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Thursday, December 12, 2013


Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.
CSMA/CD is a type of contention protocol.  Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE 802.3.


Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet was invented by engineer Robert Metcalfe. A local-area network (LAN) architecture developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology and supports data transfer rates of 10 Mbps. The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards.
A newer version of Ethernet, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second.

Difference between a hub and a switch

Difference between a hub and a switch

Hub is a networking device that allows one to connect multiple PCs to a single network. Hubs may be based on Ethernet, Firewire, or USB connections. A switch is a control unit that turns the flow of electricity on or of in a circuit. It may also be used to route information patterns in streaming electronic data sent over networks. In the context of a network, a switch is a computer networking device that connects network segments.

Physical layer. Hubs are classified as Layer 1 devices per the OSI model.
Data Link Layer. Network switches operate at Layer 2 of the OSI model.
Hubs always perform frame flooding; may be unicast, multicast or broadcast
First broadcast; then unicast & multicast as needed.
4/12 ports
Switch is multi port Bridge. 24/48 ports
A network hub cannot learn or store MAC address.
A network switch stores MAC addresses in a lookup table.
Passive Device (Without Software)
Active Device (With Software) & Networking device
Half duplex
Full duplex
Hub has one Broadcast Domain.
Switch has one broadcast domain [unless VLAN implemented]
Electrical signal or bits
Frame (L2 Switch) Frame & Packet (L3 switch)
To connect a network of personal computers together, they can be joined through a central hub.
Allow to connect multiple device and port can be manage, Vlan can create security also can apply
Collisions occur commonly in setups using hubs.
No collisions occur in a full-duplex switch.


1)It has 7 layers
2)Transport layer gurantees delivery of packets
3)Horizontal approach
4)Seperate presentation layer
5)Seperate session layer
6)Network layer provides both connectionless and connection oriented services
7)It defines the services,interfaces and protocols very clearly and makes a clear distinction between them
8)The protocol are better hidden and can be easily replaced as the technology changes
9)OSI truly is a general model
10)It has a problem of protocol filtering into a model


1)Has 4 layers
2)Transport layer does not gurantees delivery of packets
3)Vertical approach
4)No session layer, characteristics are provided by transport layer
5)No presentation layer, characteristics are provided by application layer
6)Network layer provides only connection less services
7)It does not clearly distinguishes between service interface and protocols
8)It is not easy to replace the protocols
9)TCP/IP can not be used for any other application
10)The model does not fit any protocol stack.