Networking essentials3

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network topology: The specific physical, i.e., real, or logical, i.e., virtual,

arrangement of the elements of a network. Note 1: Two networks have the same

topology if the connectionconfiguration is the same, although the networks may

differ in physical interconnections, distances between nodes, transmission rates,

and/or signal types.

BUS TOPOLOGY

A bus network uses a multi-drop transmission medium, all node on the network share a common

bus and thus share communication. This allows only one device to transmit at a time. A

distributed access protocol determines which station is to transmit. Data frames contain source

and destination addresses, where each station monitors the bus and copies frames addressed to

itself.

( a typical bus topology)

A bus topology connects each computer (nodes) to a single segment trunk (a communication

line, typically coax cable, that is referred to as the 'bus'. The signal travels from one end of the

bus to the other. A terminator is required at each to absorb the signal so as it does not reflect

back across the bus. A media access method called CSMA/MA is used to handle the collision

that occur when two signals placed on the wire at the same time. The bus topology is passive. In

other words, the computers on the bus simply 'listen' for a signal; they are not responsible for

moving the signal along.

Advantages:

Failure of one of the station does not affect others.
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Good compromise over the other two topologies as it allows relatively high rate of data

tansmittion.

Well suited for temporary networks that must be set up in a hurry.

Easy to implement and extend.

Disadvantage:

Require a network to detect when two nodes are transmitting at the same time.

Does not cope well with heavy traffic rates

Difficult to administer/troubleshoot.

Limited cable length and number of stations.

A cable brake can disable the entire network; no redundancy.

Maintenance cost may be higher in the long run.

Performance degrade as additional computers are added.

The physical topology of a network is the way in which the cables and computers

are connected to each other. The main types of physical topologies are:

Bus Star

Ring
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With a star topology, theworkstations, fileservers,

printers etc. are attached via

drop cables to a central hub or

multiport repeater.

The hub is central to a star topology and the network cannot function

without it. Other hubs may be attached to the first hub via drop cables,

creating nodes. These other hubs can themselves have computers attached

to them or even further hubs to create further nodes.

The cables used in star topologies are either shielded or unshielded twisted pair

wires (STP or UTP).

Star topologies are difficult to install, but once installed are easy to maintain and

expand. A fault in a cable only affects the computer or node it is attached to andis easy to locate.


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At the moment, the ring topology is the least used. However, it is likely to become more

widespread with the use of fibre optic cables.

physical topologyLast modified: Tuesday, December 19, 2006

The physical layout of devices on a network. Every LAN has a topology, or the way that thedevices on a network are arranged and how they communicate with each other. The way that theworkstations are connected to the network through the actual cables that transmit data -- thephysical structure of the network -- is called the physical topology. Thelogical topology, incontrast, is the way that the signals act on the network media, or the way that the data passesthrough the network from one device to the next without regard to the physical interconnectionof the devices.

A network's logical topology is not necessarily the same as its physical topology. For example,twisted pairEthernet is a logical bus topology in a physical star topology layout. While IBM'sToken Ring is a logical ring topology, it is physically set up in a star topology.

For network diagrams, seeNetwork Topology Diagrams in theQuick Reference section ofWebopedia.

In order for systems to communicate on a network, there has to be a communication path ormultiple paths between which the data can travel. To communicate with another entity, thesepaths move the information from one location to another and back. This is the function ofswitching. Switching provides communication pathways between two endpoints and manageshow data is to flow between these endpoints. Two of the more common switching methods used

today include: Packet switching

Circuit switching

Packet switching is the approach used by some computernetwork protocols to deliver dataacross a local or long distance connection. Examples of packet switching protocols areFrameRelay, IP and X.25.
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How Packet Switching Works

Packet switching entails packaging data in specially formatted units (calledpackets) that are

typically routed from source to destination using network switches and routers. Each packet

contains address information that identifies the sending computer and intended recipient.

Using these addresses, network switches and routers determine how best to transfer the

packet between hops on the path to its destination.

circuit switchingLast modified: Friday, May 03, 2002

A type of communications in which adedicatedchannel (orcircuit) is established for theduration of a transmission. The most ubiquitous circuit-switching network is the telephonesystem, which links together wire segments to create a single unbroken line for each telephonecall.

The other common communications method ispacket switching, which divides messages intopackets and sends each packet individually. The Internetis based on a packet-switching protocol,TCP/IP.

Circuit-switching systems are ideal for communications that require data to be transmitted inreal-time. Packet-switching networks are more efficient if some amount of delay is acceptable.

Circuit-switching networks are sometimes called connection-orientednetworks. Note, however,that although packet switching is essentiallyconnectionless, a packet switching network can bemade connection-oriented by using a higher-level protocol. TCP, for example, makesIP

networks connection-oriented.

