Chapter 4

Topologies and Media Access Control


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Summary

In this chapter we take a look at how the nodes can be physically arranged on a network as well how they transmit data onto the medium.  We will also look at how a node gains access to a the medium and we will look at LAN standards.


LAN Topologies and the Physical Layer

A Topology is the physical layout of object in space.  When we refer to a LAN topology we are talking about the layout of nodes on our network and how the media connects to them.  There are three basic types of topologies uses in local area networks.  Ring, Bus and Star; each of these different topologies are unique.

Ring Topology - With the ring topology all computers are connected to each other and form a complete loop

Bus Topology - A bus topology all computers are connected in a line to the same cable or segment.

Star Topology - The star topology has a hub or switch in which all computers connect.  This is the most common topology.


Data Link and Media Access Control Protocols

The physical layer tells us characteristics of the medium and connectors used in a connection as well as the types of signals being transmitted on the medium.  For example it might define a CAT V as the medium, an RJ-45 as the connector and electronic pulses for the signal. (as opposed to light in fiber or radio waves in wireless)  Once a node is physically connected to the medium it needs to have a way to send and receive messages.  The way a node is able to gain control of the medium and transmit a message is called MAC Protocol.  The data link layer of the OSI model is broken down into two parts the first is the Mac Protocol and the second is the Logical Link Control Layer (LLC)  The LLC provides I constant interface between the Data Link Layer and the Physical Layer so you will always have a constant interface between the two.

  • Data Link Protocols - Establish the rules for exchanging messages on a LAN.  Listed below are five aspects that are covered in the message exchange process.

    • Delineation of Data - This will dictate where the data begins and ends in a transmission.  There are two standard ways, framing the data with certain control characters, or a standard message format where data is identified by its position within the message.

      • Asynchronous transmission - The signal starts with an idle stream of 1's.  Once a 0 is sent this tells the receiving computer that the next x number of bits will be data (one character) and after it is done a 1 bit will tell it the signal is done and it will return the the idle state.

      • Binary synchronous transmission - More then one character is sent at a time.  Two bit patterns are reserved as the STX and ETX characters.  The STX pattern is used to mark the start of the data and the ETX is used to mark the end of the data.

    • Error Control - In Chapter 3 we learned about the VRC and LRC and CRC.  In this section the most common method of error detection is CRC because it is more reliable.  In Chapter 1 we learned that the transport layer is responsible for error detection.  This is an additional level of protection used to verify data at the data link layer.

    • Addressing - Network cards have a 48bit MAC Address embedded in them.  No two network cards have the same MAC Address.  At the Data Link layer all address is done by this address.  This is different then an IP address which is a Network Layer Address covered in Chapter 15

    • Transparency - The data portion of the message must be able to be anything.  This way it won't matter if you send text or application data (binary).  In a binary synchronous transmission if there is a ETX bit pattern in the data the computer will think it is the end of the data.  In this example the Data Link Protocol is not transparent.

    • Code Independence - You must be able to send any type of data.

  • MAC Protocols - Describe how nodes on the network gain access to the medium.

  • Contention (CSMA/CD) - With CSMA/CD the sending node tries to send a message when the media is free.  If another node transmits at the same time both nodes detect a collision.  Once a collision is detected both nodes generate a random number and wait that time period and try to transmit again.  In a network using CSMA/CD there can be a lot of collisions in a network with many nodes, this can cause it to slow down.

Collisions on a LAN

 

  • Token Passing - In a token passing network their is a token passed from node to node.  A node can only transmit when it has the token.  One computer on the network is the active monitor.  If this computer doesn't receive the token in a certain time period a new token is generated.  Their are multiple standby monitors that listen for the active monitor to send out a signal  If this signal is not received a standby monitor becomes the active monitor and generates a new token.

LAN Standards

LAN Standards define different aspects of a LAN such as Media, MAC protocol, and topology.  For example

802.3 10BaseT uses twisted pair, contention, and a star topology.  This also tells us that the network runs at 10Mbps.   I will not require you to memorize all the different 802 standards but we will talk about some of the common ones.

  • The IEEE (Institute of Electrical and Electronic Engineers) 802 standers are listed in the book, the two most common are 802.3 (Ethernet) and 802.11 (Wireless).  The 802 comes from the date the standards were first introduced (February 1980)

  • 802.3 - Ethernet is broken down into a bunch of sub-standards.  Again I will not require you to memorize them but you should be able to look at the type and know all about it.

    • 10Base-T would run at 10Mbps use baseband and twisted pair.  The way the name breaks down is simple, the first number is the speed in Mbps.  The Base or Broad explain how the media is used, i.e. baseband or broadband.  And the last part tells you one of two things, if it is a number it tells you the distance of the media in hundreds of meters, if it a letter it tells you the media type.

  • Broadband - The medium is divided into several channels allowing multiple transmissions at once.  An example of broadband would be a cable modem.

  • Baseband - The entire medium is dedicated to one message

  • 802.6 - Defines a MAN using FDDI (Fiber Distributed Data Interface) and token passing on a ring topology.  They can also use CDDI (Copper Distributed Data Interface)


More Information

CSMA/CD

 

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