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Topologies, Routing
and Deadlock
Various speeds of networks
1Gb
100Mbb BUS LAN
10Mb
MAN
1Mb WAN
100Kb
10Kb
EXCH
1Kb
0.1m 1m 10m 100m 1Km 10Km 100Km1000Km
Figure – Recap of speeds
Speed of LANs
The raw transmission rate of LANs are
high, typically being in the 1-1000 Mbps
range.
On some LANs (eg Open System LANs)
every device has the potential of connecting
to any other device on the LAN.
Smaller LANs typically operate on a
Slave/Master basis with slave PCs clustered
around a shared master filestore system.
Network Sharing
A fundamental feature of many LANs is the
network itself is shared.
The physical network medium is shared by
many machines.
In a traditional network, each machine is
usually wired into a switching device.
For example, your telephone is connected to
your local switching office.
Low Error Rates/Low Cost
On LANs, network errors are expected to be
relatively few when compared with larger
networks.
LANs are relatively inexpensive when
compared to the cost of the equipment that
connects to it.
However, Each Network Interface Unit
(NIU) still costs in the region of £80-£200.
WAN
A WAN (Wide Area Network) is a network
that is spread over multiple sites (>30Km).
WANs are not limited in size (they can even
cross the world).
Public facilities (such as the public switched
telephone network) are extensively used.
However, this means that the rate at which
data is sent is limited by the bandwidth of
these facilities.
WAN vs. LAN
When comparing WANs with LANs, the main
difference is in the data transmission rates.
Delay and error rate parameters are also
relevant to some applications.
We can view the technical facilities offered by
a WAN as a subset of those offered by a LAN.
What a WAN offers is long distance
connectivity.
Implementation of a
Computer Network
How easy a network is to use depends on the
sophistication of the software provided.
The most basic level of netware provision is
to only have programs designed for specific
tasks such as file transfer.
More sophisticated systems incorporate
network facilities in the operating system of
the computer (thus network operations
become a coherent part of the user interface).
The Most Sophisticated type
of Software Provision
The highest level of refinement is to regard
each computer as part of a single distributed
operating system.
This level of sophistication allows the user
to access files, programs, utilities and
resources as if they were on his or her own
computer.
Network Specifications
A network implies that all the computer can
communicate with each other.
This requirement can be met in a number of
ways but there are certain basic principles
common in most networks.
Network Structure
Each host computer communicates via a
Network Interface Unit (NIU).
By host computer we mean computers on
which users can run applications.
The term node is often used for the intelligent
interface that is part of each host computer (or
sometimes for the host computer itself).
The term node can also be used to mean
another computer with which the host
communicates (such as a server or a router).
Network Topology
The term topology refers to the way in which
the nodes of a network are connected.
The topology of a network will effect its
performance (it terms of speed) and its cost
(both short and long term).
Cost/resource considerations and the
environment in which the network is to be
used often determines the choice of topology.
Switching
Networks/Broadcast Networks
We can distinguish networks by the way in
which they transmit data.
WAN usually use switching networks to send
data from source to destination nodes.
LANs, however, often use broadcast networks
because they are cheaper to build and maintain.
Broadcast networks send all the data to all
nodes (which must out listen for the data meant
for them).
Common Topologies
Common topologies are:
– MESH
– STAR
– BUS
– TREE
– RING
– BACKBONE
Mesh Topology
A mesh topology is a network in which the
connections between nodes is random.
Fully connected Network Random Network with 7
with 5 nodes nodes
Mesh topologies include fully connected
networks and random networks (e.g. Internet).
Redundant connections in random networks
ensure that alternative routes exist for data.
Fully Connected Networks
Fully connected networks are the fastest
types of networks since each device directly
connects to every other device.
There is no time delay due to switching.
If there are N hosts in the network, we need
N(N-1)/2 bi-directional connections (e.g. 20
hosts needs 190 connections).
This is far too many connections (most of
which will be idle most of the time).
Sharing Connections
Without a fully connected network,
connections between nodes must be shared.
One way to do this is to allow nodes to
switch data through a random network.
We can view a random network as
interconnecting star networks.
Data is passed through this network, via
intermediate nodes, until it arrives at its
destination.
Star Networks
A star network consists of a special central
node (or hub node) to which host computers
or terminals are connected.
