Semester 1 Module 6 Ethernet Fundamentals

Andres, Wen-Yuan Liao

Department of Computer Science and Engineering

De Lin Institute of Technologyandres@dlit.edu.tw

http://www.cse.dlit.edu.tw/~andres

Overview

Describe the basics of Ethernet technology Explain naming rules of Ethernet technology Explain how Ethernet relates to the OSI mod

el Describe the Ethernet framing process and fr

ame structure List Ethernet frame field names and purposes Identify the characteristics of CSMA/CD Describe Ethernet timing, interframe spacing,

and backoff time after a collision

Define Ethernet errors and collisions Explain the concept of auto-negotiation in

relation to speed and duplex

Outline

Ethernet Fundamentals Ethernet Operation

Introduction to Ethernet

The success of Ethernet is due to the following factors: Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade

The original idea for Ethernet grew out of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other.

This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii.

A system called Alohanet was developed to allow various stations on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere.

This work later formed the basis for the Ethernet access method known as CSMA/CD.

The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX).

These standards start with the number 802. The standard for Ethernet is 802.3.

In 1995, IEEE announced a standard for a 100-Mbps Ethernet.

In 1999, 1Gbps Ethernet. Now, 10Gbps Ethernet

IEEE Ethernet naming rules

When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard.

Naming Rule : Speed : 10, 100, 1G, 10G Signal method : BASE(baseband) , BROAD(br

oadband) Medium : 2, 5, T, TX, FX, CX, SX, LX, ZX

Ethernet and the OSI model

Ethernet operates in two areas of the OSI model: The lower half of the data link layer (MAC sublaye

r) Physical layer

To guarantee minimum bandwidth and operability, standards specify the maximum number of stations per segment, maximum segment length, and maximum number of repeaters between stations.

Stations separated by bridges or routers are in different collision domains (shared resource/ segment).

Problems originating in one part of the collision domain will usually impact the entire collision domain.

Stations separated by repeaters are within the same collision domain.

Stations separated by bridges or routers are in different collision domains.

Data link sublayers contribute significantly to technology compatibility and computer communication. The MAC sublayer is concerned with the physical co

mponents that will be used to communicate the information.

The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.

Naming

To allow for local delivery of frames on the Ethernet, there must be an addressing system, a way of uniquely identifying computers and interfaces.

Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits.

MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.

The first six hexadecimal digits of the MAC address is known as the Organizational Unique Identifier (OUI).

The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer.

At the data link layer MAC headers and trailers are added to upper layer data.

The header and trailer contain control information intended for the data link layer in the destination system.

The NIC uses the MAC address to assess whether the message should be passed onto the upper layers of the OSI model.

The NIC makes this assessment without using CPU processing time, enabling better communication times on an Ethernet network.

On an Ethernet network, when one device sends data it can open a communication pathway to the other device by using the destination MAC address.

All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches.

Layer 2 framing

Framing is the Layer 2 encapsulation process.

A single generic frame has sections called fields. Each field is composed of bytes. Start Frame field Address field Length/Type field Data field Frame Check Sequence (FCS) field

Ethernet frame structure

The Ethernet II Type field is incorporated into the current 802.3 frame definition.

If the value of Length/Type field is equal to or greater than 1536 or 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated.

Ethernet frame fields

Preamble field The pattern of ones and zeroes used for timing

synchronization in 10 Mbps Ethernet. Faster versions of Ethernet are synchronous so

this timing information is unnecessary but retained for compatibility.

Start Frame Delimiter field It consists of a one-octet field that marks the end of

the timing information, and contains the bit sequence 10101011.

Destination field Destination node MAC address (can be unicast, multicast,

broadcast)

Source field Source node MAC address (can be unicast address)

Length field / type field Length of a frame in bytes. Some frames have a type field, which specifies the Layer 3

protocol.

Data field Ethernet requires each frame to be between 64

and 1518 octets. Padding if necessary. Maximum transmission unit (MTU) for Ethernet is

1500 octets

FCS field (Frame Check Sequence number) FCS contains a 4-byte CRC value. When the destination node receives the frame the

FCS number is recalculated and compared with the FCS number included in the frame.

If the two numbers are different, an error is assumed, the frame is discarded, and the source is asked to retransmit.

Outline

Ethernet Fundamentals Ethernet Operation

Media Access Control (MAC)

Categories of Media Access Control Deterministic (taking turns)

In a Token Ring network, individual hosts are arranged in a ring and a special data token travels around the ring to each host in sequence.

When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring.

Token Ring is a collisionless environment as only one host is able to transmit at any given time.

Non-deterministic (first come, first served) In a CSMA/CD system, the NIC listens for an absence

of a signal on the media and starts transmitting. If two nodes transmit at the same time a collision

occurs and none of the nodes are able to transmit.

