1Ethernet Lecture # 12 Computer Communication & Networks

Ethernet History ↗Ethernet is the most widespread LAN technology ↗Ethernet was developed at Xerox (1973) inspired by ALOHAnet which Robert Metcalfe had studied as part of his PhD dissertation ↗Standardized by Xerox/DEC/Intel in 1978 ↗Ethernet was officially accepted as IEEE standard in 1985 ↗The original Xerox Ethernet operated at 3Mbps and used thick coax cable ↗Ethernet networks up to 10Gbps now exist (switched Ethernet) 2Ethernet

Ethernet & IEEE What the IEEE standard covers- Physical layer and interface to the link layer. IEEE is the Link layer standard. Differences between Ethernet and There are some electrical and connector differences; most equipment uses IEEE There is difference in the header. DIX uses TYPE, uses LENGTH. SInce the frame is limited in size, the two coexist. 3-Most people use the DIX format. 3Ethernet

IEEE & OSI PHY MAC LLC 2- Data link 1- physical LLC = Logical Link Control MAC = Media Access Control PHY = Physical 4Ethernet

Why Ethernet became so popular ↗Easy to understand, implement, manage, and maintain ↗Low-cost network implementations ↗Topological flexibility for network installation ↗Successful interconnection and operation of products, regardless of manufacturer 5Ethernet

Frame Format Ethernet and IEEE Frame Format 6Ethernet

↗Preamble:Starts with 7 bytes of to get timing synchronized ↗SFD: indicate the frame is starting ( ), 1- byte ↗PDU or Frames: headers (addresses + length/type) ↗802.3 standard substitutes length with type field ↗The Length/EtherType: In it indicates the number of bytes of data in the frame from 0 to 1500 bytes. Frames must be at least 64 bytes long, not including the preamble, so, if the data field is shorter than 46 bytes, it must be compensated by the Pad field (padding bytes). ↗Type field is the first thing in data portion ↗Data ( ), add bits if necessary to get to 46 bytes. ↗CRC-32 FCS for error checking Preamble SFD Destination Address Source Address Length Data + Pad FCS Frame Format (Ethernet & 802.3) Type 7Ethernet

Ethernet Addresses ↗MAC address, short for Media Access Control address, a hardware address that uniquely identifies each node on a network ↗When a manufacturer creates a network capable piece of hardware they will assign the MAC address which will usually begin with a code that is tied to the manufacturer ↗It will be unique to every device, even two devices of the same type. ↗A device’s MAC address is composed of six pairs of hexadecimal numbers ↗The numbers are separated by colons as in the following example: ↗6E:51:F5:C1:11:00 ↗Once assigned, the MAC address is used to ensure that each device connected to the LAN is unique 8Ethernet

Ethernet Address ↗The first three octets identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI) ↗The following three octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness e.g. in the address 00:A0:C9:14:C8:29, the prefix 00A0C9 indicates the manufacturer is Intel Corporation 9Ethernet

Ethernet Addresses Types ↗Unicast; unique, 48-bit address assigned to each adapter ↗Examples: ↗01:23:45:67:89:AB ↗08:00:07:A9:B2:FC ↗Broadcast: all 1s ↗Multicast: first bit is 1 10Ethernet

Normal Ethernet Operation A B D Data C Address mismatch packet discarded Address mismatch packet discarded Address match packet processed Send data to node D Transmitted packet seen by all stations on the LAN (broadcast medium) 11Ethernet

Ethernet Collisions A B C D Collision Data transmission for A Data transmission for C 12Ethernet

Two stations (2 and 4) communicating using a single bus How Ethernet Works 13Ethernet

How Ethernet Works Shared Media Problem ↗Problem: demands can conflict, e.g., two hosts send simultaneously ↗Solution is a medium access control (MAC) algorithm Need method to moderate access ↗Fair arbitration ↗Good performance 14Ethernet

Need for an Access Method ↗Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other ↗Such overlaps which turn signals to noise are called Collisions ↗As traffic increases on multiple-access link, so do collisions ↗Such a network therefore needs a mechanism to coordinate traffic, minimize the number of collisions and maximizes the number of frames that are delivered successfully ↗The access mechanism used in Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 15Ethernet

Ethernet MAC – CSMA/CD (Carrier Sense Multiple Access/Collision Detection) ↗Multiple access ↗Nodes send and receive frames over a shared link ↗Carrier sense ↗Nodes can distinguish between an idle and busy link ↗Collision detection ↗A node listens as it transmits to detect collision 16Ethernet

Ethernet MAC – CSMA/CD MAC Algorithm ↗If line is idle (no carrier sensed) ↗Send immediately ↗Upper bound message size of ~1500 bytes ↗Must wait 9.6µs between back-to-back frames 17Ethernet

Ethernet MAC – CSMA/CD MAC Algorithm ↗If line is busy (carrier sensed) ↗Wait until the line becomes idle and then transmit immediately ↗Called 1-persistent (special case of p-persistent) ↗If collision detected ↗Stop sending data and jam signal ↗Try again later 18Ethernet

19 Ethernet MAC (CSMA/CD) Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yes attempts < 16 attempts == 16 Carrier Sense Multiple Access/Collision Detection

Ethernet20 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: If deterministic delay after collision, collision will occur again in lockstep If random delay with fixed mean – Few senders  needless waiting – Too many senders  too many collisions Goal: adapt retransmission attempts to estimated current load – heavy load: random wait will be longer

Ethernet Switched Ethernet ↗No need Of MAC ↗No collision because of the micro level segmentation ↗Switches performs the switching between hosts ↗Today’s Ethernet LANs are built on switch topologies 21Ethernet

Ethernet Flavors 10Base510BaseT10baseFL100BaseTX100BaseFX MediaThick coax RG-8, RG-11 TIA/EIA UTP Cat 3,4,5,5e,6 (2 pair) 62.2/125 m Multi-mode Fiber TIA/EIA UTP Cat 5 and up (2 pair) 62.2/125 m Multi-mode Fiber Or Single mode fiber Speed10 Mbps 100 Mbps Max Seg. Length 500 Meters100 Meters2000 Meters100 Meters400/2000 Meters (full/half duplex), 10 Km 22Ethernet