are on the same wire" . It means that they all share the same media - even though the "wire" might be CAT 5 UTP, which has four pairs of wire.

·         extended shared media environment - is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple-access, or more users. There are, however, negative aspects to this as well as positive aspects.

·         point-to-point network environment - is most widely used in dial-up network connections, and is the one with which you are most likely familiar. It is a shared networking environment in which one device is connected to only one other device via a link, such as you connecting to internet service provider by phone line.

Some networks are indirectly-connected, meaning that some higher layer networking devices and/or some geographical distance is between the two communicating hosts. There are two types:

·         circuit-switched - an indirectly-connected network in which actual electrical circuits are maintained for the duration of the communication. The current telephone system is still, in part, circuit-switched, although the telephone systems in many countries is now concentrating less on circuit-switched technologies.

·         packet-switched - rather than dedicating a link as an exclusive circuit connection between two communicating hosts, the source sends messages in packets. Each packet contains enough information for it to be routed to the proper destination host. The advantage is that many hosts can share the same link; the disadvantage is that conflicts can occur.

Collisions and Collision Domains in Shared Layer Environments

Some networks are directly-connected; all hosts share Layer 1. Examples are:

·         shared media environment - occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire, optical fiber, or share the same airspace, they all share the same media environment. Occasionally you may hear someone say "all the computers are on the same wire" . It means that they all share the same media - even though the "wire" might be CAT 5 UTP, which has four pairs of wire.

·         extended shared media environment - is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple-access, or more users. There are, however, negative aspects to this as well as positive aspects.

·         point-to-point network environment - is most widely used in dial-up network connections, and is the one with which you are most likely familiar. It is a shared networking environment in which one device is connected to only one other device via a link, such as you connecting to internet service provider by phone line.

Some networks are indirectly-connected, meaning that some higher layer networking devices and/or some geographical distance is between the two communicating hosts. There are two types:

·         circuit-switched - an indirectly-connected network in which actual electrical circuits are maintained for the duration of the communication. The current telephone system is still, in part, circuit-switched, although the telephone systems in many countries is now concentrating less on circuit-switched technologies.

·         packet-switched - rather than dedicating a link as an exclusive circuit connection between two communicating hosts, the source sends messages in packets. Each packet contains enough information for it to be routed to the proper destination host. The advantage is that many hosts can share the same link; the disadvantage is that conflicts can occur.

Collisions and Collision Domains in Shared Layer Environments

A situation that can occur, when two bits propagate at the same time on the same network, is a collision. A small, slow network could work out a system that allowed only two computers to send messages, each agreeing to take turns. That would mean that they could both send messages, but there would be only one bit on the system. The problem is that many computers are connected to large networks, each one wanting to communicate billions of bits every second. It's also important to remember that the "bits" are actually packets containing many bits.

Serious problems can occur as a result of too much traffic on a network. If there is only one cable that interconnects all of the devices on a network, the possibility of conflicts with more than one user sending data at the same time is very high. The same is true if segments of a network are only connected by non-filtering devices, such as repeaters. Ethernet allows only one data packet to access the cable at any one time. If more than one node attempts to transmit at the same time, a collision occurs, and the data from each device suffers damage .

The area within the network, where the data packets originated and collided, is called a collision domain, and includes all shared media environments. One wire may be connected to another wire through patch cables, transceivers, patch panels, repeaters, and even hubs. All of these Layer 1 interconnections are part of the collision domain.

Collisions and Collision Domains in Shared Layer Environments

When a collision occurs, the data packets that are involved are destroyed, bit by bit. In order to avoid this problem, the network should have in place a system that can manage the competition for the medium (contention). For example, a digital system can only recognize two voltage, light, or electromagnetic wave states. Therefore, in a collision, the signals interfere, or collide, with each other. Just as two cars cannot occupy the same space, on the same road, at the same time, neither can two signals occupy the same medium, at the same time.

