Information Technology Reference
In-Depth Information
The algorithm collision counter stops incrementing at 10, where the penalty wait time is selected
from a range of 0 to 1023 slot times before retransmission. This is pretty bad, but the algorithm will
attempt to retransmit the frame up to 16 collisions. Then it just gives up, and a higher-layer network
protocol such as TCP/IP will attempt to retransmit the packet. This is an indication that you have
some serious errors.
When a station successfully sends a frame, the collision counter (penalty) is cleared (for that frame)
and no loner must wait for the back-off time. (“Interface” statistics are not cleared, just the timer is).
Any stations with the lowest collisions will be capable of accessing the wire more quickly because
they do not have to wait.
Both Ethernet and IEEE 802.3 LANs are broadcast networks. In other words, all stations see all frames,
regardless of whether they represent an intended destination. Each station must examine received frames
to determine whether the station is a destination. If it is a destination, the frame is passed to a higher
protocol layer for appropriate processing.
Differences between Ethernet and IEEE 802.3 standards are subtle. Ethernet provides services
corresponding to Layers 1 and 2 of the OSI reference model, whereas IEEE 802.3 specifies the physical
layer (Layer 1) and the channel-access portion of the link layer (Layer 2), but does not define a logical
link control protocol. Both Ethernet and IEEE 802.3 are implemented in hardware. Typically, the
physical manifestation of these protocols is either an interface card in a host computer or circuitry on a
primary circuit board within a host computer.
Now, having said all that regarding the regular operation of traditional Ethernet and 802.3, we must
discuss where the two separate in features and functionality. The IEEE 802.3 standard was based on
traditional Ethernet, but improvements have been made to this current standard. What we have discussed
so far will not scale in today's demanding service provider and enterprise networks.
Full-Duplex Operation 10/100/1000
Everything you've read so far dealt with half-duplex operation (CSMA/CD, back-off timers, and so on).
Full-duplex mode allows stations to transmit and receive data simultaneously. This makes for more
efficient use of the available bandwidth by allowing open access to the medium. Conversely, this mode
of operation can function only with Ethernet switching hubs or via Ethernet cross-over cables between
interfaces capable of full-duplex Ethernet. Full-duplex mode expects links to be point-to-point links.
There are also no collisions in full-duplex mode, so CSMA/CD is not needed.
Autonegotiation
Autonegotiation allows Ethernet devices to automatically configure their interfaces for operation. If the
network interfaces supported different speeds or different modes of operation, they will attempt to settle
on a lower common denominator. A plain repeater cannot support multiple speeds; it knows only how to
regenerate signals. Smart hubs employ multiple repeaters and a switch plane internally to allow stations
that support different speeds to communicate. The negotiation is performed only when the system
initially connects to the hub. If slower systems are attached to the same smart hub, then faster systems
will have to be manually configured for 10 Mbps operation.
To make sure that your connection is operating properly, IEEE 802.3 Ethernet employs normal link
pulses (NLPs), which are used for verifying link integrity in a 10BaseT system. This signaling gives you
the link indication when you attach to the hub and is performed between two directly connected link
interfaces (hub-to-station or station-to-station). NLPs are helpful in determining that a link has been
established between devices, but they are not a good indicator that your cabling is free of problems.
