KIS – Cvi č enie #2

1. KIS – Cvičenie #2 Ethernet, MAC podvrstva Marián Beszédeš, B506 beszedes@ktl.elf.stuba.sk

2. Ethernet Network Elements • Ethernet LANs consist of : • network nodes • interconnecting media • The network nodes fall into two major classes: • Data terminal equipment (DTE) • Data communication equipment (DCE)

3. The network nodes • Data terminal equipment (DTE) • Devices that are either the source or the destination of data frames • devices as PCs, workstations, file servers, or print servers = end stations • Data communication equipment (DCE) • Intermediate network devices that receive and forward frames across the network • standalone devices - repeaters, network switches, routers • communications interface units such as interface cards and modems

4. RM OSI = Reference Model Open Systems Interconnection

5. Logical Relationship to the RM OSIDTE • MAC Client =IEEE Standard802.2 - Logical Link Control (LLC) Specification • Specifies the general interface between the network layer (IP, IPX, etc) and the data link layer (Ethernet, Token Ring, etc). • IEEE Standard802.3 - CSMA/CD Network (Ethernet) Specification • Specifiesthe frame format, cabling and signaling standards

6. Logical Relationship to the RM OSIDCE • MAC Client = IEEE Standard802.1- Bridge entitySpecification • LAN-to-LAN interfaces between LANs that use • the same protocol (Ethernet to Ethernet) • different protocols (Ethernet to Token Ring) • IEEE Standard802.3 - CSMA/CD Network (Ethernet) Specification • Specifiesthe frame format, cabling and signaling standards

7. The Ethernet basic data frame • Frame - format of data “packets” on the wire • Defined by the IEEE 802.3 standard • Preamble (PRE) • alternating pattern of ones and zeros • tells receiving stations that a frame is coming • provides a means to synchronize with the incoming bit stream

8. The Ethernet basic data frame • Start-of-frame delimiter (SOF) • alternating pattern of ones and zeros ending with two consecutive 1-bits • indicates that the next bit is the left-most bit in the left-most byte of the destination address

9. Ethernet – MAC Address • Individual addresses = unicast addresses • refer to a single MAC • are assigned by the NIC manufacturer from a block of addresses allocated by the IEEE • Group addresses = multicast addresses • identify the end stations in a workgroup • and are assigned by the network manager • Broadcast address • indicates all stations on the network.

10. The Ethernet basic data frame • Destination address (DA) • MAC address (Media Access Control address) • unique identifier attached to most forms of networking equipment • Used on layer 2 • Unique (48-bit address space, there are potentially 281,474,976,710,656 possible MAC addresses ) • first three octets (in transmission order) - identify an organization • next three octets are unique for the organization • Broadcast address • "FF:FF:FF:FF:FF:FF". • Multicast addresses • received by stations on a LAN which have been configured to do so • the least significant bit of their first octet set to one • Locally Administered addresses • assigned by the network administrator instead of the hardware vendor • The second bit of their first octet set to one

11. The Ethernet basic data frame • Destination address (DA) • The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network. • Source addresses (SA) • identifies the sending station. • always an individual address and the left-most bit in the SA field is always 0

12. The Ethernet basic data frame • Length/Type - Consists of 2 bytes • If the Length/Type field value is less than or equal to 1500 • Indicates the number of MAC-client data bytes that are contained in the data field of the frame • If the Length/Type field value is greater than 1536 • indicates the frame type ID if the frame is assembled using an optional format

13. The Ethernet basic data frame • Data • a sequence of n bytes of any value • n is less than or equal to 1500 • If the length of the Data field is less than 46, the Data field must be extended by adding a filler (a pad) sufficient to bring the Data field length to 46 bytes

14. The Ethernet basic data frame • Frame check sequence (FCS) • contains a 32-bit cyclic redundancy check (CRC) value • CRC value: • Is created by the sending MAC • is recalculated by the receiving MAC to check for damaged frames • is generated over the DA, SA, Length/Type, and Data fields

