Internetworking Basics

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     Chapter 6: Ethernet Fundamentals


    Communication Modes and Autonegotiation

    These notes guide discuss Ethernet Communication Modes and Autonegotiation

    Introduction

    Modes of Communication
            Simplex Mode
            Half-Duplex Mode
            Full-Duplex Mode

    Autonegotiation 
            The Autonegotiation Fast Link Pulse
            The Autonegotiation Process
            Examples of Autonegotiation

    Summary

     
     

    Introduction

    On completion of these notes you should...

    • Understand the different modes of communication, such as half-duplex and full-duplex
    • Understand the autonegotiation process
     

    Modes of Communication Overview

    Suppose you had to set up some sort of communication with say a friend. Would you prefer to use one-way communication, where only one of you could speak at one time. Or, would you prefer a two-way connection, where you could both speak simultaneously. How does Ethernet operate in this respect? Is it one-way communication or two?

    Well, Legacy Ethernet uses one-way communication. On 802.3, 10BASE-2, 10BASE-5 and 10BASE-T networks, a signal can be sent in either direction on a wire, but simultaneous transmit and receive is not allowed. This mode of communication is called half-duplex.

    However, the 802.3 Ethernet standard was modified in 1995 to include 802.3u 100 Mbps Fast Ethernet. Along with the new standard, the IEEE incorporated a feature called full-duplex mode of operation. This means that in some cases, two-way communication can be allowed, whereby signals can be transmitted and received simultaneously.

    Another feature that was also introduced was called autonegotiation, which allows different technologies, such as 10BASE-2 and 100BASE-Tx to coexist on the same network. Auto-negotiation is very useful mechanism that enables devices to negotiate the speed and mode (half or full-duplex) that would best suit both communicating parties.

     

    Duplex Modes and Autonegotiation

    Different modes of communication are possible between two parties. There is the simplex mode, in which communication occurs in one direction only; there is also half-duplex and full-duplex mode where communication can occur in both direction.

    Simplex Mode

    The simplest form of communication mode between two entities is simplex mode - where a signal can travel in one direction only. This mode of operation can be likened to analog television transmission, where you can receive but not send. Simplex mode is not used on LAN's. Two other two modes, half-duplex and full-duplex are commonly used.

    Half-Duplex Mode

    Original Legacy Ethernet communication used a half-duplex CSMA/CD mode of operation. This meant that Ethernet stations would first listen to the wire and only transmit if they did NOT detect another device already transmitting. If by bad luck two end stations transmitted at the same time, a COLLISION would occur. Each one of the guilty stations would back off for a random period of time before retransmitting. This mode of operation can be likened to the use of walkie talkies, where only one party can speak at any one time.

    Full-Duplex Mode

    When Fast Ethernet was introduced by the IEEE in the 1990's a new mode of operation was possible on switched networks; full-duplex mode. Full-duplex operation implies a point-to-point connection between the sender and the receiver and so eliminates the CSMA-CD protocol, since collisions do not occur. This mode of operation can be likened to the telephone conversations where both parties can speak at the same time.

     

    Autonegotiation

    When Fast Ethernet was introduced by the IEEE, another feature was implemented called auto-negotiation. Imagine a network where different parts implement different standards. For example you may have a 100BASE-Tx network segment connected to a 10BASE-T segment, by a 1000BASE-T switch. Since the different parts operate at different speeds and some can operate half-duplex while others can operate at full-duplex, there needs to be a mechanism whereby the different parts can agree on common speed and duplex modes that will be suitable for communication.

    Autonegotiation is a mechanism whereby devices sharing a link segment can automatically configure themselves to operate at the highest speed and duplex mode that they can both cope with. Thus, the autonegotiation standard allows devices based on different Ethernet standards, from 10BASE-T to 1000Base-T, to coexist in the network.

    ~~ The Autonegotiation Fast Link Pulse ~~

    For the 10Base-T standard, a link test mechanism is used to test for a connection. In the absence of network traffic, a 100 nanosecond pulse is sent every 16 milliseconds. This pulse is called a Normal Link Pulse (NLP).

    When the Fast Ethernet standard was introduced, the NLP used on 10BASE-T was adopted for use in the autonegotiation process. In autonegoatiation, a group of 33 pulses are sent, one after another. This group of pulses is call the Fast Link Pulse (FLP).

