What is a LAN? Ethernet A Local Area Network is.... ES/EG 4546 Simple low cost Local Area Network (LAN) The Ethernet Shared or Switched Access local (one building, group of buildings, etc) Local Area Network Supports : always controlled by one administrative authority Unicast (1 to 1) Broadcast (1 to all) usually high speed and is always shared Multicast (1 to some) G. Fairhurst often assumes other users of the LAN are trusted Divided into Two Layers Link Layer - Medium Access Control (protocol) either planned (structured ) or unstructured Physical Layer - Transmission Control (cabling) r48 2019 What is Ethernet? What is Ethernet? IEEE 802.X Protocol Suite First LAN designed at Xerox "PARC" (1972) First LAN designed at Xerox "PARC" in 1972 2 Mbps 75 Ohm Coaxial cable To share expensive laser printers Ethernet v2 followed from Digital, Intel, Xerox (DIX) in 1980 File sharing followed later IEEE 802 Committees Standardised by IEEE in 1985: IEEE 802.3 Printer PC PC PC Two variants: Thick Ethernet and Thin Ethernet 802.1 802.2 802.3 802.4 802.5 802.6 802.7 802.8 Arch. Logical CSMA/ Token Token DQDB Broad- Fibre Link CD Bus Ring MAN band Various speeds now available: Control LAN 10 Mbps (original IEEE spec) 100 Mbps (Fast Ethernet) 1000 Mbps (1 Gbps) Ethernetv2 - Blue Book (1980) 10000 Mbps (10 Gbps) Digital, Intel, Xerox (DIX) 40 Gbps, 100 Gbps, … 10 Mbps 50 Ohm Coaxial cable Ethernet 10B5 Cabling 10B5 (Thick Ethernet) Ethernet Bus Network medium (cable) Terminator Copper ! Yellow PVC Outer Coating (0.5") Conductor Ethernet trunk cable Wiring cabinet Repeater Bus Topology Bridge or Router Dielectric Insulation Braided Outer Conductor 50 Ohm Transceiver (Attachment Unit) Network Interface High performance co-axial cable AUI drop cable to each room Segment length ≤ 500m Attachment Unit 50 Ohm Good noise immunity Interface Cable terminator N-Type connector used (one end earthed) Host Computer In-Line or Vampire external transceiver (station)
10B5 (Thick Ethernet Transceiver) 10B5 (Thick Ethernet Vampire Transceiver) 10B2 (Thin Ethernet) 50 ohm ! White, Grey or Black PVC Outer Coating N-type Connectors Vampire Cable Tap terminator Copper Bolt to Tighten Block Conductor Insulated Transceiver Block 2-Part Block Spike Pierces Holds Cable Centre Core Vampire In-Line Transceiver Transceiver Dielectric Braided Outer Insulation Conductor 50 Ohm 15 pin AUI Attachment Unit Interface MAU D-Connector (AUI) Drop Cable 0 - 50 m Low cost co-axial cable Segment length ≤ 185 m Thick Wire Shorter Spikes (Yellow) Cable Flexible, easy installation Host AUI Port Cut into Outer Host AUI Port Ethernet Cable Conductor BNC connector and “T” joiner In-built or external transceiver difficult to manage (unstructured) Ethernet 10B2 Cabling 10 Base Fibre 10 Base Fibre Terminator AUI connector Wiring cabinet Used for pt-to-pt links on equipment Segment length ≤ 1 km (or more) Two diameters of fibre: Repeater Fibre Optic Cable 10BF Multimode (Thicker, Local networks) Bridge transceiver Single Mode (Thin, Longer distance) or Router All types of fibre provide: Terminator High noise immunity LED laser transmitter No electrical path Photo diode detector (protected from lightning) Pair of fibres (secure, hard to tap-into) (62.5/125 ) Uses external transceiver (i.e. connects a pair of repeaters) One bus: Easy to upgrade transceiver speed No loops or stubs! Ethernet Success Story MAC Medium Access Control Medium Access Control Frames - Data is sent on an Ethernet Network in Frames Frames Addressing Addressing Cost-Effective - All End Systems have an Ethernet MAC Address (In their “PROM”) - Each frame sent with this source address Simple to use Shared Access - Frames also carry a Destination Address, used in three ways: Familiarity to customers Broadcast, Unicast, and Multicast Standard for Internet LANs Link Layer Shared Access - Sharing network cost - Sharing the reachability 14 bytes 46 -1500 bytes 4 bytes 14 bytes 46 -1500 bytes 4 bytes 8 bytes 8 bytes destination source destination source preamble address type preamble address type packet of data to be sent CRC packet of data to be sent CRC address address
