MEDIUM ACCESS CONTROL
This
algorithm is commonly called the Ethernet‟s media
access control (MAC). It is typically implemented in hardware on the network
adaptor.
FRAME FORMAT:
Preamble
allows the receiver to synchronize with the signal. Both the source and
destination hosts are identified with a 48-bit address. Each frame contains up
to 1,500 bytes of data. A frame must contain at least 46 bytes of data, even if
this means the host has to pad the frame before transmitting it. Each frame
includes a 32-bit CRC.
ADDRESSES:
It is
usually burned into ROM. Ethernet addresses are typically printed in a form
humans can read as a sequence of six numbers separated by colons.
Each
number corresponds to 1 byte of the 6-byte address and is given by a pair of
hexadecimal digits, one for each of the 4-bit nibbles in the byte; leading 0s
are dropped.
To ensure
that every adaptor gets a unique address, each manufacturer of Ethernet devices
is allocated a different prefix that must be prep-ended to the address on every
adaptor they build.
UNICAST
MULTICAST
BROADCAST
TRANSMITTER ALGORITHM:
The
receiver side of the Ethernet protocol is simple; the real smarts are
implemented at the sender‟s side.
The transmitter algorithm is defined as follows:
When the
adaptor has a frame to send and the line is busy, it waits for the line to go
idle and then transmits immediately.
The
Ethernet is said to be a 1-persistent protocol because an adaptor with a frame
to send transmits with probability 0<=p<=1 after a line becomes idle, and
defers with probability q=1-p. Because there is no centralized control it is
possible for two (or more) adaptors to begin transmitting at the same time,
either because both found the line to be idle or because both had been waiting
for a busy line to become idle.
When this
happens, the two (or more) frames are said to collide on the network. Each
sender, because the Ethernet supports collision detection, is able to determine
that a collision is in progress. At the moment an adaptor detects that is frame
is colliding with another, it first makes sure to transmit a sure to transmit a
32-bit jamming sequence and then stops the transmission.
Thus, a
transmitter will minimally send 96 bits in the case of a collision: 64-bit
preamble plus 32-bit jamming sequence. One way that an adaptor will send only
96-bits which is sometimes called a runt frame is if the two hosts are close to
each other. Had the two hosts been farther apart, they would have had to
transmit longer, and thus send more bits, before detecting the collision.
In fact,
the worst-case scenario happens when the two hosts are at opposite ends of the
Ethernet. To know for sure that the frame it just sent did not collide with
another frame, the transmitter may need to send as many as 512 bits.
Not
coincidentally, every Ethernet frame must be at least 512 bits (64 bytes)long:
14 bytes of header plus 46 bytes of data plus 4 bytes of CRC.
Where
hosts A and B are at opposite ends of the network. Suppose host A begins
transmitting a frame at time t, as shown in (a). it takes it one link latency
(let‟s denote the latency as d) for
the frame to reach host B.
Thus, the
first bit of A‟s frame
arrives at B at time t+d, as shown in (b). Suppose an instant before host A‟s frame arrives (i.e., B still
sees and idle line), host B begins to transmit its own frame.
B‟s frame will immediately collide
with A‟s frame, and this collision will
be detected by host B(c). host B will send the 32-bit jamming sequence, as
described above.(B‟s frame
will be a runt).
Unfortunately,
host A will not know that the collision occurred until B‟s frame reaches it, which will
happen one link latency later, at time t+2xd, as shown in (d). Host A must
continue to transmit until this time in order to detect the collision. In other
words, host A must transmit for 2xd should be sure that it detects all possible
collisions.
Considering
that a maximally configured Ethernet is 2,500 m long, and that there may be up
to four repeaters between any two hosts, the round-trip delay has been
determined to be 51.2 microseconds, which on a 10-Mbps Ethernet corresponds to
512 bits.
The other
way to look at this situation is that we need to limit the Ethernet‟s maximum latency to a fairly
small value (e.g., 512micro seconds) for the access algorithm to work; hence,
an Ethernet‟s maximum
length must be something on the order of 2,500m.
Once an
adaptor has detected a collision and stopped its transmission, it waits certain
amount of time and tries again. Each time it tries to transmit but fails, the
adaptor doubles the amount of time it waits before trying again.
This
strategy of doubling the delay interval between each retransmission attempt is
a general technique known as exponential back off. More precisely, the adaptor
first delays either 0 or 51.2 microseconds, selected at random. If this effort
fails, it then waits 0, 51.2, 102.4, or 153.6 microseconds (selected randomly)
before trying again; this is kx51.2 for k=0...2^3-1, again selected at random.
In
general, the algorithm randomly selects a k between 0 and 2^n-1 and waits
kx51.2 microseconds, where n is the number of collisions experienced so far.
The adaptor gives up after a given number of tries and reports a transmit error
to the host. Adaptor typically retry up to 16 times, although the back off
algorithm caps n in the above formula at 10.
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