3Com Hub 1000 SX manual Downlinks, and Uplink, Full-Duplex, Operation, Flow Control

1-2CHAPTER 1: INTRODUCTION

Downlinks	The hub’s eight 1000BASE-SX full-duplex downlink ports (downlinks) can
and Uplink	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	Full-duplex operation allows frames to be transmitted and received
Operation	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.