Unit – IV Ethernet and Switching Techniques

Unit – IV: ETHERNET & SWITCHING TECHNIQUES

Circuit Switching , Packet Switching, Message Switching Ethernet: Overview of Ethernet 10 Base, 100 Base, Fast Ethernet, POE, FDDI, Token Ring, VLAN and its features, frame relay, CSMA-CD,CA, Flow control, Error Control, Congestion control.,Half, Full duplex communication.
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Circuit Switching and Packet Switching:

Communication via circuit switching involves that there is a dedicated communication path between two stations. That path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection.

Communication via circuit switching involves three phases:

• Circuit establishment. Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established.
• Data transfer. Data can now be transmitted from source through the network to destination. Circuit disconnect.  After some period of data transfer, the connection is terminated, usually by the action of one of the two stations.

In circuit switching network resources (bandwidth) is divided into pieces and bit delay is constant during a connection. The dedicated path/circuit established between sender and receiver provides a guaranteed data rate. Data can be transmitted without any delays once the circuit is established.

Telephone system network is the one of example of Circuit switching. TDM (Time Division Multiplexing) and FDM (Frequency Division Multiplexing) are two methods of multiplexing multiple signals into a single carrier.

Frequency Division Multiplexing : Divides into multiple bands Frequency Division Multiplexing or FDM is used when multiple data signals are combined for simultaneous transmission via a shared communication medium.It is a technique by which the total bandwidth is divided into a series of non-overlapping frequency sub-bands,where each sub-band carry different signal. Practical use in radio spectrum & optical fiber to share multiple independent signals.

Time Division Multiplexing : Divides into frames Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line. TDM is used for long-distance communication links and bears heavy data traffic loads from end user.

Packet switching:

Packet switching is a method of transferring the data to a network in form of packets. In order to transfer the file fast and efficient manner over the network and minimize the transmission latency, the data is broken into small pieces of variable length, called Packet. At the destination, all these small-parts (packets) has to be reassembled, belonging to the same file. A packet composes of payload and various control information. No pre-setup or reservation of resources is needed.

Packet Switching uses Store and Forward technique while switching the packets; while forwarding the packet each hop first store that packet then forward. This technique is very beneficial because packets may get discarded at any hop due to some reason. More than one path is possible between a pair of source and destination. Each packet contains Source and destination address using which they independently travel through the network. In other words, packets belonging to the same file may or may not travel through the same path. If there is congestion at some path, packets are allowed to choose different path possible over existing network.

Packet-Switched networks were designed to overcome the weaknesses of Circuit-Switched networks since circuit-switched networks were not very effective for small messages.

Advantage of Packet Switching over Circuit Switching :

• More efficient in terms of bandwidth, since the concept of reserving circuit is not there.
• Minimal transmission latency.
• More reliable as destination can detect the missing packet.
• More fault tolerant because packets may follow different path in case any link is down, Unlike Circuit Switching.
• Cost effective and comparatively cheaper to implement.

Disadvantage of Packet Switching over Circuit Switching :

• Packet Switching don’t give packets in order, whereas Circuit Switching provides ordered delivery of packets because all the packets follow the same path.
• Since the packets are unordered, we need to provide sequence numbers to each packet.
• Complexity is more at each node because of the facility to follow multiple path.
• Transmission delay is more because of rerouting.
• Packet Switching is beneficial only for small messages, but for bursty data (large messages) Circuit Switching is better.

Message Switching:

Message switching was a technique developed as an alternate to circuit switching, before packet switching was introduced. In message switching, end users communicate by sending and receiving messages that included the entire data to be shared. Messages are the smallest individual unit. Also, the sender and receiver are not directly connected. There are a number of intermediate nodes transfer data and ensure that the message reaches its destination. Message switched data networks are hence called hop-by-hop systems.

Message switching is advantageous as it enables efficient usage of network resources. Also, because of the store-and-forward capability of intermediary nodes, traffic can be efficiently regulated and controlled. Message delivery as one unit, rather than in pieces, is another benefit.

However, message switching has certain disadvantages as well. Since messages are stored indefinitely at each intermediate node, switches require large storage capacity. Also, these are pretty slow. This is because at each node, first there us wait till the entire message is received, then it must be stored and transmitted after processing the next node and links to it depending on availability and channel traffic. Hence, message switching cannot be used for real time or interactive applications like video conference.

