@node CSMA NetDevice
@chapter CSMA NetDevice

This is the introduction to CSMA NetDevice chapter, to complement the
Csma model doxygen.

@menu
* Overview of the CSMA model::
* Using the CsmaNetDevice::
* CSMA Tracing::
@end menu

@node Overview of the CSMA model
@section Overview of the CSMA model

The ns-3 CSMA device models a simple bus network in the spirit of Ethernet.
Although it does not model any real physical network you could ever build 
or buy, it does provide some very useful functionality.

Typically when one thinks of a bus network Ethernet or IEEE 802.3 comes to
mind.  Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision
Detection with exponentially increasing backoff to contend for the shared 
transmission medium.  The ns-3 CSMA device models only a portion of this 
process, using the nature of the globally available channel to provide 
instantaneous (faster than light) carrier sense and priority-based 
collision "avoidance."  Collisions in the sense of Ethernet never happen and
so the ns-3 CSMA device does not model collision detection, nor will any
transmission in progress be "jammed."

@subsection CSMA Layer Model

There are a number of conventions in use for describing layered 
communications architectures in the literature and in textbooks.  The most
common layering  model is the ISO seven layer reference model.  In this view
the CsmaNetDevice and CsmaChannel pair occupies the lowest two 
layers -- at the physical (layer one), and data link (layer two) positions.
Another important reference model is that specified by RFC 1122, 
"Requirements for Internet Hosts -- Communication Layers."  In this view the
CsmaNetDevice and CsmaChannel pair occupies the lowest layer -- 
the link layer.  There is also a seemingly endless litany of alternative 
descriptions found in textbooks and in the literature.  We adopt the naming
conventions used in the IEEE 802 standards which speak of LLC, MAC, MII
and PHY layering.  These acronyms are defined as:

@itemize @bullet
@item LLC:  Logical Link Control;
@item MAC:  Media Access Control;
@item MII:  Media Independent Interface;
@item PHY:  Physical Layer.
@end itemize

In this case the @emph{LLC} and @emph{MAC }are sublayers of the OSI data link 
layer and the @emph{MII} and @emph{PHY} are sublayers of the OSI physical layer.

The "top" of the CSMA device defines the transition from the network layer
to the data link layer.  This transition is performed by higher layers by 
calling either CsmaNetDevice::Send or CsmaNetDevice::SendFrom.

In contrast to the IEEE 802.3 standards, there is no precisely specified
PHY in the CSMA model in the sense of wire types, signals or pinouts.  The
"bottom" interface of the CsmaNetDevice can be thought of as as a kind
of Media Independent Interface (MII) as seen in the "Fast Ethernet" 
(IEEE 802.3u) specifications.  This MII interface fits into a corresponding
media independent interface on the CsmaChannel.  You will not find the
equivalent of a 10BASE-T or a 1000BASE-LX PHY.

The CsmaNetDevice calls the CsmaChannel through a media independent
interface.  There is a method defined to tell the channel when to start 
"wiggling the wires" using the method CsmaChannel::TransmitStart, and 
a method to tell the channel when the transmission process is done and
the channel should begin propagating the last bit across the "wire":
CsmaChannel::TransmitEnd.

When the TransmitEnd method is executed, the channel will model a single 
uniform signal propagation delay in the medium and deliver copes of the packet
to each of the devices attached to the packet via the 
CsmaNetDevice::Receive method.

There is a "pin" in the device media independent interface corresponding to 
"COL" (collision).  The state of the channel may be sensed by calling 
CsmaChannel::GetState.  Each device will look at this "pin" before 
starting a send and will perform appropriate backoff operations if required.

Properly received packets are forwarded up to higher levels from the 
CsmaNetDevice via a callback mechanism.  The callback function is
initialized by the higher layer (when the net device is attached) using
CsmaNetDevice::SetReceiveCallback and is invoked upon "proper"
 reception of a packet by the net device in order to forward the packet up
the protocol stack.

@section CSMA Channel Model

The class CsmaChannel models the actual transmission medium.
There is no fