copyright 2008-2014 kenneth m. chipps ph.d. block encoding line signaling multiplexing on wired...

Block EncodingLine SignalingMultiplexing

On Wired MediaLast Update 2014.07.10

Objectives

• Learn about the block encoding line signaling and multiplexing methods used on wired media

Guided Media

• Wired or guided media as used in data networks can be made of copper or glass as in UTP or fiber optic cable

• At the lowest level of layer 1 of the OSI model - the physical layer - the bit stream is placed onto the media

• In this case wired media made of copper or glass

Encoding and Signaling

• Speeds on Ethernet networks are determined by the block encoding and line signaling used

• Block encoding takes several bits and sends it out as a new code word using a look up table

• Line signaling converts a digital word into a serial bit stream that is then placed onto the wired media

Ethernet at Layer 1

• Since an Ethernet based local area network using copper based UTP wired media is the most common type of network we will concentrate on the block encoding and line signaling it uses to illustrate how this layer does its work

• Here is a table from a Dell white paper from 2001 that summarizes the methods at the three main Ethernet LAN speeds

Ethernet at Layer 1

• In addition when used to extend the physical size or coverage area inside a single building an Ethernet based LAN can be extended using a backbone cable

• 1000Base-SX is the most commonly used method for this application

• 1000BASE-SX uses multimode fiber optic cable to deliver gigabit speeds

Ethernet at Layer 1

• The wavelength used is from the near infrared region at 770 to 860 nanometers

• The standard distances are between 220 meters using 62.5 µm fiber and 550 meters using 50 µm fiber

• Longer distances are commonly seen when good quality fiber, optics, and terminations are used

Ethernet at Layer 1

• These then are the four methods we will examine– 10BASE-T at 10 Mbps– 100BASE-TX at 100 Mbps– 1000BASE-T at 1000 Mbps– 1000BASE-SX at 1000 Mbps

Forms of Ethernet

• Each of these four main forms or speeds of Ethernet based LANs and building backbone connections uses a different set of block encoding and line signaling

Encoding and Signaling Used

• 10BASE-T at 10 Mbps over UTP– Block Encoding

• Manchester

– Line Signaling• Differential

• 100BASE-TX at 100 Mbps over UTP– Block Encoding

• 4B/5B

– Line Signaling• MLT-3

• 1000BASE-T at 1000 Mbps over UTP– Block Encoding

• 4D-PAM-5

– Line Signaling• PAM-5

• 1000BASE-SX at 1000 Mbps over multimode fiber optic cable– Block Encoding

• 8B/10B

– Line Signaling• NRZ

Ethernet at Layer 1

• Let’s examine the copper based cabling methods first

• As it comes in off the UTP cable here is where we see the line signaling method as illustrated in the Dell white paper

Ethernet at Layer 1

Details on Each Method

• Let’s examine each of these in more detail

10BASE-T

• Block Encoding– Manchester

• Line Signaling– Differential

Manchester

• A Manchester code uses a level transition in the middle of each bit period

• For a binary 1, the first half of the period is high, and the second half is low

• For a binary 0, the first half is low, and the second half is high

• It is comprised of +/-2.5 Volt pulses, operating at frequencies of 5 MHz and 10 MHz

Differential

• A differential Manchester code also uses a transition in the middle of the bit period

• The difference lies in how the transition is interpreted

• To represent a 1, the level remains the same as it was at the end of the previous bit period and then changes at midpoint

Differential

• To represent a 0, the level changes both at the beginning of the bit period and at the end

10BASE-T Waveform

• The resulting signal looks like this

10BASE-T Waveform

100BASE-TX

• Block Encoding– 4B/5B

• Line Signaling– MLT-3

• Before the byte is transferred with MLT-3, it is first encoded using 4B/5B encoding

• With 4B/5B encoding, each hexadecimal 4-bit digit is encoded into an essentially uncorrelated 5-bit code, using the 4B/5B translation table

• For example

• The 4B/5B block encoding method takes 4 bits of data and expands it to 5 coded bits

• Four of the coded bits contain the actual data in accordance with the table

• The fifth bit is added to transmit additional information such as

– The receiving station needs to receive the clock signal in order to decode the data accurately

– Sending the clock separately would not be a very efficient use of cabling bandwidth

– The 5-bit code word can be chosen so that a clock transition is present often enough for the receiving station to regenerate the clock

• The extra bit also allows the code-words to be chosen carefully so that an equal number of logical 0s and 1s are transmitted which means the digital signal has no DC component

• The 4B/5B encoding allows some error correction capability to be incorporated into the data stream

• MLT-3 stands for Multi-Level Transition with three levels

• Those levels being three voltage levels• MLT-3 cycles between V to 0 to +V then

back to V, repeating indefinitely• A logical zero is encoded by stopping the

cycle for one transition• A logical one is encoded by continuing the

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• MLT-3 uses these voltage levels operating at a frequency of 125 MHz– -1– 0– +1