The store-and-forward method was implemented in telegraph message switching centres. Today, although many major networks and systems are packet-switched or circuit switched networks, their delivery processes can be based on message switching. For example, in most electronic mail systems the delivery process is based on message switching, while the network is in fact either circuit-switched or packet-switched.

Overview of Ethernet:

Ethernet is the technology that is most commonly used in wired local area networks (LANs). A LAN is a network of computers and other electronic devices that covers a small area such as a room, office, or building. It is used in contrast to a wide area network (WAN), which spans much larger geographical areas. Ethernet is a network protocol that controls how data is transmitted over a LAN. Technically it is referred to as the IEEE 802.3 protocol. The protocol has evolved and improved over time to transfer data at the speed of a gigabit per second.

Many people have used Ethernet technology their whole lives without knowing it. It is most likely that any wired network in your office, at the bank, and at home is an Ethernet LAN. Most desktop and laptop computers come with an integrated Ethernet card inside so they are ready to connect to an Ethernet LAN.

When a machine on the network wants to send data to another, it senses the carrier, which is the main wire connecting all the devices. If it is free, meaning no one is sending anything, it sends the data packet on the network, and all other devices check the packet to see whether they are the recipient. The recipient consumes the packet. If there is already a packet on the highway, the device that wants to send holds back for some thousandths of a second to try again until it can send.

10 Base –T:

One of several adaptations of the Ethernet (IEEE 802.3) standard for Local Area Networks (LANs). The 10Base-T standard (also called Twisted Pair Ethernet) uses a twisted-pair cable with maximum lengths of 100 meters. The cable is thinner and more flexible than the coaxial cable used for the 10Base-2 or 10Base-5 standards.

Cables in the 10Base-T system connect with RJ-45 connectors. A star topology is common with 12 or more computers connected directly to a hub.

The 10Base-T system operates at 10 Mbps and uses baseband transmission methods.

100Base-T (IEEE 802.3u) Fast Ethernet:

A networking standard that supports data transfer rates up to 100 Mbps (100 megabits per second). 100BASE-T is based on the older Ethernet standard. Because it is 10 times faster than Ethernet, it is often referred to as Fast Ethernet. Officially, the 100BASE-T standard is IEEE 802.3u.

Like Ethernet, 100BASE-T is based on the CSMA/CDLAN (Carrier Sense Multiple Access with Collision Detection) access method. There are several different cabling schemes that can be used with 100BASE-T, including:

• 100BASE-TX: two pairs of high-quality twisted-pair wires
• 100BASE-T4:four pairs of normal-quality twisted-pair wires
• 100BASE-FX: fiber optic cables

Power over Ethernet (POE):

Power over Ethernet (POE) is a networking feature that lets network cables carry electrical power over an existing data connection with a single Cat5e/Cat6 ethernet cable.

PoE technology relies on the IEEE 802.3af and 802.3at standards, which are set by the Institute of Electrical and Electronics Engineers and govern how networking equipment should operate in order to promote interoperability between devices.

PoE-capable devices can be power sourcing equipment (PSE), powered devices (PDs), or sometimes both. The device that transmits power is a PSE, while the device that is powered is a PD. Most PSEs are either network switches or PoE injectors intended for use with non-PoE switches.

Common examples of PDs include VoIP phones, wireless access points, and IP cameras.

Token Ring:

This is a 4-Mbps or 16-Mbps token-passing method, operating in a ring topology. Devices on a Token Ring network get access to the media through token passing. Token and data pass to each station on the ring. The devices pass the token around the ring until one of the computer who wants to transmit data , takes the token and replaces it with a frame. Each device passes the frame to the next device, until the frame reaches its destination. As the frame passes to the intended recipient, the recipient sets certain bits in the frame to indicate that it received the frame. The original sender of the frame strips the frame data off the ring and issues a new token.

Fast Ethernet:

This is an extension of 10Mbps Ethernet standard and supports speed upto 100Mbps. The access method used is CSMA/CD .For physical connections Star wiring topology is used. Fast Ethernet is becoming very popular as an upgradation from 10Mbps Ethernet LAN to Fast Ethernet LAN is quite easy.

