Unit I

Introduction to networks network architecture network performance Direct link networks encoding framing error detection transmission Ethernet Rings FDDI - Wireless networks Switched networks bridges

Introduction to networksA network is a set of devices (often referred to as nodes) connected by communicationlinks. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.A network is a group of connected communicating devices such as computers and printers.A network is a combination of hardware and software that sends data from one location to another. The hardware consists of the physical equipment that carries signals from one point of the network to another. The software consists of instruction sets that makepossible the services that we expect from a network.Type of ConnectionA network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. There are two possible types of connections: point-to-point and multipoint.

Point-to-Point: A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices.Multipoint:A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection.Physical Topologyphysical topology refers to the way in which a network is laid out physically.:two or more devices connect to a link; two or more links form a topology.The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring.

Mesh In a mesh topology, every device has a dedicated point-to-point link to everyother device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n - I nodes, node 2 must be connected to n - 1nodes, and finally node n must be connected to n - 1 nodes. We need n(n - 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n (n -1) /2 duplex-mode links.To accommodate that many links, every device on the network must have n 1 input/output (VO) ports.

Fully connected mesh topology

Advantages Dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices mesh topology is robust If one link becomes unusable, it does not incapacitate the entire system. Third, there is the advantage of privacy or securityWhen every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages.Disadvantages Installation and reconnection are difficult Sheer bulk of the wiring can be greater than the available space Hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office

Star Topology Star topology with four stations In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub.Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected deviceAdvantages A star topology is less expensive than a mesh topology robustness. If one link fails, only that link is affected. All other links remain active.Disadvantages Dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.Although a star requires far less cable than a mesh, each node must be linked to acentral hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).Bus TopologyA bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a networkA bus topology connecting three stations Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.Advantages Ease of installationDisadvantages Difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add newdevices. Signal reflection at the taps can cause degradation in quality. A fault or break in the bus cable stops all transmission, even between devices on the same side of the problem.Ring Topology In a ring topology, each device has a dedicated point-to-point connectionwith only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device inthe ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along A ring topology connecting six stations

Advantages A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does notreceive a signal within a specified period, it can issue an alarm. The alarm alerts thenetwork operator to the problem and its location.unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break.Hybrid Topology A network can be hybrid. For example, we can have a main star topologywith each branch connecting several stations in a bus topology A hybrid topology: a star backbone with three bus networks

Categories of NetworksThe category into which a network falls is determined by its size. A LAN normally covers an area less than 2 mi; a WAN can be worldwide. Networks of a size in between are normally referred to as metropolitan area networks and span tens of miles.Local Area NetworkUsually privately owned and links the devices in a single office, building, or campus.Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two PCs and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers. An isolated IAN connecting 12 computers to a hub in a closet Designed to allow resources to be shared between personal computers or workstation The resources to be shared can include hardware (e.g., a printer), software (e.g., an application program), or data. A common example of a LAN, found in many business environments, links a workgroup of task-related computers, for example, engineering workstations or accounting PCs. One of the computers may be given a large capacity disk drive and may become a server to clients. Software can be stored on this central server and used as needed by the whole group. In this example, the size of the LAN may be determined by licensing restrictions on the number of users per copy of software, or by restrictions on the number of users licensed to access the operating system.In addition to size, LANs are distinguished from other types of networks by theirtransmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are bus, ring, and star.Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range.Wide Area NetworkProvides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or even the whole world.A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the InternetThe switched WAN connects the end systems, which usually comprise a router (internetworking connecting device) that connects to another LAN or WAN. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (lSP). This type of WANis often used to provide Internet access.

Switched WAN

Example of a switched WAN X.25, a network designed to provide connectivity between end users. Asynchronous transfer mode (ATM) network, which is a network with fixed-size data unit packets called cells.Metropolitan Area NetworksA metropolitan area network (MAN) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. Example of a MAN Part of the telephone company network that can provide a high-speed DSL line to the customer. Cable TV network that originally was designed for cable TV, but today can also be used for high-speed data connection to the Internet.

1.3 Network Architecture- OSI Architecture- Internet Architecture

Network architecture that guides the design and implementation of networks.

Two of the most widely referenced architectures are

The OSI architecture and the Internet architecture.

1.1. Layering

In OSI Architecture, there are 7 layers which can be combined into basic four layers as shown in the below figure.

Example of a layered network system

1.2.1 Layering Characteristics

Each layer relies on services from layer below and exports services to layer above Hides implementation - layers can change without disturbing other layers

1.2.2 Features of Layering

First, it decomposes the problem of building a network into more manageable components, rather than implementing a monolithic piece of software Second, it provides a more modular design.

1.2. Protocol

Building blocks of a network architecture Each protocol object has two different interfaces service interface: defines operations on this protocol peer-to-peer interface: defines messages exchanged with peer Term protocol is overloaded specification of peer-to-peer interface module that implements this interface

Each protocol defines two different interfaces as shown in below figure.

Service and peer interfaces1.4 OSI Architecture

The ISO Open Systems Interconnection (OSI) architecture is illustrated in below figure which defines a partitioning of network functionality into seven layers, where one or more protocols implement the functionality assigned to a given layer.

Note to Remember: PDNTSPA - People Do Need Touch Steves Pet Alligator

Starting at the bottom and working up, the physical layer handles the transmission of raw bits over a communications link. The data link layer then collects a stream of bits into a larger aggregate called a frame. The network layer handles routing among nodes within a packet-switched network. At this layer, the unit of data exchanged among nodes is typically called a packet rather than a frame. The lower three layers are implemented on all network nodes, including switches within the network and hosts connected along the exterior of the network. The transport layer then implements what we have up to this point been calling a process-to-process channel. Here, the unit of data exchanged is commonly called a message rather than a packet or a frame. The session layer performs the synchronization. The presentation layer is concerned with the format of data exchanged between peers The application layer where application programs are running which interacts with the user and receives the message from the user.

Note:

Most networked devices have (at least) two addressesthe hardware or network address (also known as a MAC address) and the IP address. a MAC address and an IP address for the wireless network adapter, and a MAC address and an IP address for the wired network adapter (for plugging into an Ethernet wall port).The MAC, or Network, Address

The MAC address, also known as a network card address or physical address, uniquely identifies your computer or device to the any type of network.

Note that the Network Card Address/Physical Address/MAC Address is an alphanumeric value, which is listed in six groups of two hexadecimal digits, separated by hyphens (-) or colons (:), e.g., 01-2B-C5-67-89-ab or 01:2B:C5:67:89:ab.

The hardware MAC address is required to register a device on the any type of network, and must be entered before your device can connect to the University network. Knowing it can be very useful in network troubleshooting.

Definition:

A network address serves as a unique identifier for a computer on a network.

When set up correctly, computers can determine the addresses of other computers on the network and use these addresses to send messages to each other.

The Internet Protocol (IP) addresses consist of four bytes (32 bits) that uniquely identify all computers on the public Internet.

The Media Access Control (MAC) address. MAC addresses are six bytes (48 bits) that manufacturers of network adapters burn into their products to uniquely identify them.

Physical Layer Responsibilities(Made up of H/W)Responsible for transmitting individual bits from one node to the next

1. Defines the characteristics of interfaces and transmission media2. Defines the type of transmission media3. To transmit this stream of bits (0s and 1s), it must be encoded into signals. Physical layer only defines the type of representation of bits.4. Internet Architecture5. Defines the transmission rate. i.e., number of bits per second.6. Sender and receiver machine clock must be adjusted in order to have a same bit rate at both the sender and receiver side.

Data Link Layer Responsibilities(Made up of H/W and S/W)Responsible for transmitting frames from one to the next

1. Framing: Divides the stream of bits into frames (splits the more num. of small data units). The complete (Larger size) informations divided or splits the more num. of small data units and each has its own header and tailor with meaningful information.2. Physical addressing: It adds the header information to the frame. The header information is the address of sender and receiver.3. Flow control: It provides the flow control mechanism. The data rate received by the receiver < the data rate sent by the sender4. Error control: It provides the mechanism to find the damaged /lost/duplication of data in the transmission. The error control information is usually added to trailer part of the message5. Access control: It determines which device can use the medium when there are more number devices involved in the transmission.

Network Layer Responsibilities(Made up of H/W and S/W)Source-to-destination delivery, possibly across multiple networks

1. Logical addressing: Adding the network address in the header. 2. Routing: Transmitting the packets to the correct destination in the network.3. Forwarding: The data packets are forwarded from network layer to transport layer.

Transport Layer Responsibilities(Made up of H/W and S/W)Delivery of message from one process (running programs) to another

1. Process-to-process delivery of entire message: Delivering the message is not only to the correct destination but also to the specific Process.2. Port addressing: Inserting the port address in the header3. Segmentation and reassembly: Dividing the message into segments and each segment will have sequence number which is used by the transport layer at the receiver machine for reassembling it.4. Connection control:It may may be of any one of the two service: Connectionless or connection-oriented5. End-to-end flow control6. End-to-end error control

Session Layer Responsiblities(Made up of S/W)The services of this layer are

1. Dialogue discipline : This may be full duplex or half duplex.2. Recovery: It has checkpointing concept. So, if some failure happens between checkpoints then session layer retransmit all data since the last checkpoint. 3. Grouping: Defines the groups of data.

Presentation Layer Responsiblities(Made up of S/W)The services of this layer are

1. Data Compression : The data is compressed by using any one of the compression method.2. Encryption: The groups of data is encypted by using a secret key with the encryption algorithms.3. Data translation: The data is translated by using some mechanisms.4. Decryption: The encrypted data can be decrypted by using decryption algorithms with a secret key.

Application Layer Responsibilities(Made up of S/W)Responsible for providing services to the user

1. Enables user access to the network2. User interfaces and support for services such asi. E-Mail services forwarding and storage.ii. File transfer and accesses to and from the remote host.iii. Remote log-in: User can enter into a remote system and have the resources.iv. WWW Services.

1.5 Internet Architecture

The Internet architecture, which is also sometimes called the TCP/IP architecture

a four-layer model

At the lowest level are a wide variety of network protocols, denoted NET1, NET2, and so on. In practice, these protocols are implemented by a combination of hardware (e.g., a network adaptor) and software (e.g., a network device driver).

The second layer consists of a single protocolthe Internet Protocol (IP). This is the protocol that supports the interconnection of multiple networking technologies into a single, logical internetwork.

The third layer contains two main protocolsthe Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

TCP provides a reliable byte-stream channel, and UDP provides an unreliable datagram delivery channel

Running above the transport layer are a range of application protocols, such as FTP, TFTP (Trivial File Transport Protocol), Telnet (remote login), and SMTP (Simple Mail Transfer Protocol, or electronic mail), that enable the interoperation of popular applications.

Advantages of using IP Arch. over OSI Arch.

The Internet architecture does not imply strict layering. That is, the application is free to bypass the defined transport layers and to directly use IP or one of the underlying networks Looking closely at the internet protocol graph, it has an hourglass shapewide at the top, narrow in the middle, and wide at the bottom. That is, IP serves as the focal point for the architectureit defines a common method for exchanging packets among a wide collection of networks. It has the ability to adapt rapidly to new user demands and changing technologies.

1.6 ISO OSI Model Vs TCP/IP Model

OSI MODELTCP/IP MODEL

Seven layers: Physical layer, Data link layer, Network layer, Transport Layer, Session layer, Presentation layer, Application layer Four layers:Network layer, Transport layer, IP layer, Application layer

Each layer defines a family of functions and the functions are interdependent Each layer defines number of protocols and they are not dependent

Widely used in Local Area Network Used in internet

1.7 Network Performance

Bandwidth (also called as throughput)

Bandwidth is the amount of data that passes through a network connection over time (in a certain period of time) as measured in bps (bits per second). Throughput = Transfer Size / Transfer Time

For example, a standard dialup modem supports 56 Kbps of peak bandwidth. On a 10 Mbps network, for example, it takes 0.1 microseconds (us) to transmit each bit.

