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7. GROUPWORKING
7.7 Layer 2: data link layer
The purpose
of
a data link layer is to
provide a
transmission-
facility
which
offers a
channel
to the network layer that is free
of
transmission
errors.
In our
case
layer 2 has the following functions:
•
Error-handling: a
simple check-sum
is
added
to the layer
2
datagram. The
checksum
is
established by
a 2'complement
of
a
simple byte arithmetic check
modulus
256.
• Message
assembling/
recognition:
create and
recognize frame
boundaries.
The format for layer
2
consists of:
•
Headerbyte: is
always FF
(hex.)
•
Length bytes
(3
bytes):
size of
the layer
3
data.
•
Layer
3
data
•
Stopbyte: is
always
00
(hex.)
7.8 Layer 3: network layer
An important
aspect of
the network layer
is
to
route messages
from
source
to destination.
Within a
broadcast
environment
routing
is
very simple
because unique
addressing
is not needed.
The main
goal of
the
CD-i
network is to broadcast
data,
but for
future use unique addressing
must be
possible.
Therefore
the
network layer
for the CD-i
network is needed.
The format of
a
message
within
the CD-i protocol is
always
the
same.
The
message always starts with a
destination
address
followed by
the address of
the
source station.
When
you want
to broadcast
a message
to
all stations you
must
use destination identification 0.
7. 9 Layer 4: transport layer
The
main function
of
the transport layer,
according
to
the
OSI-
model, is to receive data from the
session
layer
and
to
split
this
data into
smaller
parts
and offer
this to the network layer.
In
our case
there is no need to
split
the data into smaller parts
so we
disable the function
of
the transport layer.
20
7. IO Layer 5: session layer
The purpose
of
the
session layer is to
handle
the dialogues
between
the
current station and all
the
other stations
.
In
our
case
the
session
layer
has
to interpret
all
received
commands
and
handle these
commands correctly.
The format of
the
session
layer
consists of four sections: kind of
message,
command,
data
and
kind
of error.
7. I I
Summary
The layout
of
the messages
on
the
different
OSI
layers is
summarized
in the figure below.
Layer
5:
Layer
4:
Layer
3:
Layer
2:
Layer
1:
header source
de.st.
kind of com-
byte
identif.
kientil.
mess. mand
header source
dest
byle
idontif.
idontlf.
header
source dest
byle
idontif.
idontif.
1
~
~1
FIGURE
3
.
Network OSI
data concerning the command
~~~~t
=·
~°:
layer Sdata
---
sum
by1e
layer4 data
c:hec:k-
S1op
sum
by1e
layer
3 data
1
=
-
1 ~
physical data
I
I
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