Collisions and Collision Domains in Shared Layer Environments

Normally, people think that collisions are bad because they decrease network performance. However, a certain amount of collisions are a natural function of a shared media environment (i.e. collision domain). This is because large numbers of computers are all trying to communicate with each other at the same time, by using the same wire. 

The history of how Ethernet handles collisions and collision domain dates back to research at the University of Hawaii. In its attempts to develop a wireless communication system for the Islands of Hawaii, university researchers developed a protocol called Aloha. This protocol was instrumental in the development of Ethernet.

Collisions and Collision Domains in Shared Layer Environments

As a networking professional, one important skill is the ability to recognize collision domains. If you connect several computers to a single medium that has no other networking devices attached, you have a shared-access situation, and you have a collision domain. Depending on the particular technology used, this situation limits the number of computers that can use that part of the medium, also called a segment.

Collisions and Collision Domains in Shared Layer Environments

Repeaters regenerate and retime bits, but they cannot filter the flow of traffic that pass through them. Data (bits) that arrive at one port of a repeater are sent out on all other ports. Using a repeater extends the collision domain, therefore, the network on both sides of the repeater is one larger collision domain.

Collisions and Collision Domains in Shared Layer Environments

You have already learned that another name for a hub is a multiport repeater. Any signal that comes in one port of the hub is regenerated, retimed, and sent out every other port. Therefore, hubs, which are useful for connecting large numbers of computers, extend collision domains. The final result is diminished network performance if all the computers on that network are demanding large bandwidths, simultaneously.

Collisions and Collision Domains in Shared Layer Environments

Both repeaters and hubs are Layer 1 devices, therefore they perform no filtering of network traffic. Extending a run of cable with a repeater, and ending that run with a hub, results in a larger collision domain.

Collisions and Collision Domains in Shared Layer Environments

The four repeater rule in Ethernet states, that no more than four repeaters or repeating hubs can be between any two computers on the network. To assure that a repeated 10BASE-T network will function properly, the following condition must be true: (repeater delays + cable delays + NIC delays) x 2 < maximum round-trip delay. Repeater delays for 10BASE-T are usually less than 2 microseconds per repeater; cable delays are near 0.55 microseconds per 100 m trip; NIC delays are about 1 microsecond per NIC; and the maximum round-trip delay (the 10BASE-T bit time of 0.1 microseconds times the minimum frame size of 512 bits) is 51.2 microseconds. For a 500 m length of UTP connected by 4 repeaters (hubs) and 2 NIC delays the total delay would be well below the maximum round-trip delay. Repeater latency, propagation delay, and NIC latency all contribute to the 4-repeater rule. Exceeding the four repeater rule can lead to violating the maximum delay limit.

When this delay limit is exceeded, the number of late collisions dramatically increase. A late collision, is when a collision happens after the first 64 bytes of the frame are transmitted. The chipsets in NICs are not required to retransmit automatically when a late collision occurs. These late collision frames add delay referred to as consumption delay. As consumption delay and latency increase, network performance decreases. This Ethernet rule of thumb is also known as the 5-4-3-2-1 rule. Five sections of the network, four repeaters or hubs, three sections of the network are "mixing" sections (with hosts), two sections are link sections (for link purposes), and one large collision domain.

Collisions and Collision Domains in Shared Layer Environments

Although repeaters and hubs are useful, inexpensive networking devices, they extend collision domains. If the collision domain becomes too large, this can cause too many collisions and result in poor network performance.   The size of collision domains can be reduced by using intelligent networking devices that break up the domains. Examples of this type of networking device are bridges, switches, and routers. This process is called segmentation.

A bridge can eliminate unnecessary traffic on a busy network by dividing a network into segments and filtering traffic based on the station address. Traffic between devices on the same segment does not cross the bridge, and does not affect other segments. This works well as long as the traffic between segments is not too heavy. Otherwise, the bridge can become a bottleneck, and actually slow down communication.