Cisco Certified Network Associate Cisco Networking Academy Turku Polytechnic
CCNA1 – Module 6 • Based on Arpanet and Alohanet (late 60’s early 70’s) • Ethernet was invented by Robert Metcalfe and David Boggs at Xerox Parc (palo alto research center) • In 1979 DIX started to develop Ethernet • IEEE standard for Ethernet (802.3) 1983
CCNA1 – Module 6 • Ethernet operates in layer 1 and layer 2 (OSI) • Layer 1 is divided in two: Physical Medium and Physical Signaling Sublayer • Layer 2 is also divided in two: Media Access Control and Logical Link Control • IEEE 802.x
CCNA1 – Module 6 • MAC – Controls traffic from LLC to layer 1 and visa versa – Place for physical address, MAC address (48bit long / 12 hex) • First 6 as Organizational Unique Identifier • Last 6 administrated by the manufacturer
– Without MAC same as without name
CCNA1 – Module 6 • Encapsulation on layer 2 is frame – Originally Ethernet and 802.3 frames were different – In later IEEE standards, they are combined in 802.3 Ethernet
CCNA1 – Module 6 • Deterministic MAC – Token Ring, logical ring, physical star – FDDI, logical ring, physical dual ring
• non-deterministic MAC – Ethernet, logical bus, physical star or extended star
CCNA1 – Module 6 • CSMA/CD – Carrier sense – Multiple access – Collision detection
• After collision, keep sending awhile, wait random time and try again
CCNA1 – Module 6 • Slot time – Transmission in 10/100 Ethernet can’t be smaller than 512 bit-times (64 octets) – Slot time for 1000 Mbps is 4096 bit-times (512 octets)
• Slot time is calculated with maximum cable lengths in largest legal network • Slot time only applies to half-duplex Ethernet links!
CCNA1 – Module 6 • Interframe spacing is always 96 bit-times – It’s the time between last bit of frame (FCS) and first bit of next frame (preample)
• Procedure after collision at 10 Mbps collision domain: picture
CCNA1 – Module 6 • Local collision in UTP – Bits in RX at the same time when sending on TX (only half dublex) or over voltage
• Remote collision – Frame is shorter than minimum length or invalid FCS checksum and doesn’t have local symptoms – Most common collision type on UTP
CCNA1 – Module 6 • Late collision – Occurs after first 64 octets – Main difference to local and remote collision is that NIC will not retransmit late collision automatically. Retransmission have to be handled with upper layers. Rest of the functions after collision are same than local and remote collision.
CCNA1 – Module 6 • Collision or runt – Simultaneous transmission occurring before slot time has elapsed • Late collision – Simultaneous transmission occurring after slot time has elapsed • Jabber, long frame and range errors – Excessively or illegally long transmission • Short frame, collision fragment or runt – Illegally short transmission
CCNA1 – Module 6 • FCS error – Corrupted transmission • Alignment error – Insufficient or excessive number of bits transmitted • Range error – Actual and reported number of octets in frame do not match • Ghost or jabber – Unusually long Preamble or Jam event
CCNA1 – Module 6 • Other errors on Ethernet network – Bad checksum or CRC error – Message don’t end on octet boundary = alignment error – Bad number of octets = range error • Less than minimum, more than maximum = out of range
– Ground loops and other wiring problems = ghosting (must be more than 72 octets long)
CCNA1 – Module 6 • Full Duplex = sending and receiving at the same time • Half Duplex = cannot send and receive at the same time, only one station may transmit at a time (mandatory for coaxial) • Auto-negotiation to both ends • If one link partner is forced to full duplex, but the other partner attempts to Auto-Negotiate, then there is certain to be a duplex mismatch. This will result in collisions and errors on that link. • Additionally if one end is forced to full duplex the other must also be forced. The exception to this is 10-Gigabit Ethernet, which does not support half duplex.
IEEE 802.x
MAC
IEEE 802.3 Ethernet
Bit time and spacing
After collision
Short and long frame