Numerous tools exist for administrators to measure the bandwidth of network connections. On LANs (local area networks), these tools include netperf and ttcp.

Likewise a traditional Ethernet network theoretically supports 100 Mbps of bandwidth, but this maximum amount cannot reasonably be achieved due to overhead in the computer hardware and operating systems.

What Is Network Latency (also called as delay)The term latency refers to any of several kinds of delays typically incurred in processing of network data.

A so-called low latency network connection is one that generally experiences small delay times, while a high latency connection generally suffers from long delays.

Network tools like ping tests and traceroute measure latency by determining the time it takes a given network packet to travel from source to destination and back, the so-called round-trip time (RTT). Round-trip time is not the only way to specify latency, but it is the most common.

Latency = Propagation+Transmit+QueuePropagation = Distance / Speed of Light Transmit = Size / Bandwidth

Light travels across different medium at different speeds: It travels at 3.0 x 108 Where Distance is the length of the wire over which the data will travel,SpeedOfLight is the effective speed of light over that wire.

1.8 Links

Links are medium that connects nodes to form a computer network.

Types of Links

The communication between the nodes is either based on a point-to-point model or a Multicast model. In the point-to-point model, a message follows a specific route across the network in order to get from one node to another. In the multicast model, on the other hand, all nodes share the same communication medium and, as a result, a message transmitted by any node can be received by all other nodes. A part of the message (an address) indicates for which node the message is intended.

All nodes look at this address and ignore the message if it does not match their own address.

Connection Types:

Connections between devices may be classified into three categories:

Simplex

This is a unidirectional connection, i.e., data can only travel in one direction. Simplex connections are useful in situations where a device only receives or only sends data (e.g., a printer).

Half-duplex

This is a bidirectional connection, with the restriction that data can travel in one direction at a time.

Full-duplex

This is a bidirectional connection in which data can travel in both directions at once. A full-duplex connection is equivalent to two simplex connections in opposite directions.

1.9 Direct Link Networks

There are five problems that must be addressed before the nodes can successfully exchange packets.

encoding problem framing problem error detection problem

Network adapter:

A Network adapter a piece of hardware that connects a node to a link. The network adapter contains a signaling component that actually encodes bits into signals at the sending node and decodes signals into bits at the receiving node.

Lets return to the problem of encoding bits into signals.

1.10 ENCODING

NRZ Non-return to zero transmits 1s as High Signal and 0s as Low signal.

Problem: Consecutive 1s (signal stays high on the links for an extended period of time) or 0s (signal stays low for a long time). There are two fundamental problems caused by long strings of 1s or 0s.

Low signal (0) may be interpreted as no signal and High signal (1) leads to baseline wander. Unable to recover clock (clock Recovery).

Baseline Wander: Specifically, the receiver keeps an average of the signal, and then uses this average to distinguish between low and high signals.

Whenever the signal is significantly lower than this average, the receiver concludes that it has just seen a 0, and likewise higher than the average is interpreted to be a 1.

The problem, if too many consecutive 1s or 0s cause this average to change, making it more difficult to detect a significant change in the signal.

Clock Recovery:

The frequent transitions from high to low and vice versa are necessary to enable clock recovery.

The clock recovery problem is that both encoding and the decoding processes are driven by a clock.

Every clock cycle the sender transmits a bit and the receiver recovers a bit.

The senders and receivers clocks have to be precisely synchronized in order for the receiver to recover the same bits the sender transmits.

If the receivers clock is even slightly faster or slower than the senders clock, then it does not correctly decode the signal.

So far clock recovery process, the receiver derives the clock from the received signal. Whenever the signal changes, such as 0 to 1 or from 1 to 0, then the receiver knows it is at a cycle boundary, and it can resynchronize itself.

NRZI (non-return to zero inverted ):Transition data if input is 1, and no transition if input is 0.

Manchester encoding: A transition occurs in the middle of the bits. 0 becomes a low to high transition and 1 high to low

Differential Manchester: a transition in the beginning of the interval to transmit 0. No transition in the beginning of the interval to transmit 1. The transition in the middle is always present.

4B/5B Encoding: Problem: consecutive zeros Idea: Every 4 bits of data is encoded in a 5-bit code, with the 5-bit codes selected to have no more than one leading 0 and no more than two trailing 0 (i.e., never get more than three consecutive 0s). Resulting 5-bit codes are then transmitted using the NRZI encoding. Achieves 80% efficiency.

1.11 Framing

Breaking sequence of bits into a frame Must determine first and last bit of the frame Typically implemented by network adapter Adapter fetches (deposits) frames out of (into) host memory

The network adaptor that enables the nodes to exchange frames.

When node A wishes to transmit a frame to node B, it tells its adaptor to transmit a frame from the nodes memory. This results in a sequence of bits being sent over the link. The adaptor on node B then collects together the sequence of bits arriving on the link and deposits the corresponding frame in Bs memory. Recognizing exactly what set of bits constitutes a framethat is, determining where the frame begins and endsis the central challenge faced by the adaptor. .

1.11.1 Approaches

There are several ways to address the framing problem.

Some of them are:

1. Byte Oriented: Special character to delineate frames, replace character in data streama. Sentinel approachb. Byte counting approach2. Bit Oriented: use a technique known as bit stuffing3. Clock Based: fixed length frames, high reliability required

1.11.1.1 Byte-Oriented Protocols

A byte-oriented approach is exemplified by the BISYNC (Binary Synchronous Communication) protocol developed by IBM. The BISYNC protocol illustrates the sentinel approach to framing; its frame format is depicted in the following figure

Sentinel Approach PPP protocol uses 0x7e=01111110 as the flag byte to delimit a frame When a 0x7e is seen in the payload, it must be escaped to keep it from being seen as an end of frame

The beginning of a frame is denoted by sending a special SYN (synchronization) character. The data portion of the frame is then contained between special sentinel characters: STX (start of text) and ETX (end of text). The SOH (start of header) field serves much the same purpose as the STX field. The frame contains additional header fields that are used for, among other things, the link-level reliable delivery algorithm

The problem with the sentinel approach, is that the ETX character might appear in the data portion of the frame. BISYNC overcomes this problem by escaping the ETX character by preceding it with a DLE (data-link-escape) character whenever it appears in the body of a frame;The DLE character is also escaped (by preceding it with an extra DLE) in the frame body. This approach is often called character stuffing because extra characters are inserted in the data portion of the frame.