Ethernet MAC Address MAC Vendor Codes (OUIs) Ethernet Frames 08:00:20:00:00:01 08:00:20:00:00:01 Protocol Data Unit, PDU (Internet Packets) 46 -1500 bytes The first 3B of address tells you the manufacturer Each Network Interface Card (NIC) has a MAC Address packet of data to be sent 080002 3Com (Formerly Bridge) Held in a manufacturer-configured PROM 080003 ACC (Advanced Computer Communications) 080005 Symbolics Symbolics LISP machines Encapsulated by Ethernet Frames to cross the LAN 080008 BBN Addresses are globally unique 080009 Hewlett-Packard Adds 26 bytes of overhead to each PDU 08000A Nestar Systems A MAC Vendor Code (OUI) + Number 08000B Unisys 14 bytes 46 -1500 bytes 4 bytes 080011 Tektronix, Inc. 8 bytes 080014 Excelan BBN Butterfly, Masscomp, Silicon Graphics About 1% of OUIs have been used. source destination address type 080017 NSC preamble packet of data to be sent CRC address 08001A Data General IEEE sells the blocks of addresses to manufacturers 08001B Data General 08001E Apollo 080020 Sun Sun machines e.g. Each block has 256 cubed addresses 080022 NBI a 46 byte packet is carried in a 72 byte frame 080025 CDC 080026 Norsk Data (Nord) That is 16 Million!! a 1200 byte PDU is carried in a 1226 byte frame. Shared Access to Ethernet Medium Using the Destination MAC address Ethernet Frame Structure 14 bytes 46 -1500 bytes 4 bytes 8 bytes All stations receive Source: A the frame, but discard Destination: B source destination address type preamble packet of data to be sent CRC the frame if the destination address address does not match the local address Sender Intended Recipient LAN address of intended recipient first bit = 0 indicates point to point first bit = 1 indicates broadcast or multicast C A D B 48 bits, expressed as 12 hexadecimal digits Shared medium delivers all frames to all computers e.g., 12:34:56:78:9A:BC A theoretical 200,000,000,000 addresses Actually 70,000,000,000... (2 bits are used) A sends a frame to B The destination station 20,000 MAC addresses for each person on the planet! which is broadcast to receives the frame and all stations forwards it to host B Each computer discards frames intended for other computers This assume that a sender knows the value of the MAC address in the destination’s PROM (we’ll find out how it does this later!) Group MAC Addresses Special MAC Addresses Multicast on Ethernet Server FF:FF:FF:FF:FF:FF 01:00:5E:00:00:FF The all 1’s Address is used to send to all NICs 1 Receiver Groups addresses Known as the broadcast destination address Have the least significant bit of the first byte to 1 Only ever used as destination address The remainder of the address carries the specific group address Client Server (destination address matches) Group addresses identify “channels” not Receivers 00:00:00:00:00:00 Sender chooses a group address to use 3 Multicast e.g. one channel may carry a specific Internet TV station The all 0’s Address is special Receivers Known as the unknown address Client NICs need to “register” to receive from a group Client Client Not (registered) (registered) (registered) Registered Only ever used as source address A computer may “register” several group addresses The NIC passes all frames with group addresses that match TV/Radio/etc Transmission (several receivers)
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