Data communication and computer network- chapter 04.pdf

Data Communications and Computer Networks
CH-04
Connecting Devices
Baessa K.
Mettu University
College of Engineering and Technology
Department of Information Technology
March 15, 2022
Baessa K. (Mettu University) 04 Data link layer March 15, 2022 1 / 18

Lecture Topics : #
1 Connecting Devices
Hubs
Link-Layer Switches
Routers

Connecting Devices
Outline
Hubs
Link-Layer Switches
Routers

Connecting Devices
Introduction
• Hosts and networks do not normally operate in isolation.
• Connecting devices are used to connect hosts together to make a
network or to connect networks together to make an internet.
• Connecting devices can operate in different layers of the Internet
model.
• three kinds of connecting devices:

Connecting Devices Hubs
Outline
Hubs
Link-Layer Switches
Routers

Hubs
• A hub is a device that operates only in the physical layer.
• Signals that carry information within a network can travel a fixed
distance before attenuation endangers the integrity of the data.
• A repeater receives a signal and, before it becomes too weak or
corrupted, regenerates and retimes the original bit pattern.
• The repeater then sends the refreshed signal.
• In the past, when Ethernet LANs were using bus topology,
• …a repeater was used to connect two segments of a LAN to overcome
the length restriction of the coaxial cable.
• Today, however, Ethernet LANs use star topology.
• In a star topology, a repeater is a multiport device, often called a hub,
• …that can be used to serve as the connecting point and
• …at the same time function as a repeater.

Hubs …
• hub forwards the packet from all outgoing ports except the one from
which the signal was received.
• …the frame is broadcast.
• All stations in the LAN receive the frame, but only station B keeps it.
• The rest of the stations discard it
• hub does not have a filtering capability
• it does not have the intelligence to find from which port the frame
should be sent out.

Hubs …
• A hub or a repeater is a physical-layer device.
• They do not have a link-layer address and they do not check the
link-layer address of the received frame.
• They just regenerate the corrupted bits and send them out from every
port.
• There are two types of hubs
1 Passive Hubs
• it does nothing except provide a pathway for the electrical signals to
travel along.
2 Active Hubs
• provide a pathway PLUS regenerates the signal before it forwards
• A Hub does not perform any processing on the data that it forwards,
nor does it perform any error checking.

Connecting Devices Link-Layer Switches
Outline
Hubs
Link-Layer Switches
Routers

Link-Layer Switches
• Operates in both the physical and the data-link layers.
• As a physical-layer device,
• …it regenerates the signal it receives.
• As a link-layer device,
• …the link-layer switch can check the MAC addresses (source and
destination) contained in the frame.

Link-Layer Switches …
Filtering
• A link-layer switch has filtering capability.
• It can check the destination address of a frame and can decide from
which outgoing port the frame should be sent.
• A link-layer switch has a table used in filtering decisions.

• Advantages of Switches
• A link-layer switch has several advantages over a hub.
• Collision Elimination
• by creating a direct path between two devices and controlling their
communication,
• …it can greatly reduce the number of collisions on the network.
• the lack of collisions enables switches to communicate with
devices in full-duplex mode.
• Connecting Heterogenous Devices
• A link-layer switch can connect devices that use different protocols
at the physical layer (data rates) and different transmission media.

• There are three kinds of switching methods:
1 Cut-through
• packet begins to be forwarded as soon as it is received (fast, but error
prone)
• the switch starts the frame processing as soon as it reads the
destination MAC address of the frame.
• To start the frame processing, a switch only needs the destination MAC
address.
• Just after reading the destination address, it checks the CAM table and
based on the match it starts the forwarding of the frame
2 Store-and-forward
• entire packet is received and error checked before being forwarded
(error-free, but slow)
• After receiving the entire frame, switch verifies whether the received
frame is in the good state or not.
• If the received frame is in the good state, only then the switch
processes it.
• If the received frame is not in the good state, the switch discards it.

3 Fragment-Free
• enough of the packet is read to check errors (Takes advantage of both).
• the switch starts the frame processing after reading the first 64 bytes of
the frame.
• 64 bytes is the minimum legal size of an Ethernet frame

Internal frame processing
• A switch has multiple ports.
• These ports are used to connect various devices with it.
• To know which device is connected to which port, the switch stores
the MAC addresses of all connected devices with their related ports in
a table, known as the CAM table.
• When a switch receives a frame on any of its port, it reads the
destination address from the frame and finds that destination address
in the CAM table.
• If the CAM table has an entry for that destination address,
• …the switch forwards that frame from the port on which the
destination address is connected.
• If the CAM table does not have an entry for that destination address,
the switch forwards that frame from all of its ports.

Connecting Devices Routers
Outline
Hubs
Link-Layer Switches
Routers

Routers
• A router is a three-layer device; it operates in the physical, data-link,
and network layers
• As a physical-layer device, it regenerates the signal it receives.
• As a link-layer device,the router checks the physical addresses (source
and destination) contained in the packet
• As a network-layer device, a router checks the network-layer addresses.
• A router can connect networks.
• a router is an internetworking device;
• …it connects independent networks to form an internetwork.
• …two networks connected by a router become an internetwork or an
internet.

Router …
• There are three major differences between a router and a repeater or
a switch
1 A router has a physical and logical (IP) address for each of its
interfaces.
2 A router acts only on those packets in which the link-layer destination
address matches the address of the interface at which the packet
arrives.
3 A router changes the link-layer address of the packet (both source and
destination) when it forwards the packet.

Data communication and computer network- chapter 04.pdf

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