• Homework
• Token Ring
• FDDI
• Network Devices
• Device Drivers

Homework (By Wed Sep 10th): Read handout. Read textbook section 3.4.

Each host has two point-to-point connections (to its neighbors); the topology of the whole is a ring. Shared access is achieved by using a token protocol

Notes:

On receipt of a token:

If the token has a packet:
  if we are the packet's destination:
    read, forward token and packet
  if this host is the packet's source:
    forward the token
else (the token has no packet)
  if we have a packet to send:
    forward the token with our packet
  else (no packet to send):
    forward the token

• There is one token in the ring at any time.
• The maximum token rotation time is independent of the number of hosts.
• Every host sees every packet.
• Distributed control.
• Distributed token generation, token removal algorithms.
• Suitable for real-time (delay-sensitive) data.
• All packets are received by all hosts in the same order.

• Dual token ring for reliability.
• Synchronous (delay-sensitive) and asynchronous (best-effort) traffic.
• Target Token Rotation Time negotiated among all hosts on the ring.
• No minimum data length.

Frame format: SOF (1), control (1), destination address (6), source address (6), data (1-4478), CRC (4), EOF (1), status (3).

Control field records synchronous/asynchronous, using 16/48 bit addresses, and 6 bits of demultiplexing.

Status field records whether the frame was correctly received.

What is the maximum size of a complete FDDI Frame? What is the minimum size?

Usually, you go with what's already there or with what you can afford.

If you have a choice:

• Determine present and future bandwidth requirements.
• Determine topology of LAN (e.g. star, ring, line).
• Select a LAN technology to provide the desired performance.
• Pick fiber (wave of the future?) vs cable (cheaper).

The network interface moves bytes to and from the network.

The bus interface moves bytes to and from the bus (very simple in the case of programmed I/O, more complex in the case of Direct Memory Access/DMA).

The controller interrupts the host, interprets commands in the control register, sets the status bits, and may do other functions such as computing the CRC.

Internal buffering may be used to hold data.

The simplest device drivers have 2 parts: the lower half runs at interrupt time, the upper half is invoked by the main part of the operating system (e.g. the write system call).

The main challenges of writing device drivers are:

• writing kernel software
• synchronizing the two halves, and
• dealing with all the possible status results of an operation.

• Put outbound data in buffer, waiting if no buffers are available. Make sure the device will interrupt when it is idle.
• Get any inbound data, or queue if none is ready.

Generally, semaphores can be used to keep track of available buffers.

• Disable interrupts.
• If called on a send interrupt, send the first available buffer if any, otherwise disable "idle" interrupts.
• If called on a receive interrupt, buffer the data if a buffer is available, otherwise discard the data.
• Enable interrupts.

In the x-kernel, if we are called on a receive interrupt, we create a thread to handle the data.