- We will now consider the
problem of
inter-connecting
heterogeneous networks.
- Example:
- Problems
- The identification problem:
- Different networks
use different
network numbers (identification)
- How can we
uniquely identify a
node on the
entire network:
(Computers need to use the same type of identifiers !!!)
- Different networks
use different
network numbers (identification)
- The heterogeneity problem:
- Different networks
use different frame formats
(Computers need to use the same format (= code) !!!)
- How can we make different networks agree on the same encoding ???
- Different networks
use different frame formats
- Scalability (same issue as in a homogeneous network)
- The identification problem:
- Fact:
- Every Ethernet address
is 48 bits
- Every Ethernet network card that has been made has a unique Ethernet (network) address
- Every Ethernet address
is 48 bits
- Therefore:
- Nodes on different Ethernet
networks have
network addresses that are
48 bits long
(we know what to use to identify a node)
- Each node (even on different networks) is uniquely identified by its Ethernet Address
Graphically illustrated:
- Nodes on different Ethernet
networks have
network addresses that are
48 bits long
(we know what to use to identify a node)
- Fact:
- An Ethernet Address is 48 bits
- A Token bus address is also 48 bits
Claim:
- Even when the address format are identical, we cannot use the Ethernet/Token bus address as identifiers !!!
Reason:
- Two (2) different nodes on the
heterogeneous network can
have the same address !!!
Example:
An Ethernet/Token bus address is no longer unique !!!
- Analogy:
- The licence plate of a
car is
unique within
one state
(e.g., some imaginary Georgia licence plate
XXX YYY is
unique in
Georgia
- But this is not true for licence plates in two different states
- The licence plate of a
car is
unique within
one state
(e.g., some imaginary Georgia licence plate
XXX YYY is
unique in
Georgia
- In a homogeneous network:
- In homogeneous networks,
all frames has
the same format:
- In homogeneous networks,
node uses the same encoding to
interprete meaning:
- In homogeneous networks,
all frames has
the same format:
- The different frame formats
used in
heterogeneous networks
will cause a Babel problem:
- In heterogeneous networks,
the fields in the
frame can be
interpreted
incorrectly:
- In heterogeneous networks,
the codes in the
control field can be
interpreted
incorrectly:
- In heterogeneous networks,
the fields in the
frame can be
interpreted
incorrectly:
- Analogy:
- Each type of
network
"speak" a
different language
- The interconnecting device
must be
smarter than
a ordinary bridge because
the
interconnecting device must perform:
- Protocol translation !!!
- Each type of
network
"speak" a
different language
- Recall: Flooding:
- When an Ethernet switch does
not know the
location of
a destination:
the Ethernet switch will flood all networks:
- When an Ethernet switch does
not know the
location of
a destination:
- Scalability:
- Network operation must minimize the use of flooding !!!
- Summary of solutions:
- Establish a
uniform addressing method
(and assignment):
- We need a world-wide Social Security Number (identification) system for every computer on the Internet
- Establish a
common packet format
- I.e.: we need a network "Esperanto" language spoken by every computer on the Internet !!!
- Minimzie
the use of
flooding:
- Avoid the use of
promiscuous interfaces
(which encourage broadcasting)
- Retain (simplified) forwarding information to all destinations on the Internet
- Avoid the use of
promiscuous interfaces
(which encourage broadcasting)
- Establish a
uniform addressing method
(and assignment):
- We will discuss
each problem/solution
in more details next
(Not neccessary in the order of the problems presented above)