Point-to-Point Protocol (PPP) is similar to BISYNC in that it uses character stuffing.The format for a PPP frame is given in Figure.

The special start-of-text character, denoted as the Flag field is 01111110. The Address and Control fields usually contain default values, and so are uninteresting. The Protocol field is used for demultiplexing. The frame payload size can be negotiated, but it is 1500 bytes by default.The Checksum field is either 2 (by default) or 4 bytes long used for error detection.

Byte counting approach

The number of bytes contained in a frame can be included as a field in the frame header.DDCMP protocol uses this approach, as illustrated in the following figure

The COUNT field specifies how many bytes are contained in the frames body. One danger with this approach is that a transmission error could corrupt the COUNT field, in which case the end of the frame would not be correctly detected.

1.11.1.2 Bit Oriented approach

The High-Level Data Link Control (HDLC) protocol developed by IBM is an example of a bit-oriented protocol.

HDLC: High-Level Data Link Control

Delineate frame with a special bit-sequence: 01111110

Its frame format is given in below figure.

Bit-oriented protocols use a technique known as bit stuffing.

Bit Stuffing:

The delimiting bit pattern used is 01111110 and is called a flag. To avoid this bit pattern occurring in user data, the transmitter inserts a 0 bit after every five consecutive 1 bits it finds. This is called bit stuffing.

Sender: any time five consecutive 1s have been transmitted from the body of the message, insert a 0. Receiver: should five consecutive 1s arrive, look at next bit(s): if next bit is a 0: remove it if next bits are 10:end of frame if next bits are 11: error

Bit stuffing Example Original Data 001111111000011111100 Bit Stuffed 00111110110000111110100 Receiver 0011111011000011111010001111110

1.11.1.3 Clock-Based Framing

This approach to framing is used by the Synchronous Optical Network (SONET) standard.

1.12 Error Detection and Correction

Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected

Types of Error Single-Bit Error Burst Error

Single-Bit Error

In a single-bit error, Only one bit is changed: 0 changed to 1, or a 1 to a 0

Burst Error Two or more bits in data unit are in error, not necessarily consecutive in order

The basic idea behind any error detection scheme is to add redundant information to a frame that can be used to determine if errors have been introduced. In other words, error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination as shown in below figure.

We say that the extra bits we send are redundant because they add no new information to the message. Instead, they are derived directly from the original message using some well-defined algorithm. Both the sender and the receiver know exactly what that algorithm is. The sender applies the algorithm to the message to generate the redundant bits. It then transmits both the message and those few extra bits. When the receiver applies the same algorithm to the received message, it should (in the absence of errors) come up with the same result as the sender. It compares the result with the one sent to it by the sender. If they match, it can conclude (with high likelihood) that no errors were introduced in the message during transmission. If they do not match, it can be sure that either the message or the redundant bits were corrupted, and it must take appropriate action, that is, discarding the message, or correcting it if that is possible.

1.13 Error Detection methods

Parity Check

1. Simple-parity check 2. Two dimensional parity check

Simple-parity check

In this parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity). The following figure illustrates this concept.

Suppose the sender wants to send the word world. In ASCII the five characters are coded as

1110111 1101111 1110010 1101100 1100100

The following shows the actual bits sent

11101110 11011110 11100100 11011000 11001001

Now suppose the word world is received by the receiver without being corrupted in transmission.

11101110 11011110 11100100 11011000 11001001

The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

Now suppose the word world is corrupted during transmission.

11111110 11011110 11101100 11011000 11001001

The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

Performance

Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.

Two dimensional parity check

In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block.

Example 1:

Example 2:Suppose the following block is sent:

10101001 00111001 11011101 11100111 10101010

1010100 1 0011100 1 1101110 1 1110011 1 --------------- 1010101 0---------------

However, it is hit by a burst noise of length 8, and some bits are corrupted.

10100011 10001001 11011101 11100111 10101010

1010001 1 1000100 1 1101110 1 1110011 1 --------------- 0001000 0--------------- When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded. 10100011 10001001 11011101 11100111 10101010

CRC Parity checks based on addition; CRC based on binary division A sequence of redundant bits (a CRC or CRC remainder) is appended to the end of the data unit These bits are later used in calculations to detect whether or not an error had occurred.CRC Steps On senders end, data unit is divided by a predetermined divisor; remainder is the CRC When appended to the data unit, it should be exactly divisible by a second predetermined binary number At receivers end, data stream is divided by same number If no remainder, data unit is assumed to be error-free Deriving the CRC A string of 0s is appended to the data unit; n is one less than number of bits in predetermined divisor New data unit is divided by the divisor using binary division; remainder is CRC CRC of n bits replaces appended 0s at end of data unit

CRC Generator function

CRC Checker function

Polynomial

The divisor number used in the CRC algorithm, which is (n+1) bit in length, can also be considered as the coefficients of a polynomial, called Generator Polynomial which is shown below.

The divisor can give n-bit length CRC remainder. For example, for the divisor 11001 the corresponding polynomial is X4+X3+1.

A polynomial is Used to represent CRC generator Cost effective method for performing calculations quickly

Standard CRC polynomials

Name Polynomial Application

CRC-8 x8 + x2 + x + 1 ATM header

CRC-10 x10 + x9 + x5 + x4 + x 2 + 1 ATM AAL

ITU-16 x16 + x12 + x5 + 1 HDLC

ITU-32 x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 LANs

A polynomial is selected to have at least the following properties:

It should not be divisible by X. It should not be divisible by (X+1).

