CSC 311. CHAPTER TEN CONNECTING NETWORKS. We have looked at several different network topologies Why do we have different types of networks? Why do we create different, distinct LAN arrangements? How do we connect them, so LANs can communicate with other LANs and WANs?. CSC 311.
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Why do we have different types of networks?
Why do we create different, distinct LAN arrangements?
How do we connect them, so LANs can communicate
with other LANs and WANs?
what we are trying to accomplish?
Are we connecting segments of a LAN, similar LANs, dissimilar LANs,
or wide area networks?
Contrary to what the author says, repeaters do not connect LANs, they connect
segments of a LAN. The figure shown below would, logically, behave as
a single Local Area Network.
All devices, on any of these so-called LANs lie in the same collision
domain, and thus, on the same LAN.
Networks today commonly use hubs and their star configuration,
nevertheless, they constitute a single LAN
For this purpose, we might use a bridge rather than a repeater.
To use a bridge, all of these LANs must be of the same type.
A bridge does not alter the frames it transmits, all networks would
be using the same MAC protocol. It is not technically correct to refer
to a device connecting two dissimilar LANs as a bridge, it is more of
a router. It does alter frames and must have the MAC protocol of both
types of networks.
frame, or whether it should be forwarded at all?
Bridges use routing tables, these can be installed on the bridge
by the systems manager or it can be a transparent bridge that will
“learn” its routing table based on the traffic.
18. 10 pts. Indicate the contents of the bridge forwarding table given the following traffic. Indicate after each message, what action the bridge would take and the current contents of the routing table.
A B C
certain configurations can cause difficulty.
What would the two routing tables look like if B sent a packet to A?
Often there may be multiple paths with different costs associated as
shown in the figure below.
In the absence of some learning protocol, this network would be
flooded with copies of frames
To solve this problem, we use a spanning tree algorithm.
From your data structures: A spanning tree corresponds to a minimal
subset of edges taken from a connected graph that connects the graph’s
We must assign a cost associated with each bridge to LAN connection, we
can then produce the following graph.
We won’t go into the details of how to use the spanning tree algorithm.
We would produce the following spanning tree.
the bridges shown connected by dotted lines are “blocking” bridges,
they will not forward packets.
Routing tables loaded into each bridge by the system administrator
Transparent bridges that “learn” the routes
ADAPTIVE ROUTING, ROUTE LEARNING, ETC.
A third method is SOURCE ROUTING:
The source specifies the route to the destination, routing info
is included in the packet.
Since Ethernet is the dominant LAN, we will assume, hereafter, that
all of our LANs are Ethernets.
Switches perform the same function as bridges.
While bridges typically connect a couple of LANs, switches may
have a couple of dozen ports which provides many more connections.
Note, that there
is only one switch
in this drawing,
hubs, with no
FULLY SWITCHED ETHERNET
Note: there are no collisions in this topology so CSMA/CD is not needed.
Use the “intelligence” of the switch to create Virtual LANs that are
really part of a single physical LAN. You might use this approach
to isolate different workgroups working on different projects
without having to physically separate them on separate LANs.
Layer Three Connections
LANs and the layer 1 and 2 connections typically cover
small geographic areas
Wide Area Networks (WAN) span the globe and need more
Much like the highway system, internets or the Internet provide
multiple paths from one point in the Internet to another. We
need a technique to select the best among these many
In the highway example, what do you do if a bridge is out?
Find an alternate route.
Networks are frequently faced with the same problem when one
of the nodes in the network fails or becomes congested.
Generalized Network Topology …. many routes
way to evaluate those routes. We generally associate a cost with each
link in the route… such as shown below.
Routing tables can be developed using this information to specify
the optimum route between any two points in the network.
the entire route to a destination, rather, the next node in the route
to that ultimate destination
This is a partial routing table for nodes A, B, and E
Who defines these routing tables and how?
The process by which a routing table is defined is called a routing algorithm
No centralized control.
Each node determines and maintains its routing information
Nodes exchange control information
Once a node determines its routing table, the node does not change it.
Nodes respond to changes in the network and update their routing
Large networks, such the Internet, can be viewed as a collection of
In such networks, we define two main categories of routing strategies:
interior and exterior
Interior routing protocols control routing among routers within an
autonomous system or domain
Exterior routing protocols control routers among routers from
different autonomous systems.
To send a
packet from X to
A, we would
use an interior
To send a packet from Z
to X, Z sends to its router
W, which sends across
subdomains to C, which
sends to A, which sends
Internet addresses are 32 bit numbers, usually shown in dotted decimal
126.96.36.199 is an example, each of the decimal numbers can be
represented by 8 bits.
The Internet uses a class address system, each such address can be
interpreted as having two parts: an Internet Protocol (IP) address which
is assigned to the network and a local device address.
The address above is an example of a class B address, the first 16 bits
(143.200), is the IP network address and the other 16 bits (128.3)
represents the address of the particular device on that network.
A class B address provides addresses for over 65,000 possible users,
so it is quite common to further subdivide these users into subnets.
Using subnet addressing, the local management might designate part of
the lower 16 bits to represent a subnet address.
Another interior routing protocol:
Open Shortest Path First (OSPF)
In this case, the “open” means the algorithm is not proprietary
can also use other factors in determining best route such as delay, bit rates,
actual dollar cost.
Border Gateway Protocol:
current version is BGP-4
used in the Internet to establish paths among routers in different
Often concerned with just getting to destination, not necessarily the cheapest
route. There may be other considerations, such as avoiding a particular
country in international traffic.
Within AS3, an interior routing protocol would be used. In leaving AS3, an exterior
routing protocol would be used. The gateway routers (A,B,C,F) , would share
information about their subnetworks.
Situation illustrated above is reassembly deadlock:
Buffers are full, packet 0 from both A and B cannot be accepted.
Reassembly deadlock can be prevented by establishing and reserving
sufficient buffer space to handle the window size for a particular
Store and forward is handled by allocating sufficient buffer space,
but… how much buffer space is enough?
One approach to deadlock is to allow it to occur and then deal with it.
It might be more cost effective to either prevent deadlock or take steps
to reduce the probability of occurrence.
Nodes divide their buffer space into groups based on number of hops,
hop count on a packet is incremented after each hop, a packet must wait
on available space in its hop count group at the previous node, so a packet is
always waiting on a higher numbered group.