CCNA – Semester1

Module 6Ethernet Fundamentals

Objectives

• Ethernet fundamentals

• Ethernet operation

Ethernet Fundamentals

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

– Bandwidth can be increased without changing underlying technology

IEEE 802 Committees802.0 SEC

802.1 High Level Interface (HILI)

802.2 Logical Link Control (LLC)

802.3 CSMA/CD Working Group

802.4 Token Bus

802.5 Token Ring

802.6 Metropolitan Area Network (MAN)

802.7 BroadBand Technical Adv. Group (BBTAG)

802.8 Fiber Optics Technical Adv. Group (FOTAG)

802.9 Integrated Services LAN (ISLAN)

802.10 Standard for Interoperable LAN Security (SILS)

801.11 Wireless LAN (WLAN)

802.12 Demand Priority

802.14 Cable-TV Based Broadband Communication Network

802.15 Wireless Personal Area Network (WPAN)

802.16 Broadband Wireless Access (BBWA)

RPRSG Resilient Packet Ring Study Group (RPRSG)

IEEE 802.3 - 10 Mbit

IEEE 802.3u - 100 Mbit

IEEE 802.3z - 1000 Mbit

Ethernet Standards

Ethernet Specifications

Designation Description

10Base-210 Mbps baseband Ethernet over coaxial cable with a maximum distance of 185 meters. Also referred to as Thin Ethernet or Thinnet or Thinwire.

10Base-T10 Mbps baseband Ethernet over twisted pair cables with a maximum length of 100 meters.

10Base-510 Mbps baseband Ethernet over coaxial cable with a maximum distance of 500 meters. Also referred to as Thick Ethernet or Thicknet or Thickwire.

100Base-FX 100 Mbps baseband Ethernet over two multimode optical fibers.

100Base-T 100 Mbps baseband Ethernet over twisted pair cable.

100Base-T4100 Mbps baseband Ethernet over four pairs of Category 3 or higher unshielded twisted pair cable.

100Base-TX100 Mbps baseband Ethernet over two pairs of shielded twisted pair or Category 4 twisted pair cable.

1000Base-CX 1000 Mbps baseband Ethernet over two pairs of 150 shielded twisted pair cable.

1000Base-LX1000 Mbps baseband Ethernet over two multimode or single-mode optical fibers using longwave laser optics.

1000Base-SX1000 Mbps baseband Ethernet over two multimode optical fibers using shortwave laser optics.

1000Base-T1000 Mbps baseband Ethernet over four pairs of Category 5 unshielded twisted pair cable.

Ethernet and the OSI model

• Divided OSI Layer 2 into two sublayers– Media Access Control (MAC) – Traditional L2 features

• Transitions down to media– Logical link control (LLC) – New L2 features

• Transitions up to the network layer

LLC sublayer

• LLC PDU includes:– DSAP: Destination service access point– SSAP: Source service access point

• Supports connection control methods specified by upper protocols

Packet

Packet

Packet

LLC PDU

Frame

MAC sublayer

• Naming.• Framing.• Media access control rules.

Packet

Packet

Packet Packet

802.3 802.5

MAC address

• MAC addresses are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.

• Presentation formats: 0000.0c12.3456 or 00-00-0c-12-34-56.

• Broadcast address: FFFF.FFFF.FFFF

Layer 2 Framing

• Framing helps obtain essential information that could not, otherwise, be obtained with coded bit streams alone. Examples of such information are: – Which computers are

communicating with one another

– When communication between individual computers begins and when it terminates

– Provides a method for detection of errors that occurred during the communication

– Whose turn it is to "talk" in a computer "conversation“

Generic Frame Format

• Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit.

Ethernet Frame Format

• At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps.

• Ethernet requires that the frame be not less than 46 octets or more than 1518 octets.

IEEE 802.3 Ethernet

Ethernet II

Ethernet Operation

Media Access Control (MAC)

• MAC refers to protocols that determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data.

• There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served).

Media Access Control (MAC) Protocols

• Ethernet (IEEE 802.3)– Logical bus topology – Physical star or extended star– Nondeterministic

• First-come, first-served • Token Ring (IEEE 802.5)

– Logical ring– Physical star topology– Deterministic

• Token controls traffic– Older declining technology

• FDDI (IEEE 802.5)– Logical ring topology– Physical dual-ring topology– Deterministic

• Token controls traffic – Near-end-of-life technology

CSMA/CD Process

Host wants to transmit

Is carrier sensed?

Assemble frame

Start transmitting

Is a collision detected?

Keep transmitting

Is transmission

done?Transmission

completed

Broadcast jam signal

attempts = attempts + 1

Attempts> too many?

Too many collisions; abort

transmission

Algorithm calculate backoff

Wait for t seconds

Ethernet Transmission Mode

• Full duplex : – send and receive simultaneously

– no collisions occur.

• Half duplex:– only send or receive at a specific moment

– transmit 64 bits of timing synchronization information that is known as the preamble to make sure collision do not occur.

Ethernet Timing

• For CSMA/CD Ethernet to operate, collision must be sensed before completing transmission of a minimum-sized frame.

• At 100 Mbps the system timing is barely able to accommodate 100 meter cables. For this reason half duplex is not permitted in 10-Gigabit Ethernet.

Slot time

• Ethernet standard specifications limit – maximum segment length

– maximum number of stations per segment

– maximum number of repeaters between segments

• Slot time >= round trip delay

Interframe Spacing

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

• After a frame has been sent, all stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame.

Error Handling

• Collisions are to resole contention for network access which results in network bandwidth loss.

• When collision occurs, the devices with data to transmit return to a listen-before-transmit mode and no device have priority to transmit data.

Types of Collisions

• The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are: – Local

– Remote

– Late

Local Collision

• Local collision is detected on the local segment when a station detects a signal on the RX pair at the same time it is sending on the TX pair.

• Local collision is sensed as over-voltagecondition on the local cable area when two signal from different stations collide.

Remote Collison

• The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum.

• This sort of collision usually results from collisions occurring on the far side of a repeated connection.

Late Collision

• Collisions occurring after the first 64 octets are called late collisions.

• Difference between late collisions and other collisions is that the Ethernet NIC will not automatically retransmit a frame that was collided late.

FCS and Beyond

• A received frame that has a bad frame check sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least 1 bit.

• High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network.

• If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system.

Ethernet Autonegotiation

• This process defines how two link partners may automatically negotiate a configuration offering the best common performance level.

• It has the additional advantage of only involving the lowest part of the physical layer.

Summary

• The basics of Ethernet technology

• How Ethernet and the OSI model interact

• Ethernet frame field names and purposes

• The characteristics and function of CSMA/CD

• Ethernet timing

• The backoff algorithm and time after a collision

• Ethernet errors and collisions

• Auto-negotiation in relation to speed and duplex