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1-2 CHAPTER 1: INTRODUCTION
Downlinks
and Uplink
The hub’s eight 1000BASE-SX full-duplex downlink ports (downlinks) can
connect to servers or clients that are equipped with Gigabit Ethernet
network interface cards (NICs) or to 10/100 Mbps switches that are
equipped with Gigabit Ethernet uplink devices.
One uplink port (uplink) accepts an optional gigabit interface connector
(GBIC) module. The following types of GBIC modules are available:
■ 1000BASE-CX coaxial transmits over short-haul shielded balanced
copper cable (known as twinaxial STP).
■ 1000BASE-LX long-wavelength transmits over multimode fiber (MMF)
or single-mode fiber (SMF) cable.
■ 1000BASE-SX short-wavelength transmits over MMF cable.
The primary use of the uplink is to connect to a network backbone
through a Gigabit Ethernet switch such as the 3Com SuperStack II
Switch 9300.
Full-Duplex
Operation
Full-duplex operation allows frames to be transmitted and received
simultaneously and, in effect, doubles the bandwidth available on a link.
For Gigabit Ethernet devices, full-duplex operation allows a peak data
transmission rate of 2 Gbps. All Hub 1000 SX ports operate only in
full-duplex mode and receive frames from attached segments at a
speed of 1 Gbps.
Each port’s input buffer places received packets in an 18 KB input FIFO to
wait in a queue for the hub’s internal bus. The hub grants ports access to
the bus in a round-robin fashion. When a port has access to the bus, it
places the packet at the head of its input queue on the bus. All the other
ports transmit the packet out. Link-level flow control manages the queue.
Flow Control The Hub 1000 SX uses IEEE 802.3x flow control to prevent input buffers
from overflowing. Flow control uses pause frames to communicate buffer
status between linked transmitting and receiving devices (transmitters
and receivers). A receiver sends a pause frame—thereby using outgoing
flow control—to tell a transmitter to stop the transmission of data frames
for a specified period, allowing the receiver’s input port buffers to empty.
When a transmitter receives pause frames—using incoming flow
control—it suspends transmission for the specified period.