Ethernet Fundamentals

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

Introduction to Ethernet

Essentially, Ethernet and IEEE 802.3 are the same standards.

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 drawing of the first Ethernet system by Bob Metcalfe of Xerox

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.

Layer 1 vs. Layer 2

• Layer 1– Cannot communicate with

the upper-layer protocols

– Cannot name or identify computers

– Can describe only streams of bits

– Cannot decide which computer will transmit data from a group in which all are trying to transmit at the same time

• Layer 2– Communicates with

upper-layer protocols using logical link control (LLC)

– Provides an addressing (or naming) process

– Uses framing to organize or group the bits

– Uses a system called Media Access Control (MAC) to control transmissions

Layer 2 deals with the limitations of Layer 1

IEEE Standard

• 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

Comparing LAN Standards

OSI Layer 1 and 2 Together Are the Access Protocols

• These are the delivery system protocols

• Independent of:– Network OS– Upper-level protocols

• TCP/IP, IPX/SPX

• Sometimes called:– Access methods– Access protocols– Access technologies– Media access– LAN protocols– WAN protocols

Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, Frame Relay, ATM, PPP, and so on

Logical Link Control (LLC)

• LLC allows part of the data link layer to function independent of LAN access technologies (protocols / methods)– Provides services to network layer protocols, while

communicating with access technologies below it. • LAN access technologies:

– Ethernet– Token Ring– FDDI

Logical Link Control (LLC)

• Participates in the data encapsulation process.– LLC PDU between Layer 3 and the MAC sublayer– Adds control information to the network layer data

to help deliver the packet. It adds two fields:• Destination Service Access Point (DSAP)• Source Service Access Point (SSAP)

• Supports both connectionless and connection-oriented upper-layer protocols

• Allows multiple higher layer protocols to share a single physical data link

Media Access Control (MAC)

• Provides MAC Addressing (naming)• Depending on access Technology

(Ethernet, Token Ring, FDDI), Provides:– Data transmission control– Collision resolution (retransmission)– Layer 2 frame preparation (data framing)– Frame Check Sequence (FCS – frame error detection)

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

Ethernet and the OSI model:

• 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 to upper layers.

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

How the NIC Uses MAC Addresses

Legacy (Broadcast) Ethernet:– Source host builds a Layer 2 data frame.

• Its own MAC is the source address.• The MAC of the target is the destination address.

– All devices on the segment see the frame.– Only the target’s NIC recognizes its MAC address

in the Destination Address field.– Target host copies and processes the frame.– Non-target hosts dispose of or ignore of the frame.

Layer 2 Address Encapsulation and De-encapsulation

Source and Destination MAC address fields are part of the frame header.

Framing

The following slides address the following topics:

• Why framing is necessary

• Frame format diagram

• Generic frame format

Why Framing Is Necessary

• Binary data is a stream of 1s and 0s.• Framing breaks the stream into decipherable

groupings:– Start and stop indicator fields– Naming or addressing fields– Data fields– Quality-control fields (FCS or CRC)

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

Generic Frame Format

• Start Frame Field (multiple bytes in size – preamble + SFD) • Address Fields (Source & Destination MAC addresses)• Type / Length Field (depending on Ethernet standard)• Data Field (46-1500 bytes)• FCS (Frame Check Sequence) Field• Frame Stop Field (unique value)

Media Access Control (MAC):

Non-Deterministic(1st come 1st served)Collisions occur and

slow transmission

Deterministic(taking turns)No Collisions

MAC rules and collision detection/backoff

(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.(transmit 32-bit jam signal)

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

Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases.

MAC rules and collision detection/backoff:

1. Host wants to transmit

2. Is carrier sensed?

3. Assemble Frame

4. Start Transmitting

5. Is a Collision detected?

6. Keep Transmitting

7. Is the transmission done?

8. Transmission complete

9. Broadcast jam signal

10. Attempts = Attempts + 1

11. Attempts > Too many?

12. Too many collisions; abort transmission

13. Algorithm calculates backoff

14. Wait for t microseconds

Chapter #6 Test Next Week

• This Class:– Chapter 5 Labs due!– Finish Labs:

• Chapter 5 no Labs for Chapter 6!

– Wall Outlet – Layout & cut

– Cable, Panel installation next week