Oct-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod6 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 1 Cisco Systems CCNA Version

Oct-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod6 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 1

Cisco Systems CCNA Version 3 Semester 1

Module 6



Objectives• Describe the basics of Ethernet technology. • Explain naming rules of Ethernet

technology. • Define how Ethernet and the OSI model

interact. • Describe the Ethernet framing process and

frame structure. • List Ethernet frame field names and

purposes. • Identify the characteristics of CSMA/CD. • Describe the key aspects of 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.


6.1 Ethernet Fundamentals6.1.1 Introduction to Ethernet 6.1.2 IEEE Ethernet naming rules 6.1.3 Ethernet and the OSI model 6.1.4 Naming 6.1.5 Layer 2 framing 6.1.6 Ethernet frame structure 6.1.7 Ethernet frame fields

6.2 Ethernet Operation6.2.1 Media Access Control (MAC) 6.2.2 MAC rules and collision detection/backoff 6.2.3 Ethernet timing 6.2.4 Interframe spacing and backoff 6.2.5 Error handling 6.2.6 Types of collisions 6.2.7 Ethernet errors 6.2.8 FCS and beyond 6.2.9 Ethernet auto-negotiation 6.2.10 Link establishment and full and half duplex


Module 6





Module 6


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

6.1.1 Introduction to Ethernet

Alohanet…original concept

Essentially, Ethernet and IEEE 802.3 are the same standards.


A drawing of the first Ethernet system by Bob Metcalfe of Xerox

http://www.ethermanage.com/ethernet/metcalfe-drawing.html


802.0 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

IEEE 802 Committees

IEEE 802.11

IEEE 802.11a

IEEE 802.11b WiFi

IEEE 802.11g


6.1.1 Introduction to Ethernet

DIX Ethernet is essentially the same as 802.3


6.1.2 IEEE Ethernet naming rules

• In BASE band signaling, the data signal is transmitted directly over the transmission medium.

• In BROADband signaling, not used by Ethernet, a carrier signal is modulated by the data signal and the modulated carrier signal is transmitted.


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

10Base-3610 Mbps baseband Ethernet over multi-channel coaxial cable with a maximum distance of 3,600 meters.

10Base-F 10 Mbps baseband Ethernet over optical fiber.

10Base-FB10 Mbps baseband Ethernet over two multi-mode optical fibers using a synchronous active hub.

10Base-FL10 Mbps baseband Ethernet over two optical fibers and can include an optional asynchronous hub.

10Base-FP10 Mbps baseband Ethernet over two optical fibers using a passive hub to connect communication devices.

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

10Broad-3610 Mbps baseband Ethernet over three channels of a cable television system with a maximum cable length of 3,600 meters.

10Gigabit Ethernet

Ethernet at 10 billion bits per second over optical fiber. Multimode fiber supports distances up to 300 meters; single mode fiber supports distances up to 40 kilometers.


Designation Description

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

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

100Base-T2100 Mbps baseband Ethernet over two pairs of Category 3 or higher unshielded 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.

100Base-X A generic name for 100 Mbps Ethernet systems.

1000Base-CX1000 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.

1000Base-X A generic name for 1000 Mbps Ethernet systems.


6.1.3 Ethernet and the OSI model


All other stations in the same collision domain see traffic that passes through a repeater.

Stations separated by bridges or routers

are in different collision domains.


http://configure.dell.com/dellstore/config.aspx?c=ca&CS=CADHS1&kc=6V450&l=en&oc=OCDIM2400C_FEAT_CH24W12






• The Media Access Control (MAC) sublayer is concerned with the physical components 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.


6.1.4 Naming

•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.•The MAC address is burned into ROM on the NIC card

48 bits or 12 Hex


6.1.5 Layer 2 framing


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

• Cyclic Redundancy Check (CRC) – performs calculations on the data.

• Two-dimensional parity – adds an 8th bit that makes an 8 bit sequence have an odd or even number of binary 1s.

• Internet checksum – adds the values of all of the data bits to arrive at a sum.

All frames contain naming information, such as the name of the source node (MAC address) and the name of the destination node (MAC address).

Most frames have some specialized fields. In some technologies, a length field specifies the exact length of a frame in bytes. Some frames have a type field, which specifies the Layer 3 protocol making the sending request.