Special Central
Node
Any host computer can connect to any other
host computer via the hub.
Star Networks
The hub switches messages through to the
appropriate destination.
The hub may also provide a translation
service for devices with different protocols.
Star Networks are vulnerable, however. If
the hub fails then the network fails.
Star Networks may require a lot of cabling
and can be expensive to install.
Bus Networks
A bus network consists of a single medium
(typically 5 pair twisted-wire cable) to
which all the host computers are connected.
Host
Computers Bi-directional
medium
Network Terminator
Interface
Units
Packets are broadcasted on the medium to
all nodes on the network.
Contention
The is an obvious danger that two host
computers may attempt to use the network
medium simultaneously.
This problem is called contention and is a
problem with all Bus topologies.
The nodes must employ Medium Access
Protocols, which function in conjunction
with other nodes, to permit access only at
times when the medium is free.
Tree Networks
A tree network (as used in LANs) is a
variant of the Bus topology.
Root
Node
Host
Computers
Communications
Link
Nodes are connected in a tree structure and
messages are broadcast across whole tree.
Tree Networks
Tree topologies have the advantage that
they are easy to expand.
Furthermore, if a fault occurs, the effected
branch can be easily isolated so that the rest
of the network is not effected.
The disadvantage is that signals can be
reflected from the ends of branches and
cause interference. For this reason, Tree
Networks are usually run at lower speeds.
Ring Topology
A Ring network consists of nodes
connected to each other to forma a closed
loop.
Host
Computers Data Flow
ACTIVE CABLE
Nodes accept data from neighbouring nodes
in the form of packets.
Ring Topology Operation
The NIUs (or, in some cases, the hosts
themselves) act as repeaters for the packets
being forwarded.
This means that the Ring can be expanded to
any size (although more hops will be required
to get the packets to their destinations).
One big advantage of Ring Topologies is that
contention is avoided since each repeater knows
if it has to forward an existing packet or is free
to accept a new one.
Dealing with Contention
Contention is dealt with by either using a
slotted ring system or a token-passing ring.
With the slotted ring system, blank fixed
sized frames are passed around the network.
These frames get filled in with data as they
pass a node that wishes to transmit.
The frame goes around the entire network
and is copied by the destination node as it
passes.
Token-Passing Ring
A token-passing ring is similar to a slotted ring
except that a ‘token’ frame is passed around
the ring.
If a ‘token’ arrives at a node, it can be
exchanged for a data packet.
The data packet is sent around the entire
network and is copied by the destination node
as it passes.
When the packet comes back to the sender, the
sender puts the token back on the network.
Removing Packets/Frames
A frame, or packet, will circulate around the
network until removed.
In some networks it is the destination node
that removes it.
In other networks it is the sender that
removes it.
The advantage of getting the sender to
remove the frame is that it allows data to be
broadcast to any number of nodes.
Housekeeping
If, for some reason, a frame is not removed
by the sender it will circulate forever and
reduced the efficiency of the Ring network.
This is not a problem with Bus networks
since terminators (or Head Ends) absorb
unwanted packets.
Devices called monitors are responsible for
housekeeping by marking frames as they
pass. If a marked frame comes back round
to the monitor, it is then removed.
Backbone Network
A backbone network connects many smaller
networks via devices called bridges.
This type of network is easy to expand and
isolates local traffic.
Transmission of Data
A network implies that we wish to transmit
data from one node to other nodes.
In switched networks, we can use four
switching methods:
– Circuit Switching (e.g. POTS technology)
– Message Switching
– Packet Switching
– Virtual Circuits (e.g. ATM technology)
Circuit Switching
We have already seen circuit switching in
POTS and it is the same in computer
networks.
A connection from node A to node B is
established and the full bandwidth of that
connection is available for communication.
The connection remains in place for the
duration of the computer conversation.
Time Sequence Diagram for
Circuit Switching
We can use a time sequence diagram to see
the order of events in circuit switching.
node switch switch node
A Establish 1 2 B
Connection Establish
Connection Establish
Connection
t Acknowledge
Acknowledge
i Connection
Acknowledge Connection
m
e Connection
Send Data
Relay Data
Relay Data
Send Data
Relay Data Relay Data
Close
Connection Close
Connection Close
Connection
Message Switching
Message switching sends complete
messages from node to node until they
reach their destination.
