Introduction to Ethernet and lab3.3 Magnus Lundberg maglun@sm.luth.se Nov 2001 Computation structures 1 Agenda   Introduction   History   Components (MAC, PHY)   Packet format   CRC   Bit order   Hubs, Switches and Routers   Questions Magnus Lundberg LTU Computation structures 2 What is (the) Ethernet   Ethernet is a network BUS   Ethernet is packet based   Ethernet is available as 10 Mbit/s - 10 Gbit/s   Ethernet is based on the CSMA/CD (Carrier Sense Multiple Access with Collision Detect access) protocol Magnus Lundberg Computation structures LTU 3 Ethernet Basics   Ethernet Packets are from 64 bytes up to 1518 bytes   Since Ethernet is a “BUS” several “Attached Units” can talk on the BUS at the same time   Collisions occur and the talking parties backoff for a time and then try again   For each failed attempt the backoff time is increased exponentially Magnus Lundberg LTU Computation structures 4 OSI model OSI Model Application L7 Presentation Session Transport Network Datalink Physical L1   On The OSI model with 7 layers , Ethernet is in the bottom two layers: Data Link Layer and Physical Layer   OSI (Open Systems Interconnection) Magnus Lundberg Computation structures LTU 5 History of Ethernet   Developed by Xerox at their PARC Research Facility by Bob Metcalfe and David Boggs   First version was 3 Mbit /s   Initial Ethernet standard developed by Digital , Intel and Xerox in the DIX consortium   Standard for 10 Mbit/s called “The Blue book” was ready in 1980   Standard was then submitted to a newly formed working group in IEEE called 802   IEEE splits the 802 working group in four parts 802.3 becomes the Ethernet working group Magnus Lundberg LTU Computation structures 6 History of Ethernet   Late 1982 DIX’s Bluebook and the IEEE 802.3 specifications merge into one standard   Ethernet was already multi vendor and more company’s endorsed the standard   Ethernet vs 802.3 CSMA/CD   Ethernet patents where inexpensive and easy to license   Ethernet was developed into several different mediums. Coaxial/UTP/Optical Magnus Lundberg LTU Computation structures 7 Components PHY MAC FPGA MII MAU MDI ✁ MAC Media Access Control ✁ PHY Physical layer device ✁ MAU Medium Attachment Unit (RJ45 connector) ✁ MII Media Independant Interface ✁ MDI Media Dependant Interface Magnus Lundberg LTU Computation structures 8 MAC - Media Access Controller   Does statistical functions : Packet Lengths, Packet collisions etc.   Does the stripping and adding of the preamble bits and the CRC calculations   Communicates with the rest of the design   Back off mechanism with random delay in case of collision   A MAC usually has two different datapaths one for internal clock (Sending) and one external (Receiving) Magnus Lundberg LTU Computation structures 9 Media Independent Interface - MII   This is the interface between the PHY and the MAC   Regardless of the underlying Media this interface always stays the same   MII is for 10 and 100 Mbit Ethernet. First bit from MAC First Nibble D0 MSB D1 D2 Second Nibble D3 D4 D5 D6 D7 D0 D1 D2 LSB D3 Magnus Lundberg LTU Computation structures 10 Media Independent Interface - MII   The MII consists of 10 different signals   Transmit Clock (TX_CLK) which runs at either 2.5 Mhz for 10 Mbit or 25 Mhz for 100 Mbit   Transmit Data (TXD ) data nibble to the PHY circuit from MAC   Transmit Enable (TX_EN) is high when data on the TXD pins are valid   Transmit Error (TX_ERR) goes high when the PHY is coding incorrectly, not used in 10 Mbit Magnus Lundberg Computation structures LTU 11 Media Independent Interface - MII   Receive Clock (RX_CLK) generated by the PHY for incoming data   Receive Data (RXD ) data from the sender decoded by the PHY to MAC   Receive Data Valid (RX_DV) asserted when there is valid data on the RXD path   Receive Error (RX_ER) asserted to indicate that something in the coding layer is wrong etc   Carrier Sense (CRS) asserted by the PHY to indicate someone else is sending data   Collision (COL) asserted by the PHY when a collision has taken/is taking place, useless if working in full duplex mode Magnus Lundberg LTU Computation structures 12 MII - Sending Packets TX_CLK TX_EN TXD COL CRS Magnus Lundberg Computation structures LTU 13 MII - Receiving Packets RX_CLK RX_DV RXD COL CRS Magnus Lundberg LTU Computation structures 14 PHY   Encodes/Decodes (Manchester coding for 10 Mbit/s)   Converts from parallel to serial   Detects collisions   Auto-negotiation Magnus Lundberg LTU Computation structures 15 Ethernet Packet Format Preamble 8 Bytes Magnus Lundberg Destination Addr. 6 Bytes Source Addr. 6 Bytes Etype/Length 2 Bytes Data 46−1500 Bytes FCS 4 Bytes LTU Computation structures 16 Preamble   Preamble is used by the PHY to “sync” its RX_CLK to the transmitters TX_CLK.   