UNIT V
NETWORK & APPLICATION LAYER
Repeaters – Bridges – Routers – Gateway – Routing algorithms – TCP/IP – Overview – Network
layer – Transport and application layers of TCP/IP – DNS – SMTP – HTTP – WWW.
Network Devices (Hub, Repeater, Bridge, Switch,
Router and Gateways)
1. Repeater – A repeater operates at the physical layer. Its job is to regenerate the signal
over the same network before the signal becomes too weak or corrupted so as to extend
the length to which the signal can be transmitted over the same network. An important
point to be noted about repeaters is that they do no amplify the signal. When the signal
becomes weak, they copy the signal bit by bit and regenerate it at the original strength. It
is a 2 port device.
A single Ethernet segment can have a maximum length of 500 meters with a maximum of
100 stations (in a cheapernet segment it is 185m). To extend the length of the network, a
repeater may be used as shown in Fig. 6.1.1. Functionally, a repeater can be considered
as two transceivers joined together and connected to two different segments of coaxial
cable. The repeater passes the digital signal bit-by-bit in both directions between the two
segments. As the signal passes through a repeater, it is amplified and regenerated at the
other end. The repeater does not isolate one segment from the other, if there is a collision
on one segment, it is regenerated on the other segment. Therefore, the two segments form
a single LAN and it is transparent to rest of the system. Ethernet allows five segments to
be used in cascade to have a maximum network span of 2.5 km. With reference of the
ISO model, a repeater is considered as a level-1 relay as depicted in Fig. 6.1.2. It simply
repeats, retimes and amplifies the bits it receives. The repeater is merely used to extend
the span of a single LAN. Important features of a repeater are as follows:
, • A repeater connects different segments of a LAN
• A repeater forwards every frame it receives
• A repeater is a regenerator, not an amplifier
• It can be used to create a single extended LAN
Figure Repeater connecting two LAN segments
Figure Operation of a repeater as a level-1 relay
2. Hub – A hub is basically a multiport repeater. A hub connects multiple wires
coming from different branches, for example, the connector in star topology
which connects different stations. Hubs cannot filter data, so data packets are
sent to all connected devices. In other words, collision domain of all hosts
connected through Hub remains one. Also, they do not have intelligence to
find out best path for data packets which leads to inefficiencies and wastage.
Hub is a generic term, but commonly refers to a multiport repeater. It can be used to
create multiple levels of hierarchy of stations. The stations connect to the hub with RJ-45
connector having maximum segment length is 100 meters. This type of interconnected set
,of stations is easy to maintain and diagnose. Figure shows how several hubs can be
connected in a hierarchical manner to realize a single LAN of bigger size with a large
number of nodes.
Figure Hub as a multi-port repeater can be connected in a hierarchical manner to form a
single LAN with many nodes
3. Bridge –A bridge operates at data link layer. A bridge is a repeater, with add
on functionality of filtering content by reading the MAC addresses of source
and destination. It is also used for interconnecting two LANs working on the
same protocol. It has a single input and single output port, thus making it a 2
port device.
The device that can be used to interconnect two separate LANs is known as a bridge. It
is commonly used to connect two similar or dissimilar LANs as shown in Fig. 6.1.4. The
bridge operates in layer 2, that is data-link layer and that is why it is called level-2 relay
with reference to the OSI model. It links similar or dissimilar LANs, designed to store
and forward frames, it is protocol independent and transparent to the end stations. The
flow of information through a bridge is shown in Fig. 6.1.5. Use of bridges offer a
number of advantages, such as higher reliability, performance, security, convenience and
larger geographic coverage. But, it is desirable that the quality of service (QOS) offered
by a bridge should match that of a single LAN. The parameters that define the QOS
include availability, frame mishaps, transit delay, frame lifetime, undetected bit errors,
frame size and priority. Key features of a bridge are mentioned below:
• A bridge operates both in physical and data-link layer
• A bridge uses a table for filtering/routing
• A bridge does not change the physical (MAC) addresses in a frame
• Types of bridges:
o Transparent Bridges o
Source routing bridges
A bridge must contain addressing and routing capability. Two routing algorithms have
been proposed for a bridged LAN environment. The first, produced as an extension of
IEEE 802.1 and applicable to all IEEE 802 LANs, is known as transparent bridge. And
, the other, developed for the IEEE 802.5 token rings, is based on source routing
approach. It applies to many types of LAN including token ring, token bus and
CSMA/CD bus.