• The level stays the same for consecutive logical ones or zeros

• A change in a bit means a change in the voltage level

• The change occurs in a circular, pattern 0V, +1, 0, -1,0, +1

• This circular, sinusoidal pattern change means that MLT-3 encoded data sine wave of a much lower frequency that the original, making it ideally suited for high speed data transmission over UTP

• To reduce, or spread out the electromagnetic emission, a scrambling process is applied before the signal is MLT-3 modulated

• The scrambler produces a non-repetitive bit sequence of the bits to be transmitted

100BASE-TX Waveform

1000BASE-T

• Block Encoding– 4D-PAM-5

• Line Signaling– PAM-5

4D-PAM-5

• In 1000BASE-T the system transmits and receives data on four wire pairs simultaneously – the 4D - using five voltage levels - PAM5 at each twisted pair

• Each 8 bits are converted to one transmission of four quinary symbols

4D-PAM-5

• When using PAM-5 or Five Level Pulse Amplitude Modulation each transmitted symbol represents one of five different levels– -2– -1– 0– +1– +2

4D-PAM-5

• Typically only 4 voltage levels are needed to generate 2 bits/symbol because there are only 4 possible combinations of 2 bits – 00, 01, 10, 11

• The transmitter can send 4 combinations of 2 bits as 4 distinct voltages and the receiver can decode each voltage level into the corresponding 2-bit combination

4D-PAM-5

• A 4-level signal has voltage transitions every 2 bit periods while a binary signal could have voltage transitions every bit period

• Therefore, the rate of transitions, or symbol rate of a 4-level signal is half the frequency of a 2 level signal

4D-PAM-5

• Thus, a 250MB/s data signal can be transmitted at a rate of 125Msymbols/s with only 4 voltage levels

• The 5th level in the Enhanced TX/T2 system allows for redundant symbol states that are used for error-correction coding

4D-PAM-5

• Consequently, each of the four pairs attains 250 Mbps throughput, thus achieving 1 Gbps aggregate throughput over the four pairs

1000BASE-T Waveform

Ethernet at Layer 1

• Now let’s look at a fiber based method• As it comes in off the fiber optic cable here

is where we see the block encoding method as illustrated in a white paper from Juniper

Ethernet at Layer 1

1000BASE-SX

• Block Encoding– 8B/10B

• Line Signaling– NRZ

8B/10B

• In 8B/10B signaling every 8-bits of data is converted into a 10-bit symbol prior to transmission over the media

• The overhead associated with the extra bits requires that a signal transmission rate of 1.25 gigabaud be used to transfer a net 1000 Mbps of data

8B/10B

• The extra bits permit a unique symbol to be assigned for each valid 8-bit data combination, while allowing additional symbols to be defined to transfer control and other information

• Control symbols are used to signal conditions such as start of packet, end of packet, and idle

8B/10B

• This conversion is done utilizing a look-up table

• NRZ - Non-Return-to-Zero line encoding is a binary code in which 1's are represented by one significant condition such as the presence or level of a signal and 0s are represented by some other significant condition such as an absence of a signal

1000BASE-SX Waveform

• The resulting signal looks like

1000BASE-SX Waveform

Wired Media Outside of a LAN

• Wired media is also used in larger networks such as campus, metropolitan, and wide area networks

• In these cases the wired media is always fiber optic cable

• Copper based cable should not be used for long runs in data networks other than those seen in the older proprietary telephone networks

• The block encoding and line signaling methods used in this longer distance networks varies by provider

• Ethernet and SONET based networks are common here

• To increase the throughput of these networks a multiplexing method is added

• To multiplex is to place more than one signal on the media at the same time

• Of course doing this requires that the signals be kept distinct in some way so they may be separated at the other end of the link

• There are several ways to do this• The two main ones used have been

– TDMA - Time Division Multiple Access– WDM – Wave Division Multiplexing

• With TDMA each user is assigned the channel for an amount of time on a fixed, rotating basis

• The time slot is of a fixed or variable length for each station on its list of active stations

• Advantages of TDMA include – More efficient use of the bandwidth– Ability to send data and voice on the same

line• One disadvantage is that the channel

remains idle if the user has no data to transmit during the assigned time slot

• Some systems are designed to sense the level of use and adjust the time slots accordingly

• These systems are also half duplex, as the channel can only send or receive, not both, during each cycle

• Wave division multiplexing is a version of frequency division multiplexing

• Except in this case colors of light are used to separate data

• There are two versions of this commonly used– CWDM – Coarse Wave Division Multiplexing– DWDM – Dense Wave Division Multiplexing

• Here is a slide from Steenbergen that shows this

• Steenbergen defines CWDM this way

• CWDM normally uses 8 colors• For example a Cisco SFP uses the

following colors– Gray– Violet– Blue– Green

– Yellow– Orange– Red– Brown

• The difference between CWDM and DWDM is merely the number of colors used

• Steenbergen offers this about DWDM

CWDM and DWDM

Sources

• The slides with the blue top are from a presentation to NANOG 57 in February 2013 by Richard Steenbergen

copyright 2008-2014 kenneth m. chipps ph.d. block encoding line signaling multiplexing on wired...

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