Start of FrameVarious technologies have different ways of doing this process, but all frames, have a beginning signaling sequence of bytes.

The Layer 3 Packet.



6.1.5 Layer 2 framing (DIX version)



< 0x600 = length

Otherwise = type


6.1.7 Ethernet frame fields

Preamble and Start Frame Delimiter (SFD) were combined into a single field, though the binary pattern was identical.

The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version, as both uses of the field were common throughout industry.

(DIX version)

All speeds of Ethernet have nearly identical frame structures.


• The Preamble is an alternating pattern of ones and zeroes used for timing synchronization in the asynchronous 10 Mbps and slower implementations of Ethernet.

• Faster versions of Ethernet are synchronous, and this timing information is redundant but retained for compatibility.

6.1.7 Ethernet frame fields


1. A Start Frame Delimiter consists of a one-octet field that marks the end of the timing information, and contains the bit sequence 10101011.

2. The Destination Address field contains the MAC destination address. It can be unicast, multicast (group), or broadcast (all nodes).

3. The Source Address field contains the MAC source address. The source address is generally the unicast address of the transmitting Ethernet node.

4. The Length/Type field supports two different uses. • If the value is less than 1536 decimal, 0x600 (hexadecimal), then the

value indicates length. • If the value is equal to or greater than 1536 decimal (0600

hexadecimal), the value indicates the protocol. The Data and Pad field may be of any length that does not cause the frame to exceed the maximum frame size.

• The maximum data transmission unit (MTU) for Ethernet is 1500 octets.. • The content of the data field is unspecified. If there is not enough user

data for the frame to meet the minimum frame length a pad is inserted. • Ethernet requires that the frame be not less than 46 octets or more

than 1518 octets. 5. A FCS contains a four byte CRC value that is created by the sending device

and is recalculated by the receiving device to check for damaged frames.





Module 6


6.2.1 Media Access Control (MAC)

Deterministic(taking turns)

Non-Deterministic(1st come 1st

served)


6.2.2 MAC rules and collision detection/backoff

1. Transmitting and receiving data packets 2. Decoding data packets and checking them for valid addresses before

passing them to the upper layers of the OSI model 3. Detecting errors within data packets or on the network

(JAM) When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all devices see the collision.The devices that were

involved in the collision do not have priority to transmit data.


6.2.2 MAC rules and collision detection/backoff


6.2.3 Ethernet timing •The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next.

•10 Mb/s=1/10 sec/Mb=.1*10-6=100*10-

9=100nanosecs

• As a rough estimate 203 cm per nanosecond is often used for calculating propagation delay down a UTP cable.

• For 100 meters of UTP, this means that it takes…• 100/.203 nanosecs = 492.6 nanosecs = approx 500 nanosecs• just under 5 bit-times for a 10BASE-T signal to travel the length the cable.


100 meters = 500 nanoseconds

Bit 1 will arrive at the end

Before bit 5 is transmitted

5 1

At 10 Mbps

1 bit = 100 nanoseconds

•The minimum Frame size is 64 Bytes x 8 bits = 512 bit times = 51,250 nsecs = about 10km

Slot Time

Ethernet specifies…• maximum segment length • maximum number of stations per segment • maximum number of repeaters between segments


100 meters = 500 nanoseconds

Bit 1 will arrive at the end

Before bit 50 is transmitted

At 100 Mbps

1 bit = 10 nanoseconds

0

•At 100 Mbps the system timing is barely able to accommodate 100 meter cables. •At 1000 Mbps special adjustments are required as an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. •For this reason half duplex is not permitted in 10-Gigabit Ethernet.

•The minimum Frame size is 64 Bytes x 8 bits = 512 bit times = 5,125 nsecs = about 1km•4 repeaters x 5 lengths of 100 meters = .5 kms + latency through the repeaters

Slot Time

Bit 64 is the end of the Preamble &SFD. Short

Collision.


6.2.4 Interframe spacing and backoff

•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. •To allow 1000-Mbps Ethernet to operate in half duplex the extension field was added when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return.


6.2.4 Interframe spacing and backoff

• 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.

• On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter.

• This interval is referred to as the spacing gap. • The gap is intended to allow slow stations time to process the previous

frame and prepare for the next frame.