Figure A bridge connecting two separate LANs
Figure Information flow through a bridge
NETWORK & APPLICATION LAYER
Repeaters – Bridges – Routers – Gateway – Routing algorithms – TCP/IP – Overview – Network
layer – Transport and application layers of TCP/IP – DNS – SMTP – HTTP – WWW.
Network Devices (Hub, Repeater, Bridge, Switch,
Router and Gateways)
1. Repeater – A repeater operates at the physical layer. Its job is to regenerate the signal
over the same network before the signal becomes too weak or corrupted so as to extend
the length to which the signal can be transmitted over the same network. An important
point to be noted about repeaters is that they do no amplify the signal. When the signal
becomes weak, they copy the signal bit by bit and regenerate it at the original strength. It
is a 2 port device.
A single Ethernet segment can have a maximum length of 500 meters with a maximum of
100 stations (in a cheapernet segment it is 185m). To extend the length of the network, a
repeater may be used as shown in Fig. 6.1.1. Functionally, a repeater can be considered
as two transceivers joined together and connected to two different segments of coaxial
cable. The repeater passes the digital signal bit-by-bit in both directions between the two
segments. As the signal passes through a repeater, it is amplified and regenerated at the
other end. The repeater does not isolate one segment from the other, if there is a collision
on one segment, it is regenerated on the other segment. Therefore, the two segments form
a single LAN and it is transparent to rest of the system. Ethernet allows five segments to
be used in cascade to have a maximum network span of 2.5 km. With reference of the
ISO model, a repeater is considered as a level-1 relay as depicted in Fig. 6.1.2. It simply
repeats, retimes and amplifies the bits it receives. The repeater is merely used to extend
the span of a single LAN. Important features of a repeater are as follows:
, • A repeater connects different segments of a LAN
• A repeater forwards every frame it receives
• A repeater is a regenerator, not an amplifier
• It can be used to create a single extended LAN
Figure Repeater connecting two LAN segments
Figure Operation of a repeater as a level-1 relay
2. Hub – A hub is basically a multiport repeater. A hub connects multiple wires
coming from different branches, for example, the connector in star topology
which connects different stations. Hubs cannot filter data, so data packets are
sent to all connected devices. In other words, collision domain of all hosts
connected through Hub remains one. Also, they do not have intelligence to
find out best path for data packets which leads to inefficiencies and wastage.
Hub is a generic term, but commonly refers to a multiport repeater. It can be used to
create multiple levels of hierarchy of stations. The stations connect to the hub with RJ-45
connector having maximum segment length is 100 meters. This type of interconnected set
,of stations is easy to maintain and diagnose. Figure shows how several hubs can be
connected in a hierarchical manner to realize a single LAN of bigger size with a large
number of nodes.
Figure Hub as a multi-port repeater can be connected in a hierarchical manner to form a
single LAN with many nodes
3. Bridge –A bridge operates at data link layer. A bridge is a repeater, with add
on functionality of filtering content by reading the MAC addresses of source
and destination. It is also used for interconnecting two LANs working on the
same protocol. It has a single input and single output port, thus making it a 2
port device.
The device that can be used to interconnect two separate LANs is known as a bridge. It
is commonly used to connect two similar or dissimilar LANs as shown in Fig. 6.1.4. The
bridge operates in layer 2, that is data-link layer and that is why it is called level-2 relay
with reference to the OSI model. It links similar or dissimilar LANs, designed to store
and forward frames, it is protocol independent and transparent to the end stations. The
flow of information through a bridge is shown in Fig. 6.1.5. Use of bridges offer a
number of advantages, such as higher reliability, performance, security, convenience and
larger geographic coverage. But, it is desirable that the quality of service (QOS) offered
by a bridge should match that of a single LAN. The parameters that define the QOS
include availability, frame mishaps, transit delay, frame lifetime, undetected bit errors,
frame size and priority. Key features of a bridge are mentioned below:
• A bridge operates both in physical and data-link layer
• A bridge uses a table for filtering/routing
• A bridge does not change the physical (MAC) addresses in a frame
• Types of bridges:
o Transparent Bridges o
Source routing bridges
A bridge must contain addressing and routing capability. Two routing algorithms have
been proposed for a bridged LAN environment. The first, produced as an extension of
IEEE 802.1 and applicable to all IEEE 802 LANs, is known as transparent bridge. And
, the other, developed for the IEEE 802.5 token rings, is based on source routing
approach. It applies to many types of LAN including token ring, token bus and
CSMA/CD bus.
Figure A bridge connecting two separate LANs
Figure Information flow through a bridge