15. Frame Transmission • How is the transmission started? • an end station MAC receives a transmit-frame request (accompanying address, data information) from the LLC sublayer • MAC assemble frame from the LLC sublayer information • MAC transfers the assembled frame into the MAC frame buffer • Half Duplex ? - CSMA/CD access method

16. CSMA/CD Access method • Each Ethernet MAC determines for itself when it will be allowed to send a frame • Carrier sense : Each station continuously listens for traffic on the medium to determine when gaps between frame transmissions occur. • Multiple access : Stations may begin transmitting any time they detect that the network is quiet (there is no traffic). • Collision detect • two or more stations in the same CSMA/CD network (collision domain) begin transmitting at approximately the same time • the bit streams from the transmitting stations will interfere (collide) with each other = all transmissions will be unreadable • each transmitting station must be capable of detecting that a collision has occurred before it has finished sending its frame • Collision detected - Solution • Each must stop transmitting as soon as it has detected the collision and then must wait a random length of time before attempting to retransmit the frame

17. CSMA/CD Access method

18. CSMA/CD Access method

19. CSMA/CD – Worst Case!!! • two most-distant stations on the network both need to send a frame • the second station does not begin transmitting until just before the frame from the first station arrives • second station- the collision will be detected almost immediately • first station – itwon’t be detected until the corrupted signal has propagated all the way back to that station • “Late collision” - If the collision reaches the transmitter, after it completed sending the entire frame (the transmitter will not detect the collision) • The collision window • maximum time that is required to detect a collision • approximately equal to twice the signal propagation time between the two most-distant stations on the network • minimum frame length and the maximum collision domain segment length are directly related to the term of collision window

20. Example : How "late collision" is avoided in 10Base5 Ethernet? • Recommendation: • minimum frame size in an Ethernet network - 64bytes or 512bits • maximum length of an Ethernet cable segment is 500 meters for 10BASE5 (Thick Ethernet) • Propagation speed of the signal in copper media • 10Mbps = 10,000,000 bits per second • speed of light in a vacuum = 300,000,000 metres/second • speed of electricity in a copper cable = 200,000,000 metres/second • (200,000,000 m/s) / (10,000,000 bits / s) = 20 m/bit Minimum size Ethernet frame consisting of 64 bytes (512 bits) will occupy 10,240 metres of cable

21. Example : How "late collision" is avoided in 10Base5 Ethernet? • Station A begins to transmit • It will have transmitted 25 bits by the time the signal reaches Station B 500 meters away (500[m] / 20[m/b]) • Station B begins to transmit at the last possible instant before Station A's signal reaches it • the collision will reach Station A 25 bit-times later (the time it takes for the signal on the wire to travel one bit-length - 20 metres in copper cable). Station A will have transmitted only 50 bits(NOT 512 !!!) when the collision reaches it

22. Example : How "late collision" is avoided in 10Base5 Ethernet? • Why 512 bits, 500m are the limit values? • signal propagation from A to B (distance around 5000m) = 256 bits transmitted from A • collision event to propagate back from B to A = 256 bits transmitted from A • Summary = Distance from A to B = 5000 m • Why 512 bits, 500m are the limit values? • Thin Ethernet Network - 5*500m segment (4 repeaters can be used) = 2500m <> 5000m • Specification is twice as strict as it needs to be:-)

23. Consequences … • 100-Mbps networks(minimum frame size = 512b): • minimum-length frame can be transmitted in approximately one-tenth of the defined collision window • Distance from A to B (network diameter)= around 200m (around 1/10 2500m) • 1000-Mbps networks(minimum frame size = 512b): • Distance from A to B (network diameter)= around 20m (around 1/100 2500m) = Not practical!!! • Solution – Change minimum frame size to 520B

24. Consequences …

25. Frame Reception • reverse process of frame transmission • destination address of the received frame is checked and matched against the station's address list • Its MAC address • its group addresses • broadcast address • address match is found • the frame length is checked • FCS is compared to the FCS that was generated during frame reception • frame length – OK, FCS match – OK => The frame is then parsed and forwarded to the appropriate upper layer

26. References • www.cisco.com • www.wikipedia.org • www.windowsnetworking.com