1. After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing).

2. The devices with data to transmit return to a listen-before-transmit mode.

3. The stations that collided invoke a back-off algorithm and stop transmitting data.

4. They must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame.

5. The devices involved in the collision do not have priority to transmit data.

6. The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions.

7. This is accomplished in part by expanding the interval from which the random retransmission time is selected on each retransmission attempt.

8. The waiting period is measured in increments of the parameter slot time.

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

6.2.5 Error handling



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


1. When network contention becomes too great, collisions can become a significant impediment to useful network operation.

2. Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal.

3. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput.

4. The majority of collisions occur very early in the frame, often before the start Frame Delimiter (SFD).

5. Collisions occurring before the SFD are usually not reported to the higher layers, as if the collision did not occur.

6. As soon as a collision is detected, the sending stations transmit a 32-bit “jam” signal that will enforce the collision.

7. This is done so that any data being transmitted is thoroughly corrupted and all stations have a chance to detect the collision.



1. A jam signal may be composed of any binary data so long as it does not form a proper checksum for the portion of the frame already transmitted.

2. The most commonly observed data pattern for a jam signal is simply a repeating one, zero, one, zero pattern, the same as Preamble.

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

4. The corrupted, partially transmitted messages are often referred to as collision fragments or runts.

5. Normal collisions are less than 64 octets in length and therefore fail both the minimum length test and the FCS checksum test.



6.2.6 Types of collisions • To create a local collision on coax cable (10BASE2 and 10BASE5), the

signal travels down the cable until it encounters a signal from the other station.

• The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The signal amplitude on the networking media increases.

• On UTP cable, such as 10BASE-T, 100BASE-TX and 1000BASE-T, a collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair.

• Since the two signals are on different pairs there is no characteristic change in the signal.

2nd JAM signal.

Collision starts. 1st stops transmitting


6.2.6 Types of collisions • A single collision is a collision that was detected while trying to transmit a frame, but on

the next attempt the frame was transmitted successfully. • Multiple collisions indicate that the same frame collided repeatedly before being

successfully transmitted.• There is no possibility remaining for a normal or legal collision after the first 64 octets of

data has been transmitted.

Most common.


6.2.7 Ethernet errors

1. Collision or runt – Simultaneous transmission occurring before slot time has elapsed

2. Late collision – Simultaneous transmission occurring after slot time has elapsed

3. Jabber, long frame and range errors – Excessively or illegally long transmission

4. Short frame, collision fragment or runt – Illegally short transmission

5. FCS error – Corrupted transmission 6. Alignment error – Insufficient or excessive number

of bits transmitted. A message that does not end on an octet boundary.

7. Range error – Actual and reported number of octets in frame do not match

8. Ghost or jabber – Unusually long Preamble or Jam event



• A long frame is one that is longer than the maximum legal size, and takes into consideration whether or not the frame was tagged.

• It does not consider whether or not the frame had a valid FCS checksum. This error usually means that jabber was detected on the network.



• A short frame is a frame smaller than the minimum legal size of 64 octets, with a good frame check sequence.

• Some protocol analyzers and network monitors call these frames “runts". In general the presence of short frames is not a guarantee that the network is failing.


6.2.8 FCS and beyond

• 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.

Alignment Error - Bits end off Octet Boundary

Range ErrorGhost

Frame Check Sequence or CRC

Error


6.2.9 Ethernet auto-negotiation

• When a series of Normal Link Pulses (NLPs) are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst.

• Each FLP burst is sent at the same timing interval as an NLP,


6.2.9 Ethernet auto-negotiation


6.2.10 Link establishment and full and half duplex

• There are two duplex modes, half and full. • For shared media, the half-duplex mode is mandatory. • All coaxial implementations are half duplex in nature and cannot

operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only.



Review• The basics of Ethernet technology • The naming rules of Ethernet technology • How Ethernet and the OSI model interact • Ethernet framing process and frame structure • Ethernet frame field names and purposes • The characteristics and function of CSMA/CD • Ethernet timing • Interframe spacing • The backoff algorithm and time after a

collision • Ethernet errors and collisions • Auto-negotiation in relation to speed and

duplex


FIN

Documents

Oct-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod6 – St. Lawrence College – Cornwall Campus, ON, Canada – Clark slide 1 Cisco Systems CCNA Version