Information Technology Reference
In-Depth Information
26
.14,
any of the computers in the local workgroup can connect directly to any other, or to
the printer, or the local disk drive. This type of switch works well if there is a lot of local
traffic, typically between a local server and local peripherals.
26.11.3 Store-and-forward switching
Store-and-forwarding techniques have been used extensively in bridges and routers, and are
now used with switches. It involves reading the entire Ethernet frame, before forwarding it,
with the required protocol and at the correct speed, to the destination port. This has the ad-
vantages of:
•
Improved error check - Bad frames are blocked from entering a network segment.
•
Protocol filtering - Allows the switch to convert from one protocol to another.
•
Speed matching - Typically, for Ethernet, reading at 10 Mbps or 100 Mbps and transmit-
ting at 100 Mbps or 10 Mbps. Also, can be used for matching between ATM (155 Mbps),
FDDI (100Mbps), Token Ring (4/16 Mbps) and Ethernet (10/100 Mbps).
The main disadvantage is:
•
System delay - As the frame must be totally read before it is transmitted there is a delay
in the transmission. The improvement in error checking normally overcomes this disad-
vantage.
26.12 Network interface card design
When receiving data, the network interface card (NIC) copies all data transmitted on the
network, decodes it and transfers it to the computer. An Ethernet NIC contains three parts:
•
Physical medium interface - the physical medium interface corresponds to the PLS and
PMA in the standard and is responsible for the electrical transmission and reception of
data. It consists of two parts: the transceiver, which receives and transmits data from or
onto the transmission media; and a code converter that encodes/decodes the data. It also
recognises a collision on the media.
•
Data link controller - the controller corresponds to the MAC layer.
•
Computer interface.
It can be split into four main functional blocks:
•
Network interface.
•
Manchester decoder.
•
Memory buffer.
•
Computer interface.
26.12.1 Network interface
The network interface must listen, recreate the waveform transmitted on the cable into a digi-