The first condition guarantees that all burst errors of a length equal to the degree of polynomial are detected.

The second condition guarantees that all burst errors affecting an odd number of bits are detected.

CRC Performance

CRC can detect all single-bit errors CRC can detect all double-bit errors (three 1s) CRC can detect any odd number of errors (X+1) CRC can detect all burst errors of less than the degree of the polynomial. CRC detects most of the larger burst errors with a high probability. For example CRC-12 detects 99.97% of errors with a length 12 or more.

Checksum generator function

Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. 10101001 00111001

The numbers are added using ones complement 10101001 00111001 ------------Sum 11100010

Complements of SumChecksum 00011101

Now the pattern sent is 10101001 00111001 00011101

Checksum checker function

Now suppose the receiver receives the pattern sent without any corruption:

10101001 00111001 00011101

The receiver performs the checksum checker function to ensure whether the received pattern is corrupted or not.

When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error. 1010100100111001 00011101 -----------Sum11111111

Complement 00000000 means that the pattern is OK.

Now suppose there is a burst error of length 5 introduced as

10101111 11111001 00011101

When the receiver adds the three sections, it gets

10101111 11111001 00011101 -----------Partial Sum 1 11000101Carry 1 -----------Sum 11000110

Complement 00111001 the pattern is corrupted.

Performance

Detects all errors involving odd number of bits, most errors involving even number of bits If one or more bits of a segment are damaged and the corresponding bits of opposite value in a second segment are also damaged, the sums of these columns will not change and the receiver will not detect a problem.

1.14 Transmission Media There are 2 basic categories of Transmission Media: Guided UnguidedGuided Transmission Media: uses a "cabling" system that guides the data signals along a specific path.

Unguided: The medium transmits the waves but does not guide

Twisted Pair Cable

The popularity can be attributed to the fact that it is lighter, more flexible, and easier to install than coaxial or fiber optic cable It is also cheaper and can achieve greater speeds than its coaxial competition. Ideal solution for most network environments. Two main types of twisted-pair cabling are: Unshielded Twisted Pair (UTP) more commonplace than STP and is used for most networks Shielded Twisted Pair (STP) used in environments in which greater resistance to EMI and attenuation is required. the greater resistance comes at a price. This extra protection increases the distances that data signals can travel over STP but also increases the cost of the cabling UTP: one or more pairs of twisted copper wires insulated and contained in a plastic cover Uses RJ-45 telephone connector STP: Same as UTP but with a aluminium/ polyester shield. Connectors are more awkward to work with

Twisted nature to reduce crosstalk. Any interference from a physically adjacent channel that corrupts the signal and causes trans- mission errors is what is known as crosstalk. UTP Categories: Categories 1 and 2 (CAT 1 and CAT 2) voice grade low data rates up to 4 Mbps Category 3 (CAT 3) suitable for most LANs up to 16 Mbps Category 4 up to 20 Mbps Category 5 Supports Fast Ethernet 100Mbps more twists per foot more stringent standards on connectors Category 5e up to 1000 Mbps Category 6 up to 1000 Mbps + Data grade UTP cable usually consists of either 4 or 8 wires, two or four pair

Coaxial Cable

Commonly referred to as coax Coax found success in both TV signal transmission as well as in network implementations. Constructed with a copper core at the centre that carries the signal, plastic insulation, braided metal shielding, and an outer plastic covering Constructed this way to avoid: Attenuation the loss of signal strength as it travels over distance Crosstalk the degradation of a signal caused by signals from other cables running close to it EMI Electromagnetic Interference Networks can use two types of coaxial cabling: thin coaxial and thick coaxial Thin coax is only .25 inches in diameter, making it fairly easy to install Disadvantages of all thin coax types are that they are prone to cable breaks

Size of Coax RG-8, RG-11 50 ohm Thick Ethernet RG-58 50 ohm Thin Ethernet RG-59 75 ohm Cable T.V.Fiber Optic

Addresses the shortcomings associated with copper-based media Use light transmissions instead of electronic pulses Advantages: Threats such as EMI, crosstalk, and attenuation become a nonissue Well suited for the transfer of data, video, and voice transmissions It is the most secure of all cable media Disadvantages: difficult installation and maintenance procedures of fiber often require skilled technicians with specialized tools the cost of a fiber-based solution limits the number of organizations that can afford to implement it incompatible with most electronic network equipment; you have to purchase fiber-compatible network hardware. Composed of a core glass fiber surrounded by cladding An insulated covering then surrounds both of these within an outer protective cover Two types of fiber-optic cable are available: single and multimode fiber multimode fiber, many beams of light travel through the cable bouncing off of the cable walls weakens the signal, reducing the length and speed the data signal can travel Single-mode fiber uses a single direct beam of light allows for greater distances and increased transfer speeds

core: inner-most section cladding: surrounding the core jacket: outermost layer, surrounding one or a bundle of cladded fibers Common types of fiber-optic cable include the following: 62.5 micron core/125 micron cladding multimode 50 micron core/125 micron cladding multimode 8.3 micron core/125 micron cladding single mode The main advantage of optical fiber is the great bandwidth it can carry.

2 Unguided Media

Provides a means for transmitting electro-magnetic signals through air but do not guide them. Also referred to as wireless transmission Wireless communications uses specific frequency bands which separates the ranges. Main types: radio waves, microwaves, Bluetooth and Infrared. Transmission and reception are achieved by means of antennas For transmission, an antenna radiates and electromagnetic radiation in the air For reception, the antenna picks up electromagnetic waves from the surrounding medium The antenna plays a key role; the characteristics of the antenna and the frequency that it receives

Reliable Transmission-Stop and Wait ARQ - Sliding Window Algorithm- Selective Repeat ARQ

Even when error-correcting codes are used some errors will be too severe to be corrected. As a result, some corrupt frames must be discarded. A link-level protocol that wants to deliver frames reliably must somehow recover from these discarded (lost) frames.

