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Straight up: How the Kentucky bourbon industry is going high tech. Show Comments. Hide Comments. So, what is a collision domain or transparent bridge?
Well, if you know what a network switch is, you're halfway there. In fact, you probably know what they both are, but maybe you forgot because these terms and concepts have gone by the wayside due to today's high-speed switching. Transparent bridges were the predecessors of today's network switches and both operate on the same premise.
A transparent bridge is an interconnection device that joins two or more Layer 2 collision-domains. A collision-domain is a segment of an Ethernet LAN. Nodes connected to the segment "share" the available bandwidth to transmit Ethernet frames. It's all clear now, right? The name "transparent bridge" refers to its operation in relation to the nodes connected to the Layer 2 segment.
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The nodes connected to the segment are unaware that the bridge or bridges are connected to the segment and forwarding frames between the different collision-domains. There are two types of bridges: translation and interconnection. Translation bridges translate different Layer 2 protocols so they can exchange frames without the need of a Layer 3 device. The basic operation of a translation bridge is to extract the data payload and re-frame it so it can be delivered. Interconnection bridges "extend" the length of the LAN by interconnecting two or more collision-domains.
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Transparent bridges all work under some basic operational premises:. First, the bridge's interfaces run in promiscuous mode examining all of the frames transmitted across the connected segments. The bridge stores the frames in memory buffers to be forwarded on to the other segments. The most common transparent bridge is a "store-and-forward" bridge.
The bridge determines where to forward the frames by building a forwarding table associating the MAC addresses of the nodes on the network with the bridge port the MAC was learned on. When the bridge sees a frame transmitted on the wire, it stores it, and performs a lookup against the bridge's forwarding database. If the destination address is on the same port the frame was seen on, it discards it. If the bridge has an entry for the destination node off of an adjacent port, it forwards the packet out of the associated bridge interface.
If the bridge does not have an entry for the destination node, the frame is forwarded out of each of the bridge's ports, except for the port the frame came in on. Once the bridge has learned all of the active nodes, the bridge only forwards frames that need to be forwarded, unlike a repeater that forwards every frame that it sees. The second is that all frames are forwarded by the bridge unmodified.
When a typical Ethernet node transmits a frame on the network, it places its own MAC in the frame's source address field. Bridges, however, do not place their own MAC addressees in the source address field of the frames they forward. As far as the source and destination are concerned, the frame was directly transmitted and never went through a bridge hence the name transparent.
We understand that a collision-domain is an Ethernet-shared media segment on which multiple nodes are connected. The bandwidth of the collision-domain is shared between the connected hosts.
While this definition is accurate, it really doesn't provide insight into what a collision-domain is aside from a networking construct. To get a better understanding, we need to quickly review how Ethernet transmits data. The idea is quite simple: Multiple nodes are connected to a common transmission medium, and these nodes take turns transmitting data. Only one host can transmit data at one time.
If two hosts transmit data at the same time a collision occurs, and both hosts stop their transmissions, wait and attempt to retransmit when the line is clear. The "Collision Detection" portion of the contention protocol which behaves as a primitive flow-control mechanism making it possible for all of the hosts to share the line is a component of the Ethernet protocol that is dependent on the proper construction of the collision-domain.
Now, the physical collision-domain is defined by three factors: physical length of the segment, the length of transmission protocol's bit period which is different for each flavor of Ethernet and propagation time also different for each flavor of Ethernet. Most engineers focus on the maximum cable length between the end station and the repeater which is the basis of all of the "X" Base-T Ethernet implementations. However, this is misleading because the actual measurement of collision-domain is based on bit periods, not cable length. This is because the X Base-T connections are made up of more than just cable, there are also repeaters and the DTEs themselves, each of which has a bit-period latency.
It is the total sum of the physical components' bit-period latency that makes up the actual size of the collision-domain. The bit-period value is the time it takes to transmit one bit of data. This value is equal to the transmission rate of the medium.
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For example, the bit period for Base T Ethernet is nanoseconds, and for Base-T it is 10 nanoseconds. Propagation time is the amount of time it takes for a DTE on one end of the cable to transmit a 64 byte bit period frame to a DTE on the other end of the cable. The propagation delay of 10 Base-T Ethernet is 51, nanoseconds; for Base-T it is nanoseconds. The total size of the collision-domain cannot exceed the length of time it takes for this function to happen before the stations complete their data transmissions. Otherwise, the transmitting stations will not detect the collision in time.
The maximum size of a collision-domain is the addition of the propagation delay, or in Ethernet parlance, "slot-time," and the "space" of time a transmitting hosts places between each frame, known as the interframe gap, which is 96 in bit time. Therefore, the maximum the collision-domain length for 10 Base-T Ethernet is 60, nanoseconds, and nanoseconds for Base-T.
So you can see, the size the collision-domain is really more a measurement of time then physical length. From a design perspective, each collision-domain is the perspective of latency. The amount of tolerable latency between 10 Base-T and Base-T is drastically different.