Chapter 9 Notes

Chapter 9 Notes – Ethernet  Overview ➢ IEEE Standards  Ethernet operates in DLL and Physical Layer  Ethernet is defined by DLL and Physical Layer protocols ➢ Ethernet Layer 1  Involves signals, bit streams that travel on media  Limitations: • Cannot communicate with upper layers • Cannot identify devices • Only recognizes streams of bits • Cannot determine the source of a transmission when multiple devices are transmitting ➢ Ethernet Layer 2  Functions • Connects to upper layers via Logical Link Control (LLC) • Uses addressing schemes to identify devices • Uses frames to organize bits into groups • Uses Media Access Control (MAC) to identify transmission sources ➢ Logical Link Control  802.2 describes LLC sublayer functions  802.3 describes MAC sublayer and Physical layer functions  Handles comm. Between upper layers and networking software & lower layers (hardware)  Frames the Network layer packet  Identifies the Network layer protocol  Remains relatively independent of the physical equipment ➢ Media Access Control  Two primary responsibilities: • Data Encapsulation • Media Access Control  Data Encapsulation provides: • Frame delimiting • Addressing • Error detection • • •

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Process – frame assembly before & frame parsing upon reception MAC layer adds header & trailer to Layer 3 PDU Encapsulation provides synch. Between the transmitting & receiving nodes • Provides DLL addressing • Error detection ♦ Cyclic redundancy check (CRC) of frame contents (trailer) Controls placement of frames on media and removal from media Initiation of frame trans. & recovery from trans. failure due to collisions Logical topology – multi-access bus

➢ Physical Implementations of Ethernet  Success of Ethernet is due to: • Simplicity and ease of maintenance • Ability to incorporate new technologies • Reliability • Low cost of installation and upgrade  Communication through the LAN ➢ Historic Ethernet  Ethernet’s shared media and collision detection techniques were adapted from the Alohanet radio network  Early Ethernet – shared bus topology  Bus topology changed to Star topology ➢ Collision Management  Legacy Ethernet – hub-based • Half-duplex comm.  Current Ethernet – switch-based • Full-duplex comm. ➢ Moving to 1Gbps & beyond  New networking services require high-bandwidth LANs  Gigabit Ethernet technology is applied beyond the enterprise LAN to MAN and WAN-based networks  The Ethernet Frame ➢ Encapsulating the Packet  Header & trailer have several sections of info. used by Ethernet protocol • Each section – field • 2 styles of Ethernet framing: ♦ IEEE 802.3 (original) ♦ IEEE 802.3 (Ethernet) ➢ Addition of a Start Frame Delimiter (SFD) ➢ Change to the Type field to include Length  Frame Size • Original ♦ Minimum 64 bytes & Maximum 1518 bytes • IEEE 802.3ac ♦ Maximum extended -> 1522 ♦ Changed to accommodate Virtual Local Area Network (VLAN)  Part Size • Preamble (7 bytes) & Start of Frame Delimiter(SFD) (1 byte) ♦ Used to synch. Between sending & receiving devices • Destination MAC Address (6 bytes) ♦ Indentifier for intended recipient ♦ Used by Layer 2 • Source MAC Address (6 bytes) ♦ ID’s frame’s originating NIC / interface • Length/Type (2 bytes)

♦ Defines exact length of frame’s data field ♦ Later used as part of FCS to insure message received properly • Header & Data (46-1500) ♦ Encapsulated packet plus padding if required • Frame Check Sequence (4 bytes) ♦ CRC Checksum ♦ If FCS calculated by receiver does not equal FCS calculated by source, frame is considered invalid & dropped ➢ Ethernet MAC Address  Created to assist in determining source & dest. address within an Eth. Network  All eth. Nodes share media  To receive & send data, each node needs unique address  Structure: • Determined by IEEE • IEEE determines 3-byte code called Organizationally Unique Identifier (OUI) • IEEE requires vendors to follow 2 simple rules: ♦ All MAC addresses assigned to a NIC / other Eth. Device must use that vendor’s assigned OUT as the first 3 bytes ♦ All MAC addresses with the same OUI must be assigned a unique value (vendor code / serial number) in the last 3 bytes • MAC addresses often ref. as burned-in address (BIA) – can’t be changed by software ➢ Hexadecimal Numbering and Addressing  Represent binary values  Decimal 0-9 = hexadecimal 0-9  Decimal 10-15 = hexadecimal A-F ➢ Another Layer of Addressing  DLL (Layer 2) • Ethernet MAC address – local media only • Enables packet to be carried by local media across each segment  Network Layer (Layer 3) • IPv4 addresses • Enables packet to be forwarded toward its destination ➢ Ethernet Unicast, Multicast & Broadcast  Unicast • Unicast MAC address is the unique address used when a frame is sent from a single trans. dev. To a single dest. dev.  Broadcast • Broadcast IP & broadcast MAC dest. addresses are used by the source to forward a packet to all hosts on a network  Multicast • Multicast IP & MAC destination addresses deliver packet/frame to specific group of member hosts  Ethernet Media Access Control  Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

CSMA/CD controls access to the shared media, if there is a collision, it is detected and frames are retransmitted ➢ CSMA/CD  Carrier sense • Listen before transmitting – monitor media for traffic • If dev. Detects a signal from another dev., it will wait for a specified amount of time before attempting to transmit  Multi-access • If one’s signals dev. Is not detected by anothers, the second device may transmit too • With 2 dev. Transmitting, their signals will go until they encounter each other • When they meet, their signals mix and the message is destroyed  Collision Detection • In listening mode, it can detect when a collision occurs on shared media  Jam Signal & Random Backoff • Once collision detected – jamming signal sent • Used to notify other devices of collision – they will invoke a backoff algorithm – causes all devices to stop transmitting for random amount of time  Hubs & Collision Domains • Hubs Only work at physical layer • Collisions can occur between devices they connect & within hubs themselves • Connected devices that access common media via hub makeup a collision domain (networking segment) ➢ Ethernet Timing  Latency (Ethernet delay) • An Ethernet frame takes a measurable time to travel from the sending device to the receiver. Each intermediary device contributes to the overall latency  Timing and Synchronization • Half-duplex mode ♦ No collision – sending device will transmit 64 bits of timing synch. info (preamble) • Ethernet with throughput speeds of 10mbps & slower – asynchronous ♦ Each receiving device will use the 8 bytes of timing information to synchronize the receive circuit to the incoming data & discard 8 bytes • Ethernet with throughput of 100mbps & greater – synchronous ♦ Timing info is not required ♦ Preamble & SFD fields still present  Bit Time – period of time required for a bit to be placed and sensed on media  Slot Time – time taken for an electronic pulse to travel the length of the maximum theoretical dist. Between 2 nodes 

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9.4.4.1