Part 1 Ethernet Fundamentals Cisco CCNA 3.0 Semester 1 Chapter 6 Part 1 Ethernet Fundamentals Karl Wick SUNY Ulster

Equal Access For All Understanding how network devices gain access to the network media is essential for understanding and troubleshooting the operation of the entire network. Ethernet LANS use shared bandwidth. Data collisions are dealt with using a process called CSMA-CD.

Timeline Early 1970’s – Univ. of Hawaii Alohanet 1980 – Ethernet by DEC, Intel and Xerox 1985 – IEEE 802.3 Standard for LANS 1995 – 100Mbps Ethernet 1998/9 – Gigabit Ethernet All are backward compatible.

Layer 2 is the “Data Link” layer The data link layer provides transit of data across a physical link. The data link layer works with source and destination MAC (Media Access Control) addresses. The data link layer creates frames from packets by adding headers and trailers.

Layer 2, the data link layer … Communicates with upper layers using Logical Link Control. Decides which computer can use the media at a given time. (CSMA-CD) Uses MAC addresses to get data to its intended destination.

The IEEE Defines network standards. These standards involve layer 1 and 2 IEEE divides layer 2 into two parts Logical Link Control - communicates up Media Access Control - control of layer 1

OSI Model vs IEEE Standard

The IEEE Standard (LLC) Defined by the IEEE 802.2 standard Functions independently from the technologies of the physical layer. Communicates between logical layer 3 and physical layer 1 Enables multiple upper level protocols to share a physical link.

The IEEE Standard (MAC) Deals with the protocols that a host must follow to access the physical media. (Which can be ethernet , token ring, FDDI, etc.) Deals with the actual physical technology. Creates orderly access to the medium.

IEEE Specifications (Services) Layer 2 LLC MAC Layer 1

Layer 1 Technologies Layer 2 LLC MAC Layer 1

Sub-layers of Layer 2 Data link sub-layers contribute significantly to technology compatibility and computer communication. The MAC sub-layer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sub-layer remains relatively independent of the physical equipment that will be used for the communication process.

A Expanded Seven Layer Model

Important Layer 2 concepts Communicates with upper layers through LLC. Uses a Flat Addressing scheme (unique addresses) Uses framing to organize data. Uses Media Access Control to handle contention for shared media (the network hardware). Think: LLC, Naming, Framing, MAC

MAC Addressing Vendor Code Serial Number

MAC Address Each MAC address in the world is supposed to be unique. MAC addresses are usually written as hexidecimal numbers. The first six digits identify the vendor The last six digits are assigned by the vendor.

MAC Address The NIC uses the MAC address to assess whether an incoming 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. An address matching either its own OR the “broadcast address” will pass upward.

Broadcast MAC Address FF FF FF FF FF FF (48 binary ones) Processed by all NICs.

Media Access Control (MAC) Deterministic: taking turns (Token Ring, FDDI) A special data token is passed around the hosts When a host holds the token it can send data Non-deterministic: first come, first served Carrier sense, multiple access, collision detect CSMA/CD uses jam signal, variable backoff

Ethernet and 802.3 LANs Are “broadcast” media. Every station sees every frame. Each station must examine each frame. Frames for the station get passed upward to the next layer. Frames for other stations are ignored. Every station reads broadcast frames (with special address of FF FF FF FF FF FF).

Flat Addressing Each station has a unique fixed address. This does not scale well to large networks

Framing - Extra Information Carried Source and Destination Addresses When communication starts and ends Whose turn it is to talk Error notices A frame is the layer 2 protocol data unit.

Generic Frame Format MAC Address Optional

Calculating the FCS Cyclic Redundancy Check (CRC) – performs calculations on the data. Two-dimensional parity – adds an 9th 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.

802.3 and Ethernet Frames (All sizes in bytes)

Fields permitted or required in an 802.3 Frame Preamble Start Frame Delimiter Destination Address Source Address Length/Type Data and Pad FCS Extension

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

Addresses The Destination Address field contains the MAC destination address. The destination address can be unicast, multicast (group), or broadcast (all nodes). The Source Address field contains the MAC source address. The source address is generally the unicast address of the transmitting Ethernet node. There are, however, an increasing number of virtual protocols in use that use and sometimes share a specific source MAC address to identify the virtual entity.