Because messages are of variable length,
they time delay for delivery is variable.
This makes message switching unsuitable
for interactive traffic.
Large messages can also cause congestion
and overflow switches with small memory.
Time Sequence Diagram for
Message Switching
The events in message switching occur as
follows: node switch switch node
A 1 2 B
Send
Send small Message Relay
Message Message Relay
Message
t
i
m Send large
e Message
Send
Message
Relay
Message
Relay
Message
Packet Switching
Packet switching is similar to message
switching except that the messages are
broken up into small datagrams called
packets (about 1Kbyte in length).
Because packets are smaller than messages,
they do not suffer the same delays as
messages do.
This makes packet switching better for
interactive traffic.
Time Sequence Diagram for
Packet Switching
Host A node switch switch node Host B
A 1 2 B
Transmit
Send Message broken Packet 1 Packet 1
Small into packets with Packet 2 Packet 1
distinct sequence Packet 2 Packet 1
Message numbers Packet 3 Packet 2 Packets
Packet 1
Packet 3 Packet 2 reassembled into
Packet 2
Received
Packet 3 message using
Packet 3 Small
t Packet 3 sequence
i numbers to order
Message
m Transmit Packet 1
e Packet 1
Packet 1 Packet 2
Packet 1 Packet 2 Message broken Send
Packet 1 Packet 2 Packet 3
Packet 3 into packets with Large
Packet 2 distinct sequence
Packets Packet 2 Packet 3 Packet 4 Message
Packet 3 Packet 4 numbers
Received reassembled into Packet 3 Packet 4 Packet 5
Large message using Packet 4 Packet 5
sequence Packet 4 Packet 5 Packet 6
Message numbers to order Packet 5 Packet 6
Packet 5 Packet 6
Packet 6
Packet 6
The time delay between sending first packet
and its delivery is much less.
Virtual Circuit
A Virtual Circuit is similar to packet
switching with the exception that all the
packets follow the same route.
The route, or virtual circuit, is established
in a similar way as a connection of a circuit
switched network.
The difference is that connections are
shared between many virtual circuits.
Time Sequence Diagram for
Virtual Circuit (LLC-2) Host B
Host A node switch switch node
A Establish 1 2 B
L_connect_req Connection Establish
Connection Establish
Connection L_connect_req
tim Acknowledge L_connect_
e Acknowledge Connection confirm
Acknowledge Connection Indicate Status L_connect_
L_connect_ Connection Indicate Status indication
confirm
Indiate Status
L_connect_
indication
Send Data
L_data_ Relay Data
connect_req L_data_
Relay Data connect_req
L_connect_dat
L_connect_dat Confirm Data
Confirm Data a_confirm
a_confirm Confirm Data
Indicate Status L_connect_
L_connect_ indication
Indiate Status Indicate Status
indication
L_disconnect_
req Close
Connection Close
Connection Close
L_disconnect_
Connection
req
L_disconnect_
L_disconnect_ Disconnect
Disconnect confirm
confirm Disconnect
Network Control
In a switched network, we need to have
mechanisms for controlling the movement
of data.
There are three main aspects to Network
Control:
– Routing
– Flow Control
– Error Control
Routing
Routing is the process of directing data
around the network.
Ideally we want to direct as much data from
the source to the destination with the
minimum delay.
How well this can be done depends a great
deal on the topology of the network .
In a simple network (such as a star
topology) there is little difficulty.
Routing in a Mesh Network
Routing becomes complicated in a random
mesh network (as widely used in WANs).
One widely used method uses routing tables
to decide the best direction to send data.
2
5
End Device
1
End Device B
3
A
Routing Table for 4 6
Node 1 Routing Table for
A5 3 2 Node 3
A2 2 A5 5 6
3
A6 4 A2 5 1
… … 3 A6 6 5
… … … …
Packet
Destination Next Node
Address Alternative
Next Node
Routing Packets (or Frames)
Packets (or Frames) enter the network with
a destination address field and a source
address field.
At each node, the destination address is
used to look up an entry in a routing table.
The routing table tells the node which is the
preferred direction to send the data.
A alternative direction is usually also
provided in case the first is unavailable.
Routing Tables
The routing tables obviously need to be
filled in.
There are a number of approaches for doing
this.
