Transcript Devices 6 - Muhammad sami

Multiplexing
FDM & TDM
Multiplexing
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When two communicating nodes are connected through
a media, it generally happens that bandwidth of media is
several times greater than that of the communicating
nodes.
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Transfer of a single signal at a time is both slow and
expensive. The whole capacity of the link is not being
utilized in this case.
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This link can be further exploited by sending several
signals combined into one. This combining of signals into
one is called multiplexing.
Frequency Division Multiplexing (FDM)
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This is possible in the case where transmission media
has a bandwidth than the required bandwidth of signals
to be transmitted.
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A number of signals can be transmitted at the same
time. Each source is allotted a frequency range in which
it can transfer it's signals, and a suitable frequency gap
is given between two adjacent signals to avoid
overlapping.
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This is type of multiplexing is commonly seen in the
cable TV networks.
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
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This is possible when data transmission rate of the media is
much higher than that of the data rate of the source.
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Multiple signals can be transmitted if each signal is allowed to
be transmitted for a definite amount of time. These time slots
are so small that all transmissions appear to be in parallel.
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Synchronous TDM: Time slots are preassigned and are
fixed. Each source is given it's time slot at every turn due to it.
This turn may be once per cycle, or several turns per cycle ,if
it has a high data transfer rate, or may be once in a no. of
cycles if it is slow. This slot is given even if the source is not
ready with data. So this slot is transmitted empty.
Synchronous TDM
Asynchronous TDM
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In this method, slots are not fixed. They are allotted
dynamically depending on speed of sources, and
whether they are ready for transmission.
Connectivity Devices
Bridges, Switches,
Routers & Gateways
Connecting Devices
Connecting Devices and the OSI Model
Why Bridges?
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It operates both the physical & data link
layers of the OSI model.
Bridges can divide a large networks into
smaller segments.
Bridges contain logic that allows them to
keep the traffic for each segment separate.
It can also provide security through this
partitioning of traffic.
A Bridge in the OSI Model
A Bridge
Why Bridges?
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When a frame enters a bridge, the bridge not only
regenerates the signal but checks the address of
the destination and forwards a new copy only to the
segment to which the address belongs.
As a bridge encounters a packet, it reads the
address contained in the frame and compares that
address with a table of all the stations on both
segments.
When it finds a match, it discovers to which segment
the station belongs and relays the packet only to
that segment.
Function of a Bridge
Why Bridges?
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Example: Last slide shows two segments joined by
a bridge.
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In figure a, A packet from station A addressed to station D
arrives at the bridge.
Station A is on the same segment at station D; therefore,
the packet is blocked from crossing into the lower segment.
Instead, the packet is relayed to the entire upper segment
and received by station D.
In figure b, a packet generated by station A is intended for
station G.
The bridge allows the packet to cross and relays it to the
entire lower segment, where it is received by station G.
Bridge Table
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Bridge filtering & forwarding is done by bridge
table.
The bridge table contains entries for some,
not all of the nodes on a LAN. An entry in the
tables contains:
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The LAN address of the node
The bridge interface that leads towards the node.
The time at which the entry for the node was
placed in the table.
Types of Bridges
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Simple bridge – is the most primitive and
least expensive type of bridge.
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It links two segments and contains a table that
lists the addresses of all the stations included in
each of them.
What makes it primitive is that these addresses
must be entered manually.
Whenever a new station is added, the table must
be modified.
Installation and maintenance of simple bridges are
time-consuming.
Types of Bridges
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Multi-port bridge – can be used to connect
more than two LANs
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In the next slide figure, the bridge has three
tables, each one holding the physical addresses
of stations reachable through the corresponding
port.
Multi-port Bridge
Types of Bridges
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Transparent bridge – or learning bridge builds it table of station
addresses on its own as it performs its bridge functions.
 The bridge table is initially empty.
 When a frame arrives on one of the interfaces and the frame’s
destination address is not in the table, then the bridge forwards
copies of the frame to the output buffers preceding all of other
interfaces.
 For each frame received, the bridge stores in its table (1) the LAN
address in the frame’s source address field, (2) the interface form
which the frame arrived & (3) the current time.
