1. Copyright 2003 www.ciscopress.com Objectives: Chapter 3: Layer 2 Ethernet  Ethernet Fundamentals Ethernet Operations Carrier Sense Multiple Access/Collision Detection Switches and switching methods

2. 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.

3. Copyright 2003 www.ciscopress.com Comparing LAN Standards

4. 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 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, HDLC, and so on

5. Copyright 2003 www.ciscopress.com 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

6. 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

7. 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 connection- oriented upper-layer protocols. Allows multiple higher-layer protocols to share a single physical data link.

8. 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.

9. Copyright 2003 www.ciscopress.com Framing Why framing is necessary Frame format diagram Generic frame format

10. Copyright 2003 www.ciscopress.com 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 Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit.

11. 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

12. 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

13. Copyright 2003 www.ciscopress.com Ethernet Operation

14. 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)

15. Copyright 2003 www.ciscopress.com Naming (MAC address) 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.

16. 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

17. Copyright 2003 www.ciscopress.com 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)

18. Copyright 2003 www.ciscopress.com FCS and Beyond A received frame that has a bad frame check sequence, also referred to as a checksum Redundancy Check (CRC) error, differs from the original transmission by at least 1 bit.

19. 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.

20. Copyright 2003 www.ciscopress.com CSMA/CD Ethernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritization of the nodes on the network. CSMA/CD Ethernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritization of the nodes on the network. Two nodes transmit at the same time 1 Nodes detect there has been a collision 2 Nodes transmit a jamming signal 3 Nodes wait a random period before retransmitting 4

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

22. Rick Graziani graziani@cabrillo.edu Switch Memory buffering switch 1111 2222 3333 4444 Abbreviated MAC addresses An Ethernet switch may use a buffering technique to store and forward frames. Buffering may also be used when the destination port is busy. The area of memory where the switch stores the data is called the memory buffer. This memory buffer can use two methods for forwarding frame: –port-based memory buffering –shared memory buffering In port-based memory buffering frames are stored in queues that are linked to specific incoming ports. Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share.

23. Rick Graziani graziani@cabrillo.edu Two switching methods Store-and-forward – The entire frame is received before any forwarding takes place. –The destination and source addresses are read and filters are applied before the frame is forwarded. –CRC Check done Cut-through – The frame is forwarded through the switch before the entire frame is received. –This mode decreases the latency of the transmission, but also reduces error detection.

24. Rick Graziani graziani@cabrillo.edu Cut-through Fast-forward – Offers the lowest level of latency. –Fast-forward switching immediately forwards a packet after reading the destination address. –There may be times when packets are relayed with errors. –Although this occurs infrequently and the destination network adapter will discard the faulty packet upon receipt.

25. Rick Graziani graziani@cabrillo.edu Cut-through Fragment-free – Fragment-free switching filters out collision fragments before forwarding begins. –Collision fragments are the majority of packet errors. –In a properly functioning network, collision fragments must be smaller than 64 bytes. –Anything greater than 64 bytes is a valid packet and is usually received without error. –Fragment-free switching waits until the packet is determined not to be a collision fragment before forwarding.

26. Rick Graziani graziani@cabrillo.edu Two switching methods Adaptive cut-through –In this mode, the switch uses cut-through until it detects a given number of errors. –Once the error threshold is reached, the switch changes to store-and-forward mode.

27. Rick Graziani graziani@cabrillo.edu Switches and broadcast domains These are logical not physical representations of what happens to these frames. Switches flood frames that are: –Unknown unicasts –Layer 2 broadcasts –Multicasts (unless running multicast snooping or IGMP) Multicast are special layer 2 and layer 3 addresses that are sent to devices that belong to that “group”.

28. Rick Graziani graziani@cabrillo.edu Symmetric and asymmetric switching Note: Most switches are now 10/100, which allow you to use them symmetrically or asymmetrically.