What is bridge in computer networking?

Segmenting a large network with a network device has numerous benefits. Among these are reduced collisions (in an Ethernet network), contained bandwidth utilization, and the ability to filter out unwanted packets. However, if the addition of the interconnect device required extensive reconfiguration of stations, the benefits of the device would be outweighed by the administrative overhead required to keep the network running. Bridges were created to allow network administrators to segment their networks transparently. This means that individual stations need not know whether there is a bridge separating them or not. It is up to the bridge to make sure that packets get properly forwarded to their destinations. This is the fundamental principle underlying all of the bridging behaviors.

Bridges work at the Data Link layer of the OSI model. Since bridges work in the Data Link layer they do not examine the network layer addresses. They just look at the MAC addresses for Ethernet and Token Ring, token bus and determine whether or not to forward or ignore a packet.

Functions of a Bridge

1. Isolates networks by MAC addresses
2. Manages network traffic by filtering packets
3. Translates from one MAC protocol to another

Now let us examine the functionality of a bridge in detail.

1. Isolates networks by MAC addresses

A bridge divides a network into separate collision domains. This reduces congestion as only frames that need to be forwarded are sent across interfaces. All transmissions between nodes connected to same segment are not forwarded and therefore do not load the rest of the network.

Thus bridges effectively improve the bandwidth of the network by reducing the unnecessary traffic in the network.

For example, if you have one segment called Segment 100: it has 50 users (in several departments) using this network segment. The Engineering Department is CAD (Computer Aided Design) - oriented, while the Accounting Department is into heavy number crunching (year end reports, month end statements, etc.). On this network, any traffic between Clients of Accounting Department and the Accounting File Server (in the Accounting Department) will be heard across the Segment 100. Likewise, any traffic between the Engineering Dept clients (to the CAD File Server) will be heard throughout the Network Segment. The result is that “Other” Department accesses to the Generic File Server are incredibly slow: this is because of the unnecessary traffic that’s being generated from other departments (Engineering and Accounting).

The solution is to use one Bridge to isolate the Accounting Department, and another bridge to isolate the Engineering Department. The Bridges will only allow packets to pass through that are not on the local segment. The bridge will first check its “routing” table to see if the packet is on the local segment. If it is, it will ignore the packet, and not forward it to the remote segment. If Client of Accounting Department sends a packet to the Accounting File Server then Bridge #1 will check its routing table (to see if the Accounting File Server is on the local port). If it is on the local port, then Bridge #1 will not forward the packet to the other segments. If a Client of Accounting Department sends a packet to the Generic File Server, Bridge #1 will again check its routing table to see if the Generic File Server is on the local port. If it is not, then Bridge #1 will forward the packet to the remote port.

2. Manages network traffic by filtering packets

Bridges listen to the network traffic, and build an image of the network on each side of the bridge. This image of the network indicates the location of each node (and the bridge’s port that accesses it). With this information, a bridge can make a decision whether to forward the packet across the bridge - if the destination address is not on the same port - or, it can decide not to forward the packet (if the destination is on the same port).

This process of deciding whether or not to forward a packet is termed “filtering packets.” Network traffic is managed by deciding which packets can pass through the bridge; the bridge filters packets.

3. Translates from one protocol to another

The MAC layer also contains the bus arbitration method used by the network. This can be CSMA/CD, as used in Ethernet, or Token Passing, as used in Token Ring. Bridges are aware of the Bus Arbitration and special translation bridges can be used to translate between Ethernet and Token Ring LANs.

Bridges physically separate a network segment by managing the traffic (that’s based on the MAC address). Bridges are store and forward devices. They receive a packet on the local segment, store it, and wait for the remote segments to be clear before forwarding the packet. The two physical types of bridges are Local and Remote Bridges.

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What are the functions of Repeaters

All types of network connections suffer from attenuation and pulse distortion. For a given cable specification and bit rate, each has a maximum length of cable. Repeaters can be used to increase the maximum interconnection length and will do the following:

Functions of Repeaters

• Clean signal pulses.
• Pass all signals between attached segments.
• Boost signal power.
• Possibly translate between two different media types (e.g., fiber – optic to twisted – pair cable)

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Stop and Wait Protocol

The sender allows one message to be transmitted, checked for errors and an appropriate ACK (Affirmative Acknowledgement) or NAK (Negative Acknowledgement) returned to the sending station. No other data messages can be transmitted until the receiving station sends back a reply, thus the name STOP and WAIT is derived from the originating station sending a message, stopping further transmission and waiting for a reply.

Its major drawback is the idle line time that results when the stations are in the waiting period. If the ACK is lost then the sending station retransmits the same message to the receiver side. The redundant transmission could possibly create a duplicate frame. A typical approach to solve this problem is the provision for a sequence number in the header of the message. The receiver can then check for the sequence number to determine if the message is a duplicate. The Stop and Wait mechanism requires a very small sequence Number, since only one message is outstanding at any time. The sending and receiving station only use a one bit alternating sequence of 0 and 1 to maintain the relationship of the transmitted message and its ACK/NAK status.

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Frequency Division & Time Division Multiplexing

Frequency Division Multiplexing (FDM)

In FDM the frequency spectrum is divided to form logical channels with each user having exclusive possession of the assigned channel.

Time Division Multiplexing (TDM)

In TDM, the users take turns (in a round robin); each one is periodically getting the entire bandwidth for the allotted time.

Television broadcasting provides an example of multiplexing. Each TV channel operates in a different frequency range, which is a portion of the allocated spectrum, with the inter-channel separation great enough to prevent interference. This system is an example of FDM. During the transmission of any program (Serial/film), there is an advertisement as well. These two alternate in time on the same frequency. This is an example of TDM.

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What is Multiplexing?

In communication, Multiplexing is a technique that transmits signals from several sources over a single communication channel. So in order to minimize the cost of communication bearer, various techniques of sharing a communication channel between several users have been devised. These are known as multiplexing techniques.

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Broadband Coaxial Cable

The other kind of coaxial cable systems uses analog transmission on standard cable television cabling. These can be used for digital data transfer also. It is cabled broadband. Although the term “broadband” comes from the telephone world, where it refers to anything wider than 4 kHz, in the computer networking world “broadband cable” means any cable network using analog transmission.

Since broadband networks use standard cable television technology, the cables can be used up to 300 MHz (and often up to 450 MHz) and can run for nearly 100 km due to the analog signaling, which is much less critical than digital signaling. To transmit digital signals on an analog network, each interface must contain electronics to convert the outgoing bit stream to an analog signal, and the incoming analog signal to a bit stream. Depending on the type of these electronics 1 bps may occupy roughly 1 Hz of bandwidth. At higher frequencies, many bits per Hz are possible using advanced modulation techniques.

Broadband systems are divided up into multiple channels. Frequently the 6MHz channels are used for television broadcasting. Each channel can be used for analog television, CD-quality audio or a digital bit stream at, say 3 Mbps, independent of the others. Television and data can be mixed on one cable.

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Baseband Coaxial Cable

Another communication transmission medium is the coaxial cable. It has better shielding than twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable is commonly used for digital transmission and is the subject of this section. The other kind, 75-ohm cable, is commonly used for analog transmission and will be described in the next section. This distinction is based on historical, rather than technical factor, (e.g., early dipole antennas had an impedance of 300 ohms, and it was easy to build 4:1 impedance matching transformers).

A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable length. For 1 km cables, a data rate 1 or 2 Gbps is feasible. Longer cables can also be used, to be widely used within the telephone systems but have not largely been replaced by fiber optics on long-haul routes. In the United States alone, 1000 km of fiber is installed every day (counting a 100 km bundle with 10 strands of fiber as 1000 km). Coaxial cables are still widely used for cable television and some local area networks.

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Twisted Pair Cables

The oldest and still most common transmission medium is twisted pair. A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form. The purpose of twisting the wires is to reduce electrical interference from similar pairs close by.

The most common application of the twisted pair is the telephone systems. Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec. can be achieved for a few kilometers in many cases. Due to their adequate performance and low cost, twisted pairs are widely used and are likely to remain so for years to come. Twisted pair cabling comes in several varieties, two of which are important for computer networks. Category 3 twisted pairs consist of two insulated wires gently twisted together. Four such pairs are typically grouped together in a plastic sheath for protection and to keep the eight wires together.

Starting around 1988, the more advanced category 5 twisted pairs were introduced. They are similar to category 3 pairs, but with more twists per centimeter and insulation, which results in less cross talk and a better quality signal over longer distances, making them more suitable for high–speed computer communication. Both of these wiring types are often referred to as UTP (Unshielded Twisted Pair), to contrast them with the bulky, expensive, shielded twisted pair cables IBM introduced in the early 1980s, but which have not proven popular outside of IBM installations.

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Relationship between Data Rate and Bandwidth

The concept of effective bandwidth is somewhat fuzzy one. It is the band within which most of the energy is confined. The term “most” in this context is somewhat arbitrary. The important issue here is that, although a given waveform may contain frequencies over a broad range, as a practical matter any transmission medium that is used will be able to accommodate only a limited band of frequencies. This, in turn, limits the data rate that can be carried on the transmission.

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Difference between Continuous & Discrete Signals

A continuous signal is one in which the signal amplitude or intensity varies in smooth fashion over time. There are no breaks or discontinuities in the signal. A discrete signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level.

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Classification of Transmission Media

Transmission media may be classified in Two broad categories namely Guided & Unguided.

In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and optical fiber.

Unguided media provide a means for transmitting electromagnetic waves but do not guide them; examples are propagation through air, vacuum and seawater.

The term direct link is used to refer to the transmission path between two devices in which signal propagate directly from transmitters to receivers with no intermediate devices, other than amplifiers or repeaters used to increase signal strength. This term can apply to both guided and unguided media.

A transmission may be

• Simplex
• Half-duplex
• Full duplex

In simplex transmission, signals are transmitted in only one direction; one station is a transmitter and the other is the receiver. In the half-duplex operation, both stations may transmit, but only one at a time. In full-duplex operation, both stations may transmit simultaneously. In the latter case, the medium is carrying signals in both directions at same time.

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Token Bus: IEEE Standard 804

IEEE Standard 804 (Token Bus) uses highly reliable cable envision equipments which is available from numerous vendors. It uses tokens to allow stations for start of transmission. It is more deterministic than IEEE Standard 802.3 although repeated losses of the token at critical moments can introduce more uncertainty than its supporters like to admit. Token Bus also supports priorities.

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Difference between OSI & TCP Reference Model

Following is the six basic differences between OSI Reference Model & TCP/IP Reference Model.

OSI Reference Model

1. Seven layers
2. It distinguishes between service, interface, and protocol.
3. Firstly description of model and protocol came next
4. Both have Network
5. Supports connectionless and connection oriented communication in network layer and only connection-oriented communication in transport layer.
6. Protocol in OSI model are better hidden and can be replaced relatively easily (No Transparency).

TCP Reference Model

1. Four layers
2. Did not clearly distinguish between service, interface and protocol
3. Protocol comes first and description of model later.
4. Transport and Application layer
5. TCP/IP has only one mode in Network layer (connection less) but supports both modes in Transport layer.
6. Protocols in TCP/IP are not hidden and thus cannot be replaced easily. (Transparency)

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TCP/IP Network Architecture & Reference Model

The TCP/IP network architecture is a set of protocols that allow communication across multiple diverse networks. The architecture evolved out of research that had the original objective of transferring packets across three different packet networks: the ARPANET packet-switching network, a packet radio network, and a packet satellite network. The military orientation of the research placed a premium on robustness with regard to failures in the network and on flexibility in operating over diverse networks. This environment led to a set of protocols that are highly effective in enabling communications among the many different types of computer systems and networks. Today, the Internet has become the primary fabric for interconnecting the world’s computers. In this section we introduce the TCP/IP network architecture and TCP/IP is the main protocol for carrying information.

Following Figure shows the TCP/IP network architecture, which consists of four layers. The application layer provides services that can be used by other applications. For example, protocols have been developed for remote login, for e-mail, for file transfer, and for network management.

TCP/IP network architecture

The application layer programs are intended to run directly over the transport layer. Two basic types of services are offered in the transport layer. The first service consists of reliable connection-oriented transfer of a byte stream, which is provided by the Transmission Control Protocol (TCP). The second service consists of best effort connectionless transfer of individual messages, which is provided by the User Datagram Protocol (UDP). This service provides no mechanisms for error recovery or flow control. UDP is used for applications that require quick but reliable delivery is not guaranteed.

The TCP/IP model does not require strict layering. In other words, the application layer has the option of bypassing intermediate layers. For example, an application layer may run directly over the Internet layer.

The Internet layer handles the transfer of information across multiple networks through the use of gateways or routers. The Internet layer corresponds to the part of the OSI network layer that is concerned with the transfer of packets between machines that are connected to different networks. It must therefore deal with the routing of packets across these networks as well as with the control of congestion. A key aspect of the Internet layer is the definition of globally unique addresses for machines that are attached to the Internet. The Internet layer provides a single service, namely, best-effort connectionless packet transfer. IP packets are exchanged between routers without a connection setup; the packets are routed independently, and so they may traverse different paths. For this reason, IP packets are also called datagrams. The connectionless approach makes the system robust; that is, if failures occur in the network, the packets are routed around the points of failure; there is no need to set up the connections. The gateways that interconnect the intermediate networks may discard packets when congestion occurs. The responsibility for recovery from these losses is passed on to the transport layer.

Finally, the network interface layer is concerned with the network-specific aspects of the transfer of packets. As such, it must deal with parts equivalent to OSI network layer and data link layer. Various interfaces are available for connecting end computer systems to specific networks such as X.25, ATM, frame relay, Ethernet, and token ring.

The network interface layer is particularly concerned with the protocols that access the intermediate networks. At each gateway the network access protocol encapsulates the IP packet into a packet of the underlying network or link. The IP packet is recovered at the exit gateway of the give n network. This gateway must then encapsulate the IP packet into new packet of the type of the next network or link. This approach provides a clear separation of the internet layer from the technology dependent network interface layer. This approach also allows the internet layer to provide a data transfer service that is transparent in the sense of not depending on the details of the underlying networks.

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Difference between Broadcasting & Multicasting

Broadcasting refers to addressing a packet to all destinations in a network whereas multicasting refers to addressing a packet to a subset of the entire network.

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Functions of Application Layer

Application Layer supports functions that control and supervise OSI application processes such as start/maintain/stop application, allocate or keep OSI resources, accounting, check point and recovering. It also supports remote job execution, file transfer protocol, message transfer and virtual terminal.

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Presentation Layer Services & Functions

Unlike all the lower layers, which are just interested in moving bits reliably from here to there, the presentation layer is concerned with the syntax and semantics of the information transmitted.

A typical example of a presentation service is encoding data in a standard agreed upon way. Most user programs do not exchange random binary bit strings, they exchange things such as people’s names, dates, amounts of money and invoices. These items are represented as character strings, integers, floating-point number, and data structures composed of several simpler items. Different computers have different codes for representing character strings (e.g., ASCII and Unicode), integers (e.g., one’s complement and two’s complement), and so on. In order to make it possible for computers with different representations to communicate, the data structure to be exchanged can be defined in an abstract way, along with a standard encoding to be used “on the wire”. The presentation layer manages these abstract data structure and converts from the representation used inside the computer to the network standard representation and back.

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Session Layer Functions & Services

The main tasks of the session layer are to provide:

• Session Establishment
• Session Release – Orderly or abort
• Synchronization
• Data Exchange
• Expedited Data Exchange

The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides enhanced services useful in some applications. A session might be used to allow a user to log into a remote time-sharing system or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic can only go one way at a time (analogous to a single railroad track), the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sides do not attempt the same operation at the same time. To manage these activities, the session layer provides tokens that can be exchanged. Only the side holding the token may perform the desired operation.

Another session service is synchronization. Consider the problem that might occur when trying to do a 2 hour file transfer between two machines with a 1 hour mean time between crashes. After each transfer was aborted, the whole transfer would have to start over again and would probably fail again the next time as well. To eliminate this problem, the session layer provides a way to insert markers after the appropriate checkpoints.

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Functions of Transport Layer

The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done efficiently, and in a way that isolates the upper layers from the inevitable changes in the hardware technology.

Transport Layer provides location and media independent end-to-end data transfer service to session and upper layers.

Functions of Transport Layer: 

• Segmentation
• Segment size determination
• Sequencing
• Packet Type determination
• Flow Control 
• Port Addressing

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Functions of Network Layer

The network layer ensures that each packet travels from its sources to destination successfully and efficiently. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are “wired into” the network and rarely changed. They can also be determined at the start of each conversation, for example a terminal session. Finally, they can be highly dynamic, being determined anew for each packet, to reflect the current network load.

When a packet has to travel from one network to another to get its destination, many problems can arise. The addressing used by the second network may be different from the first one. The second network one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.

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