CCNA 1 Chapter 5 Ethernet Fundamentals By Your

Objectives • Ethernet fundamentals • Ethernet operation Copyright 2003 www. ciscopress. com

Introduction to Ethernet • The success of Ethernet is due to its simplicity and ease of maintenance, as well as its ability to incorporate new technologies, reliability, and low cost of installation and upgrade. Copyright 2003 www. ciscopress. com

Comparing LAN Standards Copyright 2003 www. ciscopress. com

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 Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, Frame Relay, ATM, PPP, HDLC, and so on Copyright 2003 • Sometimes called: – Access methods – Access protocols – Access technologies – Media access – LAN protocols – WAN protocols www. ciscopress. com

IEEE Standard • Divided OSI Layer 2 into two sublayers – Media Access Control (MAC) – Traditional L 2 features • Transitions down to media – Logical link control (LLC) – New L 2 features • Transitions up to the network layer Copyright 2003 www. ciscopress. com

Logical Link Control (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 Copyright 2003 www. ciscopress. com

Logical Link Control (LLC) • Participates in the data encapsulation process – LLC PDU between Layer 3 and 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 connectionoriented upper-layer protocols. • Allows multiple higher-layer protocols to share a single physical data link. Copyright 2003 www. ciscopress. com

Naming • Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits. • The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the organizational unique identifier (OUI). • The remaining 6 hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer. Copyright 2003 www. ciscopress. com

Layer 2 Framing • Framing is the Layer 2 encapsulation process; a frame is the Layer 2 protocol data unit. • A single generic frame has sections called fields, and each field is composed of bytes. Copyright 2003 www. ciscopress. com

Framing • Why framing is necessary • Frame format diagram • Generic frame format

Why Framing Is Necessary • Binary data is a stream of 1 s and 0 s. • Framing breaks the stream into decipherable groupings: – – Start and stop indicator fields Naming or addressing fields Data fields Quality-control fields • Framing is the Layer 2 encapsulation process. • A frame is the Layer 2 protocol data unit. Copyright 2003 www. ciscopress. com

Generic Frame Format • • • Start Frame field Address fields (source and destination MAC) Type/Length field Data field FCS (Frame Check Sequence) field Frame Stop field Copyright 2003 www. ciscopress. com

Ethernet Frame Fields • Some of the fields permitted or required in an 802. 3 Ethernet frame are as follows: – – – – Preamble Start Frame Delimiter Destination Address Source Address Length/Type Data and Pad Frame Check Sequence (FCS) Extension Copyright 2003 www. ciscopress. com

Ethernet Operation Copyright 2003 www. ciscopress. com

Media Access Control (MAC) • Provides MAC addressing (naming) • Depending on access technology (Ethernet, Token Ring, FDDI), provides the following: – – Data transmission control Collision resolution (retransmission) Layer 2 frame preparation (data framing) Frame check sequence (FCS) Copyright 2003 www. ciscopress. com

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 Copyright 2003 www. ciscopress. com

Ethernet (CSMA/CD) Carrier sense multiple access with collision detection Copyright 2003 www. ciscopress. com

Ethernet Timing • Any station on an Ethernet network wanting to transmit a message first “listens” to ensure that no other station is currently transmitting. • If the cable is quiet, the station begins transmitting immediately. Copyright 2003 www. ciscopress. com

Interframe Spacing and Backoff Copyright 2003 www. ciscopress. com

Error Handling • Collisions are the mechanism for resolving contention for network access. • Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This affects all network nodes, possibly causing significant reduction in network throughput. Copyright 2003 www. ciscopress. com

Types of Collisions • Three types of collisions are: – Local – Remote –

Ethernet Errors • The following are the sources of Ethernet error: – Simultaneous transmission occurring before slot time has elapsed (collision or runt) – Simultaneous transmission occurring after slot time has elapsed (late collision) – Excessively or illegally long transmission (jabber, long frame and range errors) – Illegally short transmission (short frame, collision fragment or runt) – Corrupted transmission (FCS error) – Insufficient or excessive number of bits transmitted (alignment error) – Actual and reported number of octets in frame don't match (range error) – Unusually long preamble or jam event (ghost or jabber) Copyright 2003 www. ciscopress. com

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. Copyright 2003 www. ciscopress. com

Ethernet Autonegotiation • A process called autonegotiation (of speeds at half or full duplex) was developed. • 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. Copyright 2003 www. ciscopress. com

Link Establishment • There are only two ways to achieve a full-duplex link: – Through a completed cycle of autonegotiation – Or, by administratively forcing both link partners to full duplex Copyright 2003 www. ciscopress. com