Black Box Agility Zero-U ACR500DP-T, User Manual, Page: 39 / 59

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E.4 FORWARDING MODES
In essence, the job of a layer 2 switch is to transfer as fast as possible, data packets arriving at one port out to another port as
determined by the destination address. This is known as data forwarding and most switches offer a choice of methods to achieve
this. Choosing the most appropriate forwarding method can often have a sizeable impact on the overall speed of switching:
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Store and forward is the original method and requires the switch to save each entire data packet to buffer memory, run an error
check and then forward if no error is found (or otherwise discard it).
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Cut-through was developed to address the latency issues suffered by some store and forward switches. The switch begins
interpreting each data packet as it arrives. Once the initial addressing information has been read, the switch immediately
begins forwarding the data packet while the remainder is still arriving. Once all of the packet has been received, an error check
is performed and, if necessary, the packet is tagged as being in error. This checking ‘on-the-fly’ means that cut-through
switches cannot discard faulty packets themselves. However, on receipt of the marked packet, a host will carry out the discard
process.
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Fragment-free is a hybrid of the above two methods. It waits until the first 64 bits have been received before beginning to
forward each data packet. This way the switch is more likely to locate and discard faulty packets that are fragmented due to
collisions with other data packets.
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Adaptive switches automatically choose between the above methods. Usually they start out as a cut-through switches and
change to store and forward or fragment-free methods if large number of errors or collisions are detected.
So which one to choose? The Cut-through method has the least latency so is usually the best to use with Agility units. However,
if the network components and/or cabling generate a lot of errors, the Store and forward method should probably be used. On
higher end store and forward switches, latency is rarely an issue.
E.5 LAYER 2 AND LAYER 3: THE OSI MODEL
When discussing network switches, the terms Layer 2 and Layer 3 are very often used. These refer to parts of the Open System
Interconnection (OSI) model, a standardized way to categorize the necessary functions of any standard network.
There are seven layers in the OSI model (see Figure D-1) and these define the steps needed to get the data created by you
(imagine that you are Layer 8) reliably down onto the transmission medium (the cable, optical fiber, radio wave, etc.) that carries
the data to another user; to complete the picture, consider the transmission medium is Layer 0. In general, think of the functions
carried out by the layers at the top as being complex, becoming less complex as you go lower down.
Figure E-1. A representation of the seven layers defined by the OSI Model
LAYER 7 LAYER 7
LAYER 6 LAYER 6
LAYER 5 LAYER 5
LAYER 4 LAYER 4
LAYER 3 LAYER 3
LAYER 2 LAYER 2
LAYER 1 LAYER 1
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network connection