Ethernet logical bus topology physical star or extended star wired as a star

Token Ring logical ring topology physical star topology wired as a star

FDDI logical ring topology physical dual-ring topology wired as a dual-ring

MAC rules and collision detection/backoff

Ethernet is a shared-media broadcast technology.

The access method CSMA/CD used in Ethernet performs three functions: Transmitting and receiving data packets. Decoding data packets and checking them for

valid addresses before passing them to the upper layers of the OSI model.

Detecting errors within data packets or on the network.

In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must

first check to see whether the networking media is busy. If the node determines the network is busy, the node will

wait a random amount of time before retrying. If the node determines the networking media is not busy,

the node will begin transmitting and listening. The node listens to ensure no other stations are

transmitting at the same time. After completing data transmission the device will return

to listening mode.

Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will conti

nue to transmit for a short time to ensure that all devices see the collision.

Once all the devices have detected the collision a backoff algorithm is invoked and transmission is stopped.

The nodes stop transmitting for a random period of time, which is different for each device.

When the delay period expires, all devices on the network can attempt to gain access to the networking media.

When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data.

Ethernet timing

In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble.

10 Mbps and slower versions of Ethernet are asynchronous.

Asynchronous means that each receiving station will use the eight octets of timing information to synchronize the receive circuit to the incoming data, and then discard it.

100 Mbps and higher speed implementations of Ethernet are synchronous.

Synchronous means the timing information is not required, however for compatibility reasons the Preamble and SFD are present.

For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes how a transmission may be no smaller than the slot time. Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or

64 octets. Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512

octets. Slot time is calculated assuming maximum cable lengths

on the largest legal network architecture. All hardware propagation delay times are at the legal

maximum and the 32-bit jam signal is used when collisions are detected.

The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected.

For the system to work the first station must learn about the collision before it finishes sending the smallest legal frame size.

On 10-Mbps Ethernet one bit at the MAC layer requires 100 ns to transmit.

At 100 Mbps that same bit requires 10 ns to transmit and at 1000 Mbps only takes 1 ns.

As a rough estimate, 20.3 cm (8 inch) per nanosecond is often used for calculating propagation delay down a UTP cable.

For 100 meters of UTP, this means that it takes just under 5 bit-times for a 10BASE-T signal to travel the length the cable.

Interframe spacing and backoff

The minimum spacing between two non-colliding frames is also called the interframe spacing.

This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame.

Ethernet devices must allow a minimum idle period between transmission of frames known as the spacing gap.

It provides a brief recovery time between frames to allow devices to prepare for reception of the next frame.

The minimum spacing gap is 96 bit times, which is 9.6 microseconds for 10 Mb/s Ethernet, 960 nanoseconds for 100 Mb/s Ethernet, and 96 nanoseconds for 1 Gb/s Ethernet.

If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer.

Error handling

The most common error condition on an Ethernet is the collision.

Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal (32 bits), all stations have a chance to detect the collision.

Jam signal is simply a repeating one, zero, one, zero pattern, the same as Preamble.

When viewed by a protocol analyzer this pattern appears as either a repeating hexadecimal 5 or A sequence.

Types of collisions

Three types of collisions are: Local Remote Late

Ethernet errors

The following are the sources of Ethernet error: Collision or runt – Simultaneous transmission occurri

ng before slot time has elapsed Late collision – Simultaneous transmission occurring

after slot time has elapsed Jabber(含糊不清 ), long frame and range errors – E

xcessively or illegally long transmission 

Short frame, collision fragment or runt – Illegally short transmission

FCS error – Corrupted transmission Alignment error – Insufficient or excessive number of

bits transmitted Range error – Actual and reported number of octets i

n frame do not match Ghost or jabber – Unusually long Preamble or Jam e

vent

FCS and beyond

A received frame that has a bad FCS. The frame is then discarded.

High numbers of FCS errors from a single station a faulty NIC a faulty software drivers a bad cable connecting

FCS errors are associated with many stations a faulty software drivers bad cabling a faulty hub noise in the cable system

A message that does not end on an octet boundary is known as an alignment error. Bad software drivers Collision

A frame with a valid value in the Length field but did not match the actual number of octets counted in the data field of the received frame is known as a range error.

Fluke Networks has coined the term ghost to mean energy (noise) detected on the cable that appears to be a frame, but is lacking a valid SFD.

Ethernet auto-negotiation

Auto-Negotiation(協商 ) - This process defines how two link partners may automatically negotiate a configuration offering the best common performance level.

If anything interrupts communications and the link is lost, the two link partners first attempt to link again at the last negotiated speed.

If that fails, or if it has been too long since the link was lost, the Auto-Negotiation process starts over.

Link establishment and full and half duplex

Full duplex is preferred over half duplex in link establishment negotiation

Two methods of achieving a full duplex are auto-negotiation and administrative configuration

Duplex mismatch one end is forced to full duplex the other is forced to half duplex result in collisions and errors 10-Gigabit Ethernet does not support half duplex

The list is priority ranked, with the most desirable link configuration at the top.

Good luck in your exams !