Length or Type If the value is less than 1536 decimal, (060016) then the value indicates length. If the value is equal to or greater than 1536 decimal (0600H), the value indicates that the type and contents of the Data field are decoded per the protocol indicated. The length interpretation is used where the LLC Layer provides the protocol identification and indicates the number of bytes of data that follows this field. The type value specifies the upper-layer protocol to receive the data after Ethernet processing is completed.

Data and Padding The Data and Pad field may be of any length that does not cause the frame to exceed the maximum frame size. The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified pad is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. Ethernet requires that the frame be not less than 46 octets or more than 1518 octets.

Frame Check Sequence 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. Since the corruption of a single bit anywhere from the beginning of the Destination Address through the end of the FCS field will cause the checksum to be different, the coverage of the FCS includes itself. It is not possible to distinguish between corruption of the FCS itself and corruption of any preceding field used in the calculation.

Part 2 Ethernet Operations

CSMA-CD Operation

CSMA-CD Flowchart

Propagation and Timing The speed of a single bit traveling along a CAT 5 cable is about 20cm or 8 inches per nanosecond. It therefore takes about 500 nanoseconds to traverse a 100 meter long cable. Transmit time (next slide) can be far shorter than this. For Half duplex, collisions must be detected before a full frame is sent.

Transmit Time The time it takes for the NIC to put a single bit onto the wire. To allow 1000-Mbps Ethernet to operate in half duplex, an extension field was added when sending small frames, purely to keep the transmitter busy long enough for a collision fragment to return.

Transmit Time A minimum sized 46 octet frame has 46 * 8 = 370 bits. For gigabit: 370 Bits @ 1nsec = 370 ns. This is shorter than the potential propagation delay on a 100 meter line. (500nS) The receiving station ignores the extension. Half duplex is not allowed on 10Gbps connections. Half Duplex not allowed

Interframe Spacing 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. After a frame has been sent, all stations are required to wait a minimum of 96 bit-times (0.96 microseconds for 100-Mbps Ethernet ) before any station may legally transmit the next frame.

Collision! The first station detecting a collision immediately sends a “jam” signal. All other involved stations repeat this signal for a short time and data transmission stops. The line is then cleared and the two (or more) stations directly involved in the collision wait for a pseudo-random amount of time before trying to send again.

Backoff Time After a collision occurs and all stations wait the full interframe spacing before attempting to use the line. The stations that collided must wait an additional and progressively longer period of time before attempting to retransmit the collided frame. This is the Backoff time and is a random number with a maximum possible value that increases if the stations collide again.

Worst Case Scenario If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer. Such an occurrence is fairly rare and would happen only under extremely heavy network loads, or when a physical problem exists on the network.

Types of Collisions can (Full duplex) Or when abnormally high voltage levels occur (half duplex) Or switch Late collisions are not noticed by the NIC and resending of missing data must be handled by upper layers in the model.

Ethernet Errors Collision (caused) runt – Simultaneous transmission occurring before slot time has elapsed – Undersized frame. Late collision – Simultaneous transmission occurring after slot time has elapsed Jabber, long frame and range errors – Excessively or illegally long transmission  - AKA “Giant” 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 in frame do not match Ghost or jabber – Unusually long Preamble or Jam event

Solution to 6.2.7 Activity

More About Errors Alignment Error – Frame does not end on an octet boundary. Collision or bad drivers. Range Error – Number of octets in the data field does not match the number specified in the length field. FCS Error – At least one bit of transmission has changed. Ghosting Error – Noise on the line. Ground loops and wiring problems.

Ethernet auto-negotiation This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. Normal Link Pulse – A pulse sent about every 16 milliseconds, whenever a station is not engaged in transmitting a message. Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners Called FLP (Fast link pulse).

Ethernet auto-negotiation The burst communicates the capabilities of the transmitting station to its link partner. After both stations have interpreted what the other partner is offering, both switch to the highest performance common configuration and establish a link at that speed. 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 Auto-Negotiation is optional for most Ethernet implementations. Speed and duplex setting may be forced by the administrator. In the event that link partners are capable of sharing more than one common technology, refer to the list at the right for order of preference.

Bonus Stuff

Quiz Question 3 Which of the following is an example of non-deterministic LAN technology? Ethernet FDDI IEEE 802.5 Token Ring By extension: The other three ARE deterministic technologies.

General Info When Cisco prints a number like 0X600 or 0X2102 Just ignore the x. It signifies a hexadecimal value instead of a decimal value.

Layer Two Components

Common Topologies star, of course

The End