    Since 10BASE-T has no knowledge of FLP's and can only understand NLP pulses, each FLP group of pulses are sent at the same rate as a single NLP, once every 16 milliseconds. Then, 10BASE-T can just interpret the group of pulses in an FLP burst to be a single NLP pulse. This is to maintain backward compatibility.

    Each FLP group of pulses, contains 33 pulses. 17 are clock pulses that are alternated with 16 data pulses. First a clock pulse is sent, then a data pulse may be sent, then another clock pulse, and so on. The clock pulses are uses for timing and all 17 pulses are always present.

    The 16 data pulses found between each clock pulse, may or may not be present, depending on the information being transmitted by the sender. The data pulses contain a code word, specifying the technology ability of the sender, 100BASE-Tx, for example. The diagram below illustrates this.

    The diagram shows how different code words represent different technology standards. Additional bits include the Acknowledge bit and the Next Page bit. Once both sides of the link have exchanged FLP's and have decoded the code word from it's link partner, each side then knows what the other side is capable of. Both link partners then send three NLP's with the Acknowledge bit set indicating they have decoded the technology code word. Then both sides either send additional NLP pages - if the Next Page bit is set, or they both stop. Using the technology configuration information that has been exchanged in the code word, they then decide on and enable the highest common performance mode possible.

    ~~ The Autonegotiation Process~~

    When a station first attempts autonegotiation with a link partner, it sends out a series of FLP pulses. If the link partner supports Autonegotiation and is in autonegotiation mode, then the negotiation process can start between the link partners, resulting in the establishment of a link, where the highest common performance mode possible is used.

    If however, autonegotiation exists at only one end of a link because the link partner does not support the autonegotiation mechanism, the station must examine the signal it is receiving, to try and discover which technology the link partner supports. This is known as parallel detection and gives autonegotiation the ability to be compatible with any device that does not support autonegotiation. The parallel detection function can detect 10BASE-T, 100BASE-TX, or 100BASE-T4 technologies.

    The picture below shows that my NIC is set to use autonegotiation. You can also see that it is possible for me to fix it at a particular mode, ranging from 100Mbps Full Duplex to 10Mbps Half Duplex.

    Fixing it at a particular mode is risky though. Suppose I set it to 100Mbps full duplex and a link partner attempts to establish a connection. Since autonegotiation defaults to half-duplex mode but I have set my NIC to a fixed full-duplex mode then the link will fail.

    Autonegotiation only works when...

    • both sides are in autonegotiation mode and can negotiate the best common configuration
    • one side is autonegotiation mode and the link partner is in a fixed half-duplex mode.

    It is possible for both sides to establish a full-duplex mode and will occur when...

    • both sides are in an autonegotiation mode and decide that both sides are capable of operating in full-duplex mode.
    • both sides are in a fixed full-duplex mode that match

    ~~ Examples of Autonegotiation ~~

    Scenario 1

    Suppose a DTE can operate at 10/100/1000 Mbps but is configured to autonegotiate. Suppose the link partner is a legacy device, operating at 10BASE-T. How will a link be established between them?

    What will the resulting Link Mode be?   

    Scenario 2

    Suppose a DTE can operate at 10/100/1000 Mbps but is configured to autonegotiate. Suppose the link partner is configured to operate at 100BASE-Tx in full-duplex mode. How will a link be established between them?

    What will the resulting Link Mode be?  

    Scenario 3

    Suppose a DTE can operate at 10/100/1000 Mbps but is configured to operate at a fixed mode of 1000 Mbps full duplex. Suppose the link partner is a 1000BASE-Tx device operating in full-duplex mode. How will a link be established between them?

    What will the resulting Link Mode be?

    Scenario 4

    Suppose a DTE can operate at 10/100/1000 Mbps but is configured to autononegotiate. Suppose the link partner can operate at 10/100/1000 Mbps and is also configured to autonegoatiate. How will a link be established between them?

    What will the resulting Link Mode be?

      

    Summary

    On completing these notes you should have learned the following key points:-

    • The half-duplex mode of communication means that signals can travel in either direction but not in both directions simultaneously.
    • The full-duplex mode of communication means that signals can travel in either direction simultaneously.
    • Autonegotiation allows devices to link to each other and automatically discover and enable the highest performance that both ends of the link can operate at.
    • The autonegotiation process uses Fast Link Pulses, containing code words, to discover the technology of link partners.
     
     

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