Protocol layering and data Each layer takes data from above adds header information to create new data unit passes new data unit to layer below source destination application transport network link physical application transport network link physical M H t n l message M H t n l segment datagram frame

Protocol Headers Pre DA SA 0800h … version H L 6 TCP Header Data FCS Ether Type 0800h … version H L Protocol 6 TCP Header Data FCS Data Link Header IP Header Trailer 1: Internet Control Message Protocol (ICMP) 2: Internet Group Management Protocol (IGMP) 6: Transmission Control Protocol (TCP) 17: User Datagram Protocol (UDP) 89: Open Shortest Path First (OSPF) 0x0800 Internet Protocol, Version 4 (IPv4) 0x0806 Address Resolution Protocol (ARP) 0x8100 IEEE 802.1Q-tagged frame 0x86DD Internet Protocol, Version 6 (IPv6) 0x8847 MPLS unicast 0x8848 MPLS multicast

Ethernet “dominant” LAN technology: First widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch

Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! has become a legacy technology

10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub hub nodes

Ethernet hubs and switches By definition of the term, Ethernet hubs: Operate solely at Ethernet Layer 1 Repeat (regenerate) electrical signals to improve cabling distances Forward signals received on a port out all other ports (no buffering) Switches have the same cabling and signal regeneration benefits as hubs, but switches do a lot more—including sometimes reducing or even eliminating collisions by buffering frames When switches receive multiple frames on different switch ports, they store the frames in memory buffers to prevent collisions

Collision Domains A collision domain is a set of devices that can send frames that collide with frames sent by another device in that same set of devices Before the advent of LAN switches, Ethernets were either physically shared (10BASE2 and 10BASE5) or shared by virtue of shared hubs and their Layer 1 “repeat out all other ports” logic Ethernet switches greatly reduce the number of possible collisions, both through frame buffering and through their more complete Layer 2 logic

Ethernet hubs and switches

Collision detection

CSMA/CD The original Ethernet specifications expected collisions to occur on the LAN The media is shared Any electrical signal induced onto the wire could collide with a signal induced by another device When two or more Ethernet frames overlap on the transmission medium at the same instant in time, a collision occurs The collision results in bit errors and lost frames The original Ethernet specifications defined the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) algorithm to deal with the inevitable collisions CSMA/CD minimizes the number of collisions

CSMA/CD A device with a frame to send listens until the Ethernet is not busy (in other words, the device cannot sense a carrier signal on the Ethernet segment). When the Ethernet is not busy, the sender begins sending the frame. The sender listens to make sure that no collision occurred. If there was a collision, all stations that sent a frame send a jamming signal to ensure that all stations recognize the collision. After the jamming is complete, each sender of one of the original collided frames randomizes a timer and waits that long before resending. (Other stations that did not create the collision do not have to wait to send.) After all timers expire, the original senders can begin again with Step 1

Ethernet : Framing and Addressing In many documents, the word frame refers to the bits and bytes that include the Layer 2 header and trailer, along with the data encapsulated by that header and trailer The term packet is most often used to describe the Layer 3 header and data, without a Layer 2 header or trailer Ethernet’s Layer 2 specifications relate to the creation, forwarding, reception, and interpretation of Ethernet frames The original Ethernet specifications were owned by the combination of Digital Equipment Corp., Intel, and Xerox—hence the name “Ethernet (DIX)”

Ethernet : Framing and Addressing Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as hardware addressing or MAC addressing Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network There are four different types of Ethernet frames available: Ethernet_II IEEE 802.3 IEEE 802.2 SNAP

Ethernet : Framing and Addressing

Ethernet : Framing and Addressing Following are the details of the different fields in the 802.3 and Ethernet frame types: Preamble An alternating 1,0 pattern provides a 5MHz clock at the start of each packet, which allows the receiving devices to lock the incoming bit stream. Start Frame Delimiter (SFD)/Synch The preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011.

Ethernet : Framing and Addressing Destination Address (DA) This transmits a 48-bit value using the least significant bit (LSB) first. The DA is used by receiving stations to determine whether an incoming packet is addressed to a particular node. DA may be unicast, broadcast or multicast. Source Address (SA) The SA is a 48-bit MAC address used to identify the transmitting device, and it uses the LSB first. Broadcast and multicast address formats are illegal within the SA field. Length or Type 802.3 uses a Length field, but the Ethernet frame uses a Type field to identify the Network layer protocol. 802.3 cannot identify the upper-layer protocol.

Ethernet : Framing and Addressing Data This is a packet sent down to the Data Link layer from the Network layer. The size can vary from 64 to 1500 bytes. Frame Check Sequence (FCS) FCS is a field at the end of the frame that’s used to store the CRC.

Ethernet type II