This is usually accomplished using a combination of two fundamental mechanisms1. acknowledgments 2. timeouts An acknowledgment (ACK for short) is a small control frame that a protocol sends back to its peer saying that it has received an earlier frame. By control frame we mean a header without any data. If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame. This action of waiting a reasonable amount of time is called a timeout.

Piggybacking: To improve the use of network bandwidth, an acknowledgment method known as piggybacking is often used. In piggybacking, instead of sending a separate acknowledgment frame, the receiver waits until it has data frame to send to the sender and embeds the acknowledgment in that data frame.Thus the link bandwidth can be utilized better also it increases the speed of data transmission.

Propagation delay is defined as the delay between transmission and receipt of packets between hosts. Propagation delay can be used to estimate timeout period

The general strategy of using acknowledgments and timeouts to implement reliable delivery is sometimes called automatic repeat request (ARQ).

There are four different ARQ algorithms:

Stop-and-Wait ARQ Sliding Window ARQ Go back N ARQ Selective Repeat ARQ

Stop and Wait ARQ

Sender doesnt send next frame until hes sure receiver has last packet The data frame/Ack. Frame sequence enables reliability. They are sequenced alternatively 0 and 1 Sequence numbers help avoid problem of duplicate frames If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame The sender also starts retransmission when the timeout occurs.

a) Normal operation b) The Original frame is lost c) The ACK is lost d) Timeout occurs

Disadvantage

The link capacity can not be utilized effectively since only one data frame or ACK frame can e sent at a time

Sliding Window Algorithm

The sliding window algorithm works as follows:

1. The sender assigns a sequence number, denoted SeqNum, to each frame.

2. The sender maintains three variables: a. SWS denotes the send window size, gives the upper bound on the number of outstanding (unacknowledged) frames that the sender can transmit; b. LAR denotes the sequence number of the last acknowledgment received; and c. LFS denotes the sequence number of the last frame sent.

3. The sender maintains the following invariant:LFS LAR SWSThis situation is illustrated in below figure

When an acknowledgment arrives, the sender moves LAR to the right, thereby allowing the sender to transmit another frame.

The sender associates a timer with each frame it transmits, and it retransmits the frame should the timer expire before an ACK is received.

4. The receiver maintains the following three variables:a. RWS denotes the receive window size, gives the upper bound on the number of out-of-order frames that the receiver is willing to accept; b. LAF denotes the sequence number of the largest acceptable frame; c. LFR denotes the sequence number of the last frame received.

5. The receiver also maintains the following invariant: LAF LFR RWS

6. if LFR SWS since its impossible for more than SWS frames to arrive out of order.

Advantages

Reliable transmission: The algorithm can be used to reliably deliver messages across an unreliable network Frame Order: The sliding window algorithm can serve is to preserve the order in which frames are transmitted. Since each frame has a sequence number. Flow control: The receiver not only acknowledges frames it has received, but also informs the sender of how many frames it has room to receive The link capacity can be utilized effectively since multiple frames can be transmitted at a time

Selective Repeat ARQ:

Upon encountering a faulty frame, the receiver requests the retransmission of that specific frame. Since additional frames may have followed the faulty frame, the receiver needs to be able to temporarily store these frames until it has received a corrected version of the faulty frame, so that frame order is maintained.

Go Back N ARQ:

A simpler method, Go-Back-N, involves the transmitter requesting the retransmission of the faulty frame as well as all succeeding frames (i.e., all frames transmitted after the faulty frame).

Advantages

The advantage of Selective Reject over Go-Back-N is that it leads to better throughput, because only the erroneous frames are retransmitted.

Go-Back-N has the advantage of being simpler to implement and requiring less memory.

Ethernet Token ring FDDI - Wireless LAN Bridges and Switches

Ethernet (802.3) Most successful local area networking technology of the last 20 years. The Ethernet is a working example of the more general Carrier Sense Multiple Access with Collision Detect (CSMA/CD).

CSMA/CD: Ethernets Media Access Control (MAC) policy CS = carrier sense Send only if medium is idle MA = multiple access, i.e. a set of nodes send and receive frames over a shared link. CD = collision detection Stop sending immediately if collision is detected

Physical Properties:

Addresses:Unique, 48-bit unicast address assigned to each adapterExample: 8:0:e4:b1:2The address will be used for broadcast all 1s in the addressThe address is multicast if the first bit is 1

Bandwidths: 10Mbps, 100Mbps, 1Gbps

Max bus length: 2500m

500m segments with 4 repeaters Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or switch Detects line state and sends/receives signals Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the medium

Transceiver: A small device directly attached to the tap It detects when the line is idle and drives the signal when the host is transmitting. It also receives incoming signals. The transceiver is, in turn, connected to an Ethernet adaptor, which is plugged into the host.

Repeater: Multiple Ethernet segments can be joined together by repeaters. A repeater is a device that forwards digital signals, much like an amplifier forwards analog signals. Any signal placed on the Ethernet by a host is broadcast over the entire networkEthernet standards

10Base2 - can be constructed from a thinner cable called as thin-net, 200m length10Base5 - can be constructed from a thick cable called as thick-net, 500m length10BaseT - can be constructed from twisted pair cable; 10 in 10Base2 means that the network operates at 10 Mbps, Base refers to the fact that the cable is used in a baseband system, 2 means that a given segment can be no longer than 200 m. T stands for twisted pair, limited to less than 100 m in lengthAccess Method: CSMA/CD

Carrier Sense: This protocol is applicable to a bus topology. Before a station can transmit, it listens to the channel to see if any other station is already transmitting. If the station finds the channel idle, it attempt to transmit; Otherwise, it waits for the channel to become idle.

If two or more stations find the channel idle and simultaneously attempt to transmit. This is called a collision. When collision occurs, the station should suspend transmission and re-attempts after a random period of time. Use of a random wait period reduces the chance of the collision recurring. The following flow chart depicts this technique.

