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Understanding Ethernet Frames and Technologies

Learn about Ethernet algorithms, protocols, technologies, frame structures, and common issues to enhance your network management skills.

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Understanding Ethernet Frames and Technologies

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  1. MAC algorithm from the receiver side • Senders manage all access control • Receivers merely read frames with acceptable address • Addressed to host • Broadcast address • Addressed to multicast group to which host belongs • All frames if host is in promiscuous mode

  2. Fast Ethernet (100 Mbps) • It has a technology which is very similar to 10 Mbps Ethernet • It uses a different physical layer encoding (4B5B) • Many NICs can support 10/100 Mbps • They may be used at both speeds

  3. Gigabit Ethernet (1000 Mbps) • It is compatible with lower speeds • It uses standard framing and CSMA/CD algorithm • Distances are very limited • It is typically used for backbones and inter-router connection

  4. Experiences with Ethernet • Ethernet networks work at best with light loads • Employment over 30% is considered to be heavy • network capability is wasted by collisions • Most networks are limited to about 200 hosts • Specification allows for up to 1024 • Most networks are much shorter • 5 to 10 microsecond RTT • Transport level flow control helps reduce load (number of back to back packets) • Ethernet is not expensive; it is fast and easy to administer

  5. Ethernet problems • Top usage is quite low (like Aloha) • Peak throughput is particularly low with: • More hosts • More collisions necessary to identify the single sender • Smaller packet sizes • More frequent arbitration • Longer links • It takes longer to notice collisions • Efficiency is improved by avoiding these conditions

  6. Reasons for Ethernet success • There are LOTS of LAN protocols • Price • Performance • Availability • Ease of use • Scalability

  7. Frame Ethernet revisited (1) • Preamble: 62 bit • SFD (Start of Frame Delimiter): 2 bit • Destination address: 6 byte • Source address: 6 byte • Length or frame type field: 2 byte • Data: from 46 to 1500 byte • FCS (Frame Check Sequence): 4 byte

  8. Frame Ethernet revisited (2) • Preamble: a sequence of alternating 1's and 0's used by the receiver to acquire bit-level synchronization • Start of Frame Delimiter: two consecutive 1 bits used to acquire byte alignment • Destination address: expected receiver address. Broadcast address is all 1s • Source address: unique address of the sender station

  9. FINE Frame Ethernet revisited (3) • Length or frame type: it is data byte number for IEEE 802.3 and the packet type for Ethernet I&II • Data: short packets must be padded up to 46 byte • Frame Check Sequence: it is a 32-bit cyclic redundancy check which is calculated using the AUTODIN II polynomial

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