The Preamble consists of 8 bytes of alternating ones and zeros   The two last bits of the preamble field are both high and are called the synch sequence, it is used to signal that the preamble ends and the Destination address begins. Magnus Lundberg Computation structures LTU 17 Destination and Source address   Both Addresses are 6 bytes each and consists of two parts: – 3 Bytes Vendor code (0x00003F = Syntrex Inc) – 3 Bytes Serial number (0x0001000)   This address are sometimes callad MAC-address   Stored in ROM in a NIC   0xFFFFFFFFFFFF is the broadcast address. Magnus Lundberg LTU Computation structures 18 Type/Length   Length / Etype field is either a type field or length.   if the value is smaller than 0x0600 it contains length, otherwise it is an Ethernet Type – 0x0800 is IPv4 Datagram – 0x0806 is ARP – 0x81DD is IPv6 Datagram – 0x0101 is experimental(below 0x0600 ?) Magnus Lundberg Computation structures LTU 19 Data   Minimum data size 46 Bytes   Maximum data size 1500 Bytes   If actual data is less than 46 Bytes the packet is padded with zeros Magnus Lundberg LTU Computation structures 20 Frame Check Sequence (FCS)   4 Bytes used to detect bit faults in the packet   Computed with the CRC-32 algorithm   Computed on the Destination Address, Source Address, type and data fields. Magnus Lundberg LTU Computation structures 21 CRC theory ✞ (1) ✄ ✆ ✡ ☎ ✂ ✝ ✠ M DA, SA, type and data fields, k bits long. F The FCS field, n bits long. T M and F concatenated.(k+n bits long). P The CRC Polynomial. A pattern of n+1 bits. ✞ ✆ ☎ (2) ✝ ✌ ✄ ✠ ☞ ☛ ☛ ✞ ✄ ✆ ☎ ✂ ✝ ✌ ✠ Magnus Lundberg (3) LTU Computation structures 22 ✞ ✞ ✆ ✆ ☎ ☎ (4) ✂ ✝ ✝ ✌ ✌ ✌ ✌ ✌ ✌ ✠ ✠ ✄ ✄ ✄ ✄ ✠ ✠ ✠ ✠ ☞ ☞ ☛ ☛ ☛ ☛ ☛ ☛ ☛ But any binary number added to itself in a modulo 2 field yields zero so: ✄ ✍ ✌ ✌ ✠ ✂ (5) ✄ ☞ ☛ Magnus Lundberg LTU Computation structures 23 CRC polynomial The CRC polynomial for Ethernet is: CRC = ✓ ✙ ✓ ✓ ✑ ✓ ✓ ✓✑ ✑ ✑ ✑ ✑ ✑ ✘ ✗ ✝ ✝ ✝ ✕ ✒ ✎ ✒ ✖ ✎ ✚ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ Magnus Lundberg LTU Computation structures 24 CRC in hardware ✙ The VHDL code for the 1-bit wide data bus implementation with is shown below: polynomial ✑ ✘ ✄ ☛ ✚ ✝ ✝ ✝ ✝ ✠ ✠ ✠ ✜ ✛ crc(0) [...]... all signals are sent to all computers on network, all users share the same bandwidth   Switches = Each signal is connected to a specific link to a switch The switch receives the packet looks where it is heading and sends it to the appropriate port   Router = Is working on higher level than the Ethernet Protocol, it usually works on IP level and looks at the IP address instead of the Ethernet Address Magnus... structures 20 Frame Check Sequence (FCS)   4 Bytes used to detect bit faults in the packet   Computed with the CRC-32 algorithm   Computed on the Destination Address, Source Address, type and data fields Magnus Lundberg LTU Computation structures 21 CRC theory ✞ (1) ✄ ✆ ✡ ☎ ✂ ✝ ✠ M DA, SA, type and data fields, k bits long F The FCS field, n bits long T M and F concatenated.(k+n bits long) P The CRC Polynomial... 0xFFFFFFFFFF initially   The resulting CRC must be inverted and reflected Magnus Lundberg LTU Computation structures 26 Bit Order Byte-order Bit-order Bit-order Highest Byte MSB Lowest Byte LSB MSB LSB Preamble Synch DA SA Type Data 62 Bits 2 Bits 6 Bytes 6 Bytes 2 Bytes 46-1500 Bytes FCS 4 Bytes   Note that the Bit order is reversed compared to the Byte order Magnus Lundberg LTU Computation structures... Computation structures 22 ✞ ✞ ✆ ✆ ☎ ☎ (4) ✂ ✝ ✝ ✌ ✌ ✌ ✌ ✌ ✌ ✠ ✠ ✄ ✄ ✄ ✄ ✠ ✠ ✠ ✠ ☞ ☞ ☛ ☛ ☛ ☛ ☛ ☛ ☛ But any binary number added to itself in a modulo 2 field yields zero so: ✄ ✍ ✌ ✌ ✠ ✂ (5) ✄ ☞ ☛ Magnus Lundberg LTU Computation structures 23 CRC polynomial The CRC polynomial for Ethernet is: CRC = ✓ ✙ ✓ ✓ ✑ ✓ ✓ ✓✑ ✑ ✑ ✑ ✑ ✑ ✘ ✗ ✝ ✝ ✝ ✕ ✒ ✎ ✒ ✖ ✎ ✚ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✝ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ ✠ Magnus... Ethernet Protocol, it usually works on IP level and looks at the IP address instead of the Ethernet Address Magnus Lundberg LTU Computation structures 30 Clip-art Every presentation needs some clip-art Ethernet Switch ✥ ✥ ✦ ✣✤ ✦ NIC ✩ ✪ ✩ ✪ ★ ✧★ ✧ ✫ ✬ ✫ ✬ ✫ ✬ ✫ ✬ ✭✮ Magnus Lundberg LTU Computation structures 31 Questions ? Magnus Lundberg LTU ... structures History of Ethernet   Late 1982 DIX’s Bluebook and the IEEE 802.3 specifications merge into one standard   Ethernet was already multi vendor and more company’s endorsed the standard   Ethernet. .. with Collision Detect access) protocol Magnus Lundberg Computation structures LTU Ethernet Basics   Ethernet Packets are from 64 bytes up to 1518 bytes   Since Ethernet is a “BUS” several “Attached... with layers , Ethernet is in the bottom two layers: Data Link Layer and Physical Layer   OSI (Open Systems Interconnection) Magnus Lundberg Computation structures LTU History of Ethernet   Developed