19-09-2012, 01:23 PM
Ethernet
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A Brief History
The original Ethernet was developed as an experimental coaxial cable network in the 1970s by Xerox
Corporation to operate with a data rate of 3 Mbps using a carrier sense multiple access collision detect
(CSMA/CD) protocol for LANs with sporadic but occasionally heavy traffic requirements. Success with
that project attracted early attention and led to the 1980 joint development of the 10-Mbps Ethernet
Version 1.0 specification by the three-company consortium: Digital Equipment Corporation, Intel
Corporation, and Xerox Corporation.
The original IEEE 802.3 standard was based on, and was very similar to, the Ethernet Version 1.0
specification. The draft standard was approved by the 802.3 working group in 1983 and was
subsequently published as an official standard in 1985 (ANSI/IEEE Std. 802.3-1985). Since then, a
number of supplements to the standard have been defined to take advantage of improvements in the
technologies and to support additional network media and higher data rate capabilities, plus several new
optional network access control features.
Throughout the rest of this chapter, the terms Ethernet and 802.3 will refer exclusively to network
implementations compatible with the IEEE 802.3 standard.
Ethernet Network Topologies and Structures
LANs take on many topological configurations, but regardless of their size or complexity, all will be a
combination of only three basic interconnection structures or network building blocks.
The simplest structure is the point-to-point interconnection, shown in Figure 7-1. Only two network units
are involved, and the connection may be DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in
point-to-point interconnections is known as a network link. The maximum allowable length of the link
depends on the type of cable and the transmission method that is used.
The Basic Ethernet Frame Format
The IEEE 802.3 standard defines a basic data frame format that is required for all MAC implementations,
plus several additional optional formats that are used to extend the protocol’s basic capability. The basic
data frame format contains the seven fields shown in Figure 7-6.
• Preamble (PRE)—Consists of 7 bytes. The PRE is an alternating pattern of ones and zeros that tells
receiving stations that a frame is coming, and that provides a means to synchronize the
frame-reception portions of receiving physical layers with the incoming bit stream.
• Start-of-frame delimiter (SOF)—Consists of 1 byte. The SOF is an alternating pattern of ones and
zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the
left-most byte of the destination address.
• Destination address (DA)—Consists of 6 bytes. The DA field identifies which station(s) should
receive the frame. The left-most bit in the DA field indicates whether the address is an individual
address (indicated by a 0) or a group address (indicated by a 1). The second bit from the left indicates
whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a
1). The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined
group of stations, or all stations on the network.
• Source addresses (SA)—Consists of 6 bytes. The SA field identifies the sending station. The SA is
always an individual address and the left-most bit in the SA field is always 0.
• Length/Type—Consists of 2 bytes. This field indicates either the number of MAC-client data bytes
that are contained in the data field of the frame, or the frame type ID if the frame is assembled using
an optional format. If the Length/Type field value is less than or equal to 1500, the number of LLC
bytes in the Data field is equal to the Length/Type field value. If the Length/Type field value is
greater than 1536, the frame is an optional type frame, and the Length/Type field value identifies the
particular type of frame being sent or received.
• Data—Is a sequence of n bytes of any value, where n is less than or equal to 1500. If the length of
the Data field is less than 46, the Data field must be extended by adding a filler (a pad) sufficient to
bring the Data field length to 46 bytes.
• Frame check sequence (FCS)—Consists of 4 bytes. This sequence contains a 32-bit cyclic
redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the
receiving MAC to check for damaged frames. The FCS is generated over the DA, SA, Length/Type,
and Data fields.
Full-Duplex Transmission
An Optional Approach to Higher Network Efficiency
Full-duplex operation is an optional MAC capability that allows simultaneous two-way transmission
over point-to-point links. Full duplex transmission is functionally much simpler than half-duplex
transmission because it involves no media contention, no collisions, no need to schedule retransmissions,
and no need for extension bits on the end of short frames. The result is not only more time available for
transmission, but also an effective doubling of the link bandwidth because each link can now support
full-rate, simultaneous, two-way transmission.
Transmission can usually begin as soon as frames are ready to send. The only restriction is that there
must be a minimum-length interframe gap between successive frames, as shown in Figure 7-9, and each
frame must conform to Ethernet frame format standards.
Encoding for Signal Transmission
In baseband transmission, the frame information is directly impressed upon the link as a sequence of
pulses or data symbols that are typically attenuated (reduced in size) and distorted (changed in shape)
before they reach the other end of the link. The receiver’s task is to detect each pulse as it arrives and
then to extract its correct value before transferring the reconstructed information to the receiving MAC.
Filters and pulse-shaping circuits can help restore the size and shape of the received waveforms, but
additional measures must be taken to ensure that the received signals are sampled at the correct time in
the pulse period and at same rate as the transmit clock:
• The receive clock must be recovered from the incoming data stream to allow the receiving physical
layer to synchronize with the incoming pulses.
• Compensating measures must be taken for a transmission effect known as baseline wander.
Clock recovery requires level transitions in the incoming signal to identify and synchronize on pulse
boundaries. The alternating 1s and 0s of the frame preamble were designed both to indicate that a frame
was arriving and to aid in clock recovery. However, recovered clocks can drift and possibly lose
synchronization if pulse levels remain constant and there are no transitions to detect (for example, during
long strings of 0s).
Baseline wander results because Ethernet links are AC-coupled to the transceivers and because AC
coupling is incapable of maintaining voltage levels for more than a short time. As a result, transmitted
pulses are distorted by a droop effect similar to the exaggerated example shown in Figure 7-12. In long
strings of either 1s or 0s, the droop can become so severe that the voltage level passes through the
decision threshold, resulting in erroneous sampled values for the affected pulses.