If line is idle send immediately upper bound message size of 1500 bytes minimum frame is 64 bytes (header + 46 bytes of data)

If line is busy wait until idle and transmit immediately

If collision send jam signal, and then stop transmitting frame delay for exponential Back off time and try again

Exponential Back off 1st time: 0 or 51.2us (us means micro seconds) 2nd time: 0, 51.2, or 102.4us 3rd time51.2, 102.4, or 153.6us nth time: k x 51.2us, for randomly selected k=0..2^n - 1 give up after several tries (usually 16)

Transmitter Algorithm:

When the adapter has a frame to send and the line is idle; it transmits the frame immediately.When the adapter has a frame to send and the line is busy; it waits for the line to go idle and then transmits immediately.

1-Persistent protocol:An adaptor with a frame to send transmits with probability 1 whenever a busy line goes idle. P-Persistent protocol:An adaptor with a frame to send transmits with probability 0 TTRT, then Token is late, so that station does not transmit data If measured TRT < TTRT, then Token is early; station holds token for difference between TTRT and measured TRT and can transmit data.

Division into traffic classes

Traffic is divided into two classes1. Synchronous traffic Traffic is delay sensitive station transmits data whether token is late or early But synchronous cannot exceed one TTRT in one TRT 2. Asynchronous traffic Station transmits only if token is early

Token Maintenance

Every node monitors ring for valid token. If operations are correct, a node must observe a token or a data frame every so often.Claim frames When Greatest idle time = Ring latency + frame transmission time, and if nothing seen during this time, a node suspects something is wrong on the ring and sends a claim frame. Then, node bid (propose) for the TTRT using the claim frame. The bidding process A node can send a claim frame without having the token when it suspects failure If claim frame came back, node knows that its TTRT bid is the lowest. And now it is responsible for inserting token on the ring. When a node receives a claim frame, it checks to see if the TTRT bid is lower than its own. If yes, it resets local definition of TTRT and simply forwards the claim frame. Else, it removes the claim frame and enters the bidding process Put its own claim frame on ring. When there are ties, highest address wins.FDDI Analysis

In the worst case: First async. traffic use one TTRT worth of time. Next sync. traffic use one TTRT worth of time.So, TRT at a node = 2 * TTRT It is important to note that if Sync. traffic was transmitted first and used TTRT, no async. traffic can be sent.

Difference between Token Ring and FDDI

Token RingFDDI

Shielded twisted pair 4, 16 Mbps No reliability specified Differential Manchester Centralized clock Access control: Token Passing It uses delayed release of token Optical Fiber 100 Mbps Reliability specified (dual ring) 4B/5B encoding Distributed clocking Access control: Timed Token algorithm Early release of token is used

Wireless LAN - IEEE 802.11

802.11 was designed to run over three different physical mediatwo based on spread spectrum radio and one based on diffused infrared.The idea behind spread spectrum is to spread the signal over a wider frequency band than normal, so as to minimize the impact of interference from other devicesFrequency Hopping Spread Spectrum (FHSS) Frequency hopping is a spread spectrum technique that involves transmitting the signal over a random sequence of frequencies; that is, first transmitting at one frequency, then a second, then a third, and so on. The receiver uses the same algorithm as the senderand initializes it with the same seedand hence is able to bound with the transmitter to correctly receive the frame.Direct Sequence Spread Spectrum (DSSS) The DSSS encoder spreads the data across a broad range of frequencies using a mathematical key. The receiver uses the same key to decode the data. It sends redundant copies of the encoded data to ensure reception.Infrared (IR)The Infrared utilizes infrared light to transmit binary data using a specific modulation technique. The infrared uses a 16-pulse position modulation (PPM). Access control: CSMA/CA(Carrier Sense Multiple Access /Collision Avoidance)

a wireless protocol would follow exactly the same algorithm as the Ethernetwait until the link becomes idle before transmitting and back off should a collision occur.

Consider the situation depicted in figure, where each of four nodes is able to send and receive signals that reach just the nodes to its immediate left and right. For example, B can exchange frames with A and C but it cannot reach D, while C can reach B and D but not A. a carrier sensing scheme is used. a node wishing to transmit data has to first listen to the channel for a predetermined amount of time to determine whether or not another node is transmitting on the channel within the wireless range. If the channel is sensed "idle," then the node is permitted to begin the transmission process. If the channel is sensed as "busy," the node defers its transmission for a random period of time. This is the essence of both CSMA/CA and CSMA/CD. In CSMA/CA, once the channel is clear, a station sends a signal telling all other stations not to transmit, and then sends its packet.Assume that node A has data to transfer to node B. Node A initiates the process by sending a Request to Send frame (RTS) to node B. The destination node (node B) replies with a Clear to Send frame (CTS). After receiving CTS, node A sends data. After successful reception, node B replies with an acknowledgement frame (ACK). If node A has to send more than one data fragment, it has to wait a random time after each successful data transfer and compete with adjacent nodes for the medium using the RTS/CTS mechanism.To sum up, a successful data transfer (A to B) consists of the following sequence of frames:1. Request To Send frame (RTS) from A to B Clear To Send frame (CTS) from B to A Data frame (Data) from A to B Acknowledgement frame (ACK) from B to A.

The following flow graph explains the CSMA/CA technique

Hidden nodes problem

Suppose both A and C want to communicate with B and so they each send it a frame. A and C are unaware of each other since their signals do not carry that far. These two frames collide with each other at B, but A and C is not aware of this collision. A and C are said to be hidden nodes with respect to each other.

Solution for the hidden node problem

When node A wants to send a packet to node B Node A first sends a Request-to-Send (RTS) to B On receiving RTS Node B responds by sending Clear-to-Send (CTS) provided node B is able to receive the packet When a node C sees a CTS, it should keep quiet for the duration of the transfer

Exposed node problem

A related problem, called the exposed node problem, occurs under the following circumstances.

B talks to A C wants to talk to D C senses channel and finds it to be busy So, C stays quiet

B is sending to A in figure. Node C is aware of this communication because it hears Bs transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear Bs transmission.

Solution for Exposed Terminal Problem

Sender transmits Request to Send (RTS) Receiver replies with Clear to Send (CTS) If Neighbors See CTS - Stay quiet See RTS, but no CTS then O.K to transmit

Reliability

When node B receives a data packet from node A, node B sends an Acknowledgement (ACK)

If node A fails to receive an ACK Retransmit the packetFrame Format

The frame contains the source and destination node addresses, each of which are 48 bits long; up to 2312 bytes of data; and a 32-bit CRC. The Control field contains three subfields of interest (not shown): a 6-bit Type field that indicates whether the frame carries data, is an RTS or CTS frame. Addr1 identifies the target node, and Addr2 identifies the source node. Addr3 identifies the intermediate destination.

Switches

forwards packets from input port to output port port selected based on address in packet header adding more hosts will not deteriorate older connections

Advantagesit covers large geographic area (tolerate latency)it supports large numbers of hosts (scalable bandwidth)

Types

I. Datagram switchingII. Virtual Circuit switchingIII. Source Routing switching

I Datagram Switching No connection setup phase Each packet forwarded independently Sometimes called connectionless model Packets may follow different paths to reach their destination Receiving station may need to reorder Switches decide the route based on source and destination addresses in the packet Analogy: postal system Each switch maintains a forwarding table

Switching table for switch2

II Virtual Circuit Switching Sometimes called connection-oriented model Relationship between all packets in a message or session is preserved Single route is chosen between sender and receiver at beginning of session Call setup establishes virtual circuit; call teardown deletes the virtual circuit All packets travel the same route in order Approach is used in WANs, Frame Relay, and ATM Analogy:phone call Each switch maintains a VC table The VC table in a single switch contains a Virtual circuit identifier (VCI) an incoming interface on which packets for this VC arrives an outgoing interface on which packets for this VC leave potentially different VCI for outgoing packets

III Source Routing

A third approach to switching that uses neither virtual circuits nor conventional datagrams is known as source routing. The name derives from the fact that all the information about network topology that is required to switch a packet across the network is provided by the source host. Assign a number to each output of each switch and to place that number in the header of the packet. The switching function is then very simple: For each packet that arrives on an input, the switch would read the port number in the header and transmit the packet on that output. There will be more than one switch in the path between the sending and the receiving host. In such a case the header for the packet needs to contain enough information to allow every switch in the path to determine which output the packet needs to be placed on.

In this example, the packet needs to traverse three switches to get from host A to host B. At switch 1, it needs to exit on port 1, at the next switch it needs to exit at port 0, and at the third switch it needs to exit at port 3. Thus, the original header when the packet leaves host A contains the list of ports (3, 0, 1), where we assume that each switch reads the rightmost element of the list. To make sure that the next switch gets the appropriate information, each switch rotates the list after it has read its own entry. Thus, the packet header as it leaves switch 1 en route to switch 2 is now (1, 3, 0); switch 2 performs another rotation and sends out a packet with (0, 1, 3) in the header.

Bridges and Extended LANsA class of switches that is used to forward packets between shared-media LANs such as Ethernets. Such switches are sometimes known by the name of LAN switches; historically they have also been referred to as bridges. Operate in both physical and data link layers

LANs have physical limitations (e.g 2500m). Bridge is used to connect two or more LANs as shown below It uses Store and forward technique.

Extended LANs

a collection of LANs connected by one or more bridges is usually said to form an extended LAN.

Learning Bridges/Transparent bridge

A bridge maintains a forwarding table to forward the packet that it receives. The forwarding table contains two fields. One is host address filed and another one is used for storing the port number of bridge on which the host is connected. For example,

Each packet carries a global address, and the bridge decides which output to send a frame on by looking up that address in a table. Bridge inspects the source address in all the frames it receives and records the fact in the table. When a frame is received by the bridge, it opens the frame to see the destination address and then it checks the destination address in the forwarding table. Suppose if the destination address is available in the table then it forwards the frame to the respective one of its output port which is mentioned for that destination host in the table. Suppose if the destination address is not in the table then it floods the frame to all of its output port and then it uses the source address of the frame to update the table. Thus the bridge learns the table entry to decide whether to forward/discard the frame or to update the table and the algorithms are listed below:

Learning bridge example

Spanning Trees It ensures the topology has no loops Spanning tree Sub-graph that covers all vertices but contains no cycles Links not in the spanning tree do not forward frames

Constructing a Spanning Tree Need a distributed algorithm Switches cooperate to build the spanning tree

Key ingredients of the algorithm Switches need to elect a root The switch with the smallest identifier Each switch identifies if its interface is on the shortest path from the root and it exclude from the tree if not Messages (Y, d, X) From node X Claiming Y is the root And the distance is d

Steps in Spanning Tree Algorithm

Initially, each switch thinks it is the root Switch sends a message out every interface identifying itself as the root with distance 0 Example: switch X announces (X, 0, X) Switches update their view of the root Upon receiving a message, check the root id If the new id is smaller, start viewing that switch as root Switches compute their distance from the root Add 1 to the distance received from a neighbor Identify interfaces not on a shortest path to the root and exclude them from the spanning tree

Example From Switch #4s Viewpoint

Switch #4 thinks it is the root Sends (4, 0, 4) message to 2 and 7 Then, switch #4 hears from #2 Receives (2, 0, 2) message from 2 and thinks that #2 is the root And realizes it is just one hop away Then, switch #4 hears from #7 Receives (2, 1, 7) from 7 And realizes this is a longer path So, prefers its own one-hop path And removes 4-7 link from the tree Switch #2 hears about switch #1 Switch 2 hears (1, 1, 3) from 3 Switch 2 starts treating 1 as root And sends (1, 2, 2) to neighbors Switch #4 hears from switch #2 Switch 4 starts treating 1 as root And sends (1, 3, 4) to neighbors Switch #4 hears from switch #7 Switch 4 receives (1, 3, 7) from 7 And realizes this is a longer path So, prefers its own three-hop path And removes 4-7 Iink from the tree