There are two basic classes of routing
algorithm:
– shortest-path algorithms try to find the least
cost path between source and destination.
– bifurated routing algorithms attempt to evenly
spread out the load across the entire network.
Effects of Routing on Quality
of Service (QOS)
Routing effects two Quality of Service
(QOS) parameters: transit delay and
throughput.
Networks are often described in terms of
common qualitative and quantitative
parameters known collectively as QOS
parameters.
QOS parameters are worth a brief
discussion in themselves.
Quality of Service
There are 8 parameters commonly used to
describe networks (typically WANs).
– Connection establishment delay is the time it
takes between a request for a connection from
host A to host B and receiving confirmation.
– Connection establishment failure probability is
the chance that a connection is not confirmed
within a reasonable time.
– Throughput is the amount of user data (usually
in bytes) successfully transmitted from A to B
every second.
Quality of Service
– Transit delay is the time between sending data
and it being received by the destination host.
– Residual error ratio is the proportion of packets
that are lost or garbled.
– Protection is how easy it is for a third party to
listen to or corrupt data in the network.
– Priority is if it is possible to earmark some
connections as having a greater priority than
others.
– Resilience is the probability that a connection is
terminated due to congestion or network faults.
Quality of Service for LANs
Although LANs can use the same
parameters for Quality of Service, their
small size and general reliability means that
greater emphasis is placed on parameters
such as priority.
Measuring Throughput and
Transit Delay
We can measure throughput and transit
delay by examining acknowledgements sent
back to the source host from the destination
host (this is called feedback).
The number of acknowledgements received
back in a second will tell us the throughput
and the difference in the time fields of the
packet (or frame) and the acknowledgement
tells us the transit delay.
Feedback in Flow Control
To avoid congesting various parts of the
network, the number of packets offered to
any sub-net should match its throughput.
Delay
Offered Flow Control Throughput Routing
Load
Rejected
Load
Feedback can be used to dynamically
control the number of packets (or frames)
given to the sub-net. This is flow control.
Transit Delay vs. Throughput
Ideally we want the throughput to equal the
offered load. Excessive offered loads result
in a portion of the packets (or frames) being
rejected (either discarded or sent via an
alternative route).
Achieving maximum throughput in a
network often comes at the cost of greater
transit delay. Flow control is usually
designed to find a balance between them.
Effects of Good Routing
While precise balance between throughput
and transit delay is determined by flow
control, the effects of good routing within a
network are to improve throughput and
reduce transit delay.
More Flow
Delay Control Pressure
(microseconds)
Poor Routing
Good Routing
Throughput (packets per second)
Deterministic Routing
If the routing tables used by the nodes are
fixed, then the routing method is known as
fixed routing or deterministic routing.
Here routing tables are change only
occasionally (typically when there has been
a change to the network).
Any modifications to the tables are usually
managed centrally or from regional centres.
There is an advantage to this since routes
can be assigned to users as they log on.
Adaptive Routing
An improved scheme is to use traffic flow
data on neighbouring nodes in order to
dynamically decide the best route to send
packets (or frames).
This approach reduces the chance of
packets being rejected.
The snag is that the traffic flow data itself
has to be transmitted and this traffic reduces
the amount of user data that can be sent.
Isolated Adaptive Routing
Despite the snag adaptive routing is used in
many WANs. One form of adaptive routing
measures queue sizes and (together with a
weight) chooses to use alternative routes on
a probabilistic basis. Weight = 3
F
F Primary Route (3+1)
The ratio of F
F
E
(W1+E1): A Output either route is taken B
Queues
(W2+E2) E
is used. E=Empty
E
E
F Secondary Route (1+3)
F=Full
F
Weight = 1
Distributed Adaptive Routing
A different algorithm is used in ARPANET.
Here the object is to find the path of least
delay under the prevailing conditions.
The average delay for each of the
neighbouring nodes is measured and, every
10 seconds, this information in propagated
to other nodes.
In this way, a dynamic database is formed
of the current fastest route in the local area
of the network.
Distributed Adaptive Routing
The delay incurred as a result of the
updating process can be considerable.
If one packet is used by each node during
each time period to update its routing table,
then the cost can very high.
In earlier versions of ARPANET, updates
were performed every 0.67 seconds. This
used up about 50% of the network capacity.
