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 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 ( bytes) FCS (Frame Check Sequence) Field Frame Stop Field (unique value)

Media Access Control (MAC): Non-Deterministic (1 st come 1 st 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 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