User guide

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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:
• 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).
• 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-y’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.
• Fragment-free is a hybrid of the above two
methods.Itwaitsuntiltherst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.
• 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 AdderLink Infinity 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.
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standardisedwaytocategorisethe
necessary functions of any standard network.
TherearesevenlayersintheOSImodelandthese
define the steps needed to get the data created by
you(imaginethatyouareLayer8)reliablydown
LAYER
7L
AYER 7
LAYER
6L
AYER 6
LAYER
5L
AYER 5
LAYER
4L
AYER 4
LAYER
3L
AYER 3
LAYER
2L
AYER 2
LAYER
1L
AYER 1
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network connection
SowhyareLayer2andLayer3ofparticular
importance when discussing AdderLink Infinity?
Because the successful transmission of data relies
upon fast and reliable passage through network
switches – and most of these operate at either
Layer2orLayer3.
The job of any network switch is to receive each
incoming network packet, strip away only the first
few wrappers to discover the intended destination
then rewrap the packet and send it in the correct
direction.
In simplified terms, the wrapper that is added
atLayer2(bythesendingsystem)includesthe
physical address of the intended recipient system,
i.e. the unique MAC address (for example,
09:f8:33:d7:66:12)thatisassignedtoevery
networking device at manufacture. Deciphering
recipients at this level is more straightforward
than at Layer 3, where the address of the recipient
is represented by a logical IP address (e.g.
192.168.0.10)andrequiresgreaterknowledge
of the surrounding network structure. Due to
their more complex circuitry, Layer 3 switches are
moreexpensivethanLayer2switchesofasimilar
build quality and are used more sparingly within
installations.
onto the transmission medium (the cable, optical
fibre, 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.
As your data travel down from you towards
the transmission medium (the cable), they are
successively encapsulated at each layer within a
new wrapper (along with a few instructions), ready
fortransport.Oncetransmissionhasbeenmade
to the intended destination, the reverse occurs:
Each wrapper is stripped away and the instructions
examined until finally only the original data are left.