Circuit Switching

In this networking method, a connection called a circuit is set up between two devices,which is used for the whole communication. Information about the nature of the circuit ismaintained by the network. The circuit may either be a fixed one that is always present,or it may be a circuit that is created on an as-needed basis. Even if many potentialpaths through intermediate devices may exist between the two devices communicating,only one will be used for any given dialog. This is illustrated in Figure 1.
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Figure 1: Circuit Switching

In a circuit-switched network, before communication can occur between two devices, acircuit is established between them. This is shown as a thick blue line for the conduit ofdata from Device A to Device B, and a matching purple line from B back to A. Once setup, all communication between these devices takes place over this circuit, even though

there are other possible ways that data could conceivably be passed over the network ofdevices between them. Contrast this diagram to Figure 2.

The classic example of a circuit-switched network is the telephone system. When youcall someone and they answer, you establish a circuit connection and can pass databetween you, in a steady stream if desired. That circuit functions the same wayregardless of how many intermediate devices are used to carry your voice. You use it

for as long as you need it, and then terminate the circuit. The next time you call, you geta new circuit, which may (probably will) use different hardware than the first circuit did,depending on what's available at that time in the network.

TCP Connections: The Three-Way Handshake

The three-way handshake inTransmission Control Protocol (also called the three messagehandshake) is the method used to establish and tear downnetworkconnections. Thishandshaking technique is referred to as the 3-way handshake or as "SYN-SYN-ACK" (or moreaccurately SYN, SYN-ACK, ACK). The TCP handshaking mechanism is designed so that twocomputers attempting to communicate can negotiate the parameters of the networkconnectionbefore beginning communication. This process is also designed so that both ends can initiate andnegotiate separate connections at the same time.

3-Way Handshake Description

Below is a (very) simplified description of theTCP 3-way handshake process. Refer to thediagram on the right as you examine the list of events on the left.
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EVENT DIAGRAM

Host A sends a TCP SYNchronize packet to Host B

Host B receives A's SYN

Host B sends a SYNchronize-ACKnowledgement

Host A receives B's SYN-ACK

Host A sendsACKnowledge

Host B receives ACK. TCP connection is ESTABLISHED.

SYNchronize and ACKnowledge messages are indicated by a bit inside the TCP header of thesegment.

TCP knows whether the networkconnection is opening, synchronizing or established by usingthe SYNchronize and ACKnowledge messages when establishing a networkconnection.

When the communication between two computers ends, another 3-way communication is

performed to tear down the TCP connection. This setup and teardown of a TCPconnection ispart of what qualifies TCPa reliableprotocol.

Note that UDPdoes not perform this 3-way handshake and for this reason, it is referred to as anunreliable protocol.

Protocols Encapsulated in TCP

Note that FTP, Telnet, HTTP, HTTPS,SMTP, POP3, IMAP, SSH and any other protocol thatrides overTCP also has a three way handshake performed as connection is opened. HTTPwebrequests, SMTP emails, FTP file transfers all manage the messages they each send. TCP handlesthe transmission of those messages.

TCP rides on top ofInternet Protocol (IP)which is why it is calledTCP/IP (TCP over IP). TCP

segments are passed inside the payload section of the IP packets. IP handles addressing androuting and gets the packets from one place to another, butTCP handles the actualcommunication between hosts.

Three-way Handshake

If a source host wishes to use an IP application such as active FTP for instance, it selects a portnumber which is greater than 1023 and connects to the destination station on port 21. The TCPconnection is set up via three-way handshaking:

This begins with a SYN (Synchronise) segment (as indicated by the code bit) containinga 32-bit Sequence numberA called the Initial Send Sequence (ISS) being chosen by,and sent from, host 1. This 32-bit sequence numberA is the starting sequence number ofthe data in that packet and increments by 1 for every byte of data sent within the segment,i.e. there is a sequence number for each octet sent. The SYN segment also puts the valueA+1 in the first octet of the data.

Host 2 receives the SYN with the Sequence numberA and sends a SYN segment with itsown totally independent ISS numberB in the Sequence number field. In addition, it sendsan increment on the Sequence number of the last received segment (i.e. A+x where x is
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the number of octets that make up the data in this segment) in its Acknowledgment field.This Acknowledgment number informs the recipient that its data was received at theother end and it expects the next segment of data bytes to be sent, to start at sequencenumberA+x. This stage is aften called the SYN-ACK. It is here that the MSS is agreed.

Host 1 receives this SYN-ACKsegment and sends an ACKsegment containing the next

sequence number (B+y where y is the number of octets in this particular segment), this iscalled Forward Acknowledgement and is received by Host 2. The ACKsegment isidentified by the fact that the ACKfield is set. Segments that are not acknowledgedwithin a certain time span, are retransmitted.

TCP peers must not only keep track of their own initiated Sequence numbers but also thoseAcknowledgment numbers of their peers.

Closing a TCP connection is achieved by the initiator sending a FIN packet. The connection onlycloses when an ACKhas been sent by the other end and received by the initiator.

Maintaining a TCP connection requires the stations to remember a number of different

parameters such as port numbers and sequence numbers. Each connection has this set ofvariables located in a Transmission Control Block (TCB).

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