 When a frame arrives on one of the interfaces and the frame’s
destination address is in the table, then the bridge forwards the
frame to the appropriate interface.
 The bridge deletes an address in the table, if no frames are
received with that address as the source address after some
period of time (the aging time).
Types of Bridges
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Spanning Tree Algorithm – Bridges are
normally installed redundantly, which means
that two LANs may be connected by more
than one bridge.
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In this case, if the bridges are transparent bridges,
they may create a loop, which means a packet
may be going round and round, form one LAN to
another and back again to the first LAN.
To avoid this situation, bridges today use what is
called the spanning tree algorithm.
Types of Bridges
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Another solution to prevent loops in the LANs
connected by bridges is source routing.
In which, the source of the packet defines the
bridges and the LANs through which the packet
should go before reaching the destination.
Spanning Tree Problem Figure
Host A
Segment 1
Br 1
Br 2
Segment 2
Host B
Two LANs connected by two bridges
Spanning Tree Problem
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This situation occurs due to three factors:
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Transparent or Learning bridges are being used
that do not have information about the location of
host until they receive at least one packet from
them.
The bridges are not aware of the existence of
other bridges.
Graph has been created instead of tree.
Switches
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The switch has a buffer for each link (network) to
which it is connected.
When it receives frame, it stores the frame in the
buffer of the receiving link and checks the address
to find the out-going link.
Switches are based on two different strategies
(called fabrics):
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Store-and-forward – stores the frame in the input buffer
until the whole packet has arrived.
Cut-through-switch – forwards the packet to the output
buffer as soon as the destination address is received.
Switch
Routers
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Routers relay packets among multiple
interconnected networks.
Routers normally act on network layer.
Routers take decision on the basis of layer 3
(network layer) logical addresses (IP
address).
A Router in the OSI Model
Routers in an Internet
Routers
Routing Concepts:
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Least-Cost Routing:
 Which path does router choose to send packet over a network?
 The decision of least-cost routing is based on efficiency; which of
the available pathways are the cheapest or shortest? A value is
assigned to each links.
The term shortest, in this context, can mean either of two things
depending on the protocol:
 In some cases, it means the route requiring the smallest number
of relays, or hops; for example, a direct link from A to D would be
considered shorter than the route A-B-C-D even if the actual
distance covered by the latter is the same or less.
 In other cases, shortest means fastest, most reliable or most
secure etc. etc.
Routers
Routing Concepts:
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When shortest means the pathways requiring the
smallest number of relays, it is called hop-count
routing, in which every link is considered to be of
equal length and given the value one.
Routers
Routing Concepts:
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Non-adaptive versus Adaptive Routing:
 Non-adaptive routing: In some routing protocols, once a pathway
to a destination has been selected, the router sends all packets
for that destination along that one route. In other words, the
routing decisions are not made based on the condition or
topology of the networks.
 Adaptive routing: In which, a router may select a new route for
each packet in response to changes in condition and topology of
the networks.
Packet Lifetime (or TTL):
 Each packet is marked with a lifetime in a network, usually the
number of hops that are allowed before a packet is considered
lost and, accordingly destroyed.
 Each router to encounter the packet subtracts 1 from the total
before passing it on, when the lifetime total reaches 0, the packet
is destroyed.
A Gateway in the OSI Model
Gateways
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Gateways potentially operates in all seven
layers of the OSI model.
A gateway is a protocol converter.
In contrast with router, a gateway can accept
a packet formatted for one protocol (e.g.,
AppleTalk) and convert it to a packet
formatted for another protocol (e.g., TCP/IPI)
before forwarding it.
A gateway is generally software installed
within a router.
A Gateway
Multi-protocol Routers
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A router by default is a single-protocol device,
which means, if two LANs are to be
connected through a router, they should use
the same protocol at the network layer.
However, multi-protocol routers have been
designed to route packets belonging to two or
more protocols.
Single-Protocol versus
Multi-protocol Router
Brouters
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A brouter (bridge/router) is a single-protocol
or multi-protocol router that sometimes acts
as a router and sometimes as a bridge.
Brouter