FDDI (Fiber Distributed Data Interface):

FDDI provides data speed at 100Mbps which is faster than Token Ring and Ethernet LANs . FDDI comprise two independent, counter-rotating rings : a primary ring and a secondary ring. Data flows in opposite directions on the rings. The counter-rotating ring architecture prevents data loss in the event of a link failure, a node failure, or the failure of both the primary and secondary links between any two nodes. This technology is usually implemented for a backbone network.

VLANs and Features:

VLANs have the primary role to enable easier configuration and management of large corporate networks built around many bridges.

Virtual LAN is software that is employed to provide multiple networks in single hub by grouping terminals connected to switching hubs. It is a LANs that is grouped together by logical addresses into a virtual LAN instead of a physical LAN through a switch. The switch can support many virtual LANs that operate with having different network addresses or as subnets. Users within a virtual LAN are grouped either by IP address or by port address, with each node attached to the switch via a dedicated circuit. Users also can be assigned to more than one virtual LAN.

The VLAN can be defined as a broadcast domain in which the broadcast address reaches all stations belonging to the VLAN. Communications within the VLAN can be secured, and between those two controlled separate VLANs.

A router is generally required to establish communication between VLANs.

Features of VLANs:

VLANs provide a number of features:

• Simplified administration for the network manager: One of the best things about virtualization is that it simplifies management. By logically grouping users into the same virtual networks, you make it easy to set up and control your policies at a group level. When users physically move workstations, you can keep them on the same network with different equipment. Or if someone changes teams but not workstations, they can easily be given access to whatever new VLANs they need.
• Improved security: Using VLANs improves security by reducing both internal and external threats. Internally, separating users improves security and privacy by ensuring that users can only access the networks that apply to their responsibilities. External threats are also minimized. If an outside attacker is able to gain access to one VLAN, they’ll be contained to that network by the boundaries and controls you have in place to segment it from your others.
• Easier fault management: Troubleshooting problems on the network can be simpler and faster when your different user groups are segmented and isolated from one another. If you know that complaints are only coming from a certain subset of users, you’ll be able to quickly narrow down where to look to find the issue.
• Improved quality of service: VLANs manage traffic more efficiently so that your end users experience better performance. You’ll have fewer latency problems on your network and more reliability for critical applications.

Frame relay:

Frame relay is a packet-switching telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between endpoints in wide area networks (WANs).

Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the endpoints, which speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. Switched virtual circuits (SVC), by contrast, are temporary connections that are destroyed after a specific data transfer is completed.

Frame relay supports multiplexing of traffic from multiple connections over a shared physical link. It uses hardware components including frame routers, bridges, and switches to package data into individual frame relay messages. Each connection uses a 10-bit data link connection identifier (DLCI) for unique channel addressing.

There are two connection types:

• Permanent virtual circuits (PVC) for persistent connections intended to be maintained for long periods even if no data is actively transferred.
• Switched virtual circuits (SVC) for temporary connections that last only for a single session.

Carrier Sense Multiple Access (CSMA)

This method was developed to decrease the chances of collisions when two or more stations start sending their signals over the datalink layer. Carrier Sense multiple access requires that each station first check the state of the medium before sending.

Vulnerable Time:

Vulnerable time = Propagation time (Tp)

The persistence methods can be applied to help the station take action when the channel is busy/idle.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD):

In CSMA/CD, a station monitors the medium after it sends a frame to see if the transmission was successful.If succcessful, the station is finished, if not, the frame is sent again.

In the diagram, A starts send the first bit of its frame at t1 and since C sees the channel idle at t2, starts sending its frame at t2. C detects A’s frame at t3 and aborts transmission. A detects C’s frame at t4 and aborts its transmission. Transmission time for C’s frame is therefore   and for A’s frame is  .

So, the frame transmission time (Tfr) should be at least twice the maximum propagation time (Tp). This can be deduced when the two stations involved in collision are maximum distance apart.

The entire process of collision detection can be explained as follows:

Throughput and Efficiency – The throughput of CSMA/CD is much greater than pure or slotted ALOHA.

•           For 1-persistent method throughput is 50% when G=1.

•           For non-persistent method throughput can go upto 90%.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA):

The basic idea behind CSMA/CA is that the station should be able to receive while transmitting to detect a collision from different stations. In wired networks, if a collision has occurred then the energy of received signal almost doubles and the station can sense the possibility of collision. In case of wireless networks, most of the energy is used for transmission and the energy of received signal increases by only 5-10% if collision occurs. It can’t be used by station to sense collision. Therefore CSMA/CA has been specially designed for wireless networks.