Now they are performed every 10 seconds
only when there are significant changes.
Flooding
We can dispense with a formal systematic
routing algorithm altogether by using flooding
or random routing techniques.
In flooding, every packet is sent down every
connection to a node except the one on which
the packet was received.
This ensures that the data gets through to
every node and will arrive in the shortest time.
This type of routing produces an excessive
amount of traffic, however.
Random Routing
In random routing, the choice of output path
is any path on which the packet (or frame)
did not arrive.
The choice is made randomly and so
ensures that traffic is evenly spread across
the network.
The only problem is that the packet may
take a long time to find the destination
node.
Flow Control and Congestion
Flow Control simply means the methods
used to keep network traffic moving.
This means taking measures to avoid sending
packets (or frames) towards bottlenecks in
the network.
If more traffic offered to part of the network
that it can handle, then buffers in it will
overflow and packets will be lost.
Congestion is when part of a network has so
many packets, none of them can move at all!
Flow Control and Transit
Delays
Flow control is often used on host machines
to hold back the transmission of packets
until the network is able to accept them
without risking congestion or overflow.
This keeps the traffic on the network down
but does not reduce the transit delay since
the delay is simply shifted to the host
machine rather than the network.
The only way to avoid dissatisfaction is to
ensure there is sufficient network capacity.
Keeping Users Out
Another way to avoid overflowing the
network is to prevent user from logging in
when the network is near to full capacity.
This approach tends only to cause another
type of dissatisfaction.
When delays do become to long, things just
get worse:
– packets (or frames) get discarded
– the timers on hosts run out and they retransmit.
A Brief Word about Timers
In many data link protocols, the source host
expects to receive acknowledgements (ACK or
NAK signals) from the destination host when
packets (or frames) arrive.
If this does not happen within a certain time
then the source host assumes that the packet
was lost or corrupted in transit and retransmits
the packet.
• Note: ACK means that the packet was received intact
and NAK means that it was not (please retransmit the
packet).
Time Sequence Diagrams for
Timer
Host A Host
Host Host Node Node
Node Node B
A B
Send Frame Relay Frame Send Frame Relay Frame
Relay Frame Frame Lost 2Propagatio
T 2Propagation
n delay
I delay
Relay Send Retransmit
m Relay
ACK/NAK ACK/NAK Frame
e ACK/NAK
Relay Frame
Retransmit if Relay Frame
Relay Frame
NAK Relay Frame
Send ACK
Relay ACK
Relay ACK
ACK (or NAK)
arrives back in time ACK (or NAK) does
not arrive back
Buffer Overflow
Consider a situation where two sessions are
passing packets through the same node.
End Device
X
Slow Link
2
Fast Link 5
End Device
1 B
End Device
A 3
4 6
End Device
Y
Node 3 will temporarily store the packets it
receives in its buffers.
Buffer Overflow
Imagine now that the buffer is full. The
node will only be able to accept a new
packet once it has sent an old one.
Now imagine that one of the incoming links
is 10 times faster than the other one (not an
uncommon situation).
Every time a space is made in the butter, the
node is 10 times more likely to accept a new
packet from the faster link.
Connection Blocked by Buffer
Overflow
The effect is to almost block the other
connection since most of its packets will be
rejected by the central node.
This problem can be overcome by using
more complex nodes that have separate
buffers for all the incoming links and sends
packets out with equal priority (say by using
a round-robin selection from each buffer).
Deadlock
Another problem that can cause congestion
in a network is that of deadlock.
Imagine two nodes, P and Q, are
transmitting packets to one another.
Now imagine that both their buffers are full.
P Q
(Full) (Full)
Both are unable to receive packets so no
packets move.
Finally…..Overcoming Deadlock
This is a simple example of deadlock. There may be many nodes
involved (typically in cycles) where each is unable to transmit packets
to the next.
We can overcome this by having separate buffers for different priority
packets.
The more links a packet travels over, the greater its priority.
Deadlocks do not occur because it is unlikely that packets will
overflow all the buffers.
If a packet gets a priority greater than n-1 then the packet is discarded
(it is assumed to have travelled in a loop). This also ensures that a
deadlock cannot occur.
Another (less certain method) is to allow deadlocks to occur and
regularly discard very old packets to try to clear them.
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