- A series
of frames
(or messages in general)
is
received reliably
if and only if:
- Each frame (messages)
is received
exactly once
by the receiver
- The frames (messages) were delivered in the same order as transmitted
- Each frame (messages)
is received
exactly once
by the receiver
- Received:
- A frame is received = the frame has arrived at the receiver without errors
(Lost = not received)
- Delivered:
- A (data) frame is
delivered =
the (data) frame has been
passed on to
the user (program) for
further processing
- A (data) frame is
delivered =
the (data) frame has been
passed on to
the user (program) for
further processing
- Important note:
- A data frame can be
received but
not delivered
Example:
- A transmits
4 frames to
B:
Frame 3 suffered some bit errors (and will be discarded)
- Then:
frames 1, 2, 4 have been
received:
- But
only
the frames 1, 2 can be
delivered:
because we must deliver the frame #3 before we can deliver the frame #4 !!!
- A transmits
4 frames to
B:
- A data frame can be
received but
not delivered
- Types of
errors:
- Frame loss:
- The frame was not received or
- The frame was
received,
but contains
bit errors
(so it was discarded)
- Duplicate reception:
- The same frame was received more than once
- Out-of-order reception:
- Frames were received in a different order than the transmission order
- Frame loss:
- Example:
- A transmits
frame 1 and was
received:
- B sends back
the acknowledgement for
frame 1,
but the acknowledgement is
lost:
- A waits for
the acknowledgement....
(that is not gonna come)
- A will
the time out.... and
retransmit the frame:
This frame 1 will be a duplicate frame
- A transmits
frame 1 and was
received:
- Example:
- A transmits
2 frames:
- Frame 1 is
lost:
- B receives
frame 2
first:
- A times out and
transmits:
B will receive the frame 1, 2 in the wrong order
- A transmits
2 frames:
- Fact 2:
- A series of frames (messages) transmitted over
multiple links
can
be received
in a different
order as transmitted:
(An later frame takes a faster route to the destination...
The different frames must take different routes for this to happen)
- A series of frames (messages) transmitted over
multiple links
can
be received
in a different
order as transmitted: