LocalTalk

LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps.

Connection between a NIC and a network

The type of connection used between a NIC and a network depends on that network technology.In some technologies,the NIC contains most of the necessary hardware,and attaches directly to the network medium using a single cable or optical fiber.In many other technologies,the NIC does not contain all the electronic circuitry needed to attach directly to the network.Instead,the cable from a NIC attaches to an additional electronic component that then attaches to the network.Surprisingly,the exact details of the connection between a NIC and a network are not determined by the technology that means a given network technology can support multiple wiring schemes.Let take the example of Ethernet technology in which technology i.e Ethernet remains same while the wiring scheme changes.Wiring scheme can be Thick Ethernet Wiring,Thin Ethernet Wiring and Twisted pair Ethernet Wiring.

Thin Ethernet Wiring

A second generation of Ethernet wiring uses a thinner,more flexible cable than the original thick wiring.Formally named as a 10 Base2 and informally named as thin wire Ethernet or Thinnet.This wiring scheme differs from Thicknet wiring scheme in three important ways.First,Thinnet generally costs less to install and operate than Thicknet.Second,because the hardware that performs the transceiver function is built into the NIC,no external transceivers are needed.Third,Thinnet does not use an AUI cable to attach the NIC to a communication medium.Instead,Thinnet attachs directly to the back of each computer using BNC connector.In a Thinnet installation,a coaxial cable stretches between each pair of machines.The cable doesn't need to follow a straight line-it may lie loosely on the tabletop between computers,run under floor,or run in a conduit.Although,wiring for a thin Ethernet appears to be completely different than the wiring for a thick Ethernet,the two schemes share several important properties.Both thick and thin cables are coaxial,meaning that they shield signals from outer interference.Both thick and thin cables requires termination,and both use the bus topology.Most important,because two wiring systems have similar electrical characteristics,that means signals propagate along the cable is in the same way.

Twisted Pair Ethernet

This type of Ethernet wiring is known as Third Generation of Ethernet wiring.This generation is completely differ from its previous two generation that are thick and thin Ethernet because it does not use a coaxial cable.This generation Ethernet typically known as twisted pair Ethernet or TP Ethernet and its first version was formally known as 10BaseT.The wiring for Fast Ethernet is known as 100BaseT,and wiring for Gigabit Ethernet is known as 1000BaseT.Instead of shared medium,the new technology extends the idea used with connection multiplexing in which an electronic device serves as the center of the network.
The basic electronic device is known as an Ethernet Hub.Like the other wiring schemes,twisted pair requires each computer to have a network interface card and a direct connection from NIC to the network.The connection uses unshielded twisted pair wiring with RJ-45 connectors which are the largest versions of the modular connectors used with the telephones.The connector on one end of a twisted pair cable plugs into the network interface on a computer and the connector on the other end plugs into the hub.
Thus,each computer has a dedicated connection to the hub device.Electronic components in hub emulate a physical cable,making the entire system operate like a conventional Ethernet.For ex:A computer attached to a hub must have a physical Ethernet Address that means each computer must use the CSMA/CD to access the network and the standard Ethernet Frame Format.In fact,software does not distinguish between thick Ethernet,thin Ethernet and twisted pair Ethernet because the network interface handles the details and hides any differences.

Art of Protocol Design

Although the techniques needed to solve specific problems are well known,protocol design is on-trivial for two reasons.First to make communication efficient,details must be chosen carefully that means small design errors can result in incorrect operation,unnecessary packets or delays.For example:sequence numbers are usually stored in a fixed field in a packet header.The field must be large enough so sequence numbers are not reused frequently,but small enough to avoid wasting unnecessary bandwidth.Similarly,a higher level protocol can cause excessive overhead if the protocol chooses a non-optimal message size.

Second,protocol mechanism can interact in unexpected ways.For example:Consider the interaction between flow control and congestion control mechanism.A sliding window scheme aggressively use more of the underlying network bandwidth to improve throughput.A congestion control mechanism does,the opposite by reducing the number of packets being inserted to prevent the networking from collapsing.

The balance between sliding window and congestion control can be tricky,and a design that does both well is difficult.A protocol that is too aggressive about using bandwidth can adjust the underlying network and a protocol that is too conservative can result in lower throughput then necessary.More important,designs that attempts to switch from aggressive to conservative whenever congestion occurs tend to oscillate - then slowly increase their use of bandwidth until the network begins to experience congestion,decrease use until the network becomes stable and then begin to increase again.

Understanding Protocol Suites

After knowing the importance of protocol in communication,designers have chosen to divide the communication problem into sub pieces and to design a protocol for each of the sub piece instead of having a single giant protocol that specifies complete details for all possible forms of communications.Doing so makes each protocol easier to design,analyze,implement and test.The main advantage of dividing communication software into multiple protocols increases flexibility because it allows subsets of protocols to be used as added.The division into separate protocols must be chosen carefully to ensure the resulting communication system is efficient and effective.To avoid duplication of effort,each protocol should handle part of the communication problem not handled by other protocols.

To make efficient implementation possible,protocols should be designed so that,they can share data structures and information.Finally,the combination of protocols should handle all possible hardware failures or other exceptional conditions.Now the main question that arises in front of us is that how can one guarantee that protocols will work together.The answer of this question lies in an overall design plan that means instead of developing each protocol in isolation,protocols are designed and developed in complete,cooperative sets called suites or families.Each protocol in a suite solves one part of the communication problem;together they solve the entire communication problem.Furthermore,the entire suite is designed to make interactions among protocol efficient.

Need for Protocols

As all we know that basic communication hardware consists of mechanisms that can transfer bits from one point to another.However,using raw hardware to communicate is analogous to programming by entering 1s and 0s but it is cumbersome and inconvenient.To aid programmers,computers attached to a network use complex software that provides a convenient,high level interface for applications.The software handles most low level communication details and problems automatically,making it possible for applications to communicate easily.Thus,most application programs rely on network software to communicate that means hey do not interact with network hardware directly.All parties involved in a communication must agree on a set of rules to be used when exchanging messages and such type of agreement is known as protocol.This term is also applied to computer communications meaning of which "a set of rules that specify the format of messages and the appropriate action required for each message is known as a network protocol or a computer communication protocol."The software that implement such rules is called as protocol software.