These are three type of strategies:

1. InterFrame Space (IFS):When a station finds the channel busy, it waits for a period of time called IFS time. IFS can also be used to define the priority of a station or a frame. Higher the IFS lower is the priority.
2. Contention Window: It is the amount of time divided into slots.A station which is ready to send frames chooses random number of slots as wait time.
3. Acknowledgements: The positive acknowledgements and time-out timer can help guarantee a successful transmission of the frame.

The entire process for collision avoidance can be explained as follows:

Flow Control and Congestion Control:

Flow Control and Congestion Control are the traffic controlling methods in different situations.

The main difference between flow control and congestion control is that, In flow control, Traffics are controlled which are flow from sender to a receiver. On the other hand, In congestion control, Traffics are controlled entering to the network.

The difference between flow control and congestion control is as shown below:

FLOW CONTROL	CONGESTION CONTROL
In flow control, Traffics are controlled which are flow from sender to a receiver.	In this, Traffics are controlled entering to the network.
Data link layer and Transport layer handle it.	Network layer and Transport layer handle it.
In this, Receiver’s data is prevented from being overwhelmed.	In this, Network is prevented from congestion.
In flow control, Only sender is responsible for the traffic.	In this, Transport layer is responsible for the traffic.
In this, Traffic is prevented by slowly sending by the sender.	In this, Traffic is prevented by slowly transmitting by the transport layer.

Error Control in TCP:

TCP protocol has methods for finding out corrupted segments, missing segments, out-of-order segments and duplicated segments.

Error control in TCP is mainly done through use of three simple techniques :

Checksum: Every segment contains a checksum field which is used to find corrupted segment. If the segment is corrupted, then that segment is discarded by the destination TCP and is considered as lost.

Acknowledgement: TCP has another mechanism called acknowledgement to affirm that the data segments have been delivered. Control segments that contain no data but has sequence number will be acknowledged as well but ACK segments are not acknowledged.

Retransmission: When a segment is missing, delayed to deliver to receiver, corrupted when it is checked by receiver then that segment is retransmitted again. Segments are retransmitted only during two events: when the sender receives three duplicate acknowledgements (ACK) or when a retransmission timer expires.

Retransmission after RTO : TCP always preserve one retransmission time-out (RTO) timer for all sent but not acknowledged segments. When the timer runs out of time, the earliest segment is retransmitted. Here no timer is set for acknowledgement. In TCP, RTO value is dynamic in nature and it is updated using round trip time (RTT) of segments. RTT is the time duration needed for a segment to reach receiver and an acknowledgement to be received to the sender.

Retransmission after Three duplicate ACK segments : RTO method works well when the value of RTO is small. If it is large, more time is needed to get confirmation about whether a segment has delivered or not. Sometimes one segment is lost and the receiver receives so many out-of-order segments that they cannot be saved. In order to solve this situation, three duplicate acknowledgement method is used and missing segment is retransmitted immediately instead of retransmitting already delivered segment. This is a fast retransmission because it makes it possible to quickly retransmit lost segments instead of waiting for timer to end.

Transmission Modes:

Transmission mode means transferring of data between two devices. It is also known as communication mode. Buses and networks are designed to allow communication to occur between individual devices that are interconnected.

There are three types of transmission mode:

·         Simplex Mode

·         Half-Duplex Mode

·         Full-Duplex Mode

Simplex Mode:

In Simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit, the other can only receive. The simplex mode can use the entire capacity of the channel to send data in one direction.

Example: Keyboard and traditional monitors. The keyboard can only introduce input, the monitor can only give the output.

Half-Duplex Mode:

In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. The half-duplex mode is used in cases where there is no need for communication in both direction at the same time. The entire capacity of the channel can be utilized for each direction.

Example: Walkie- talkie in which message is sent one at a time and messages are sent in both the directions.

Full-Duplex Mode

In full-duplex mode, both stations can transmit and receive simultaneously. In full_duplex mode, signals going in one direction share the capacity of the link with signals going in other direction, this sharing can occur in two ways:

Either the link must contain two physically separate transmission paths, one for sending and other for receiving.

Or the capacity is divided between signals travelling in both directions.

Full-duplex mode is used when communication in both direction is required all the time. The capacity of the channel however must be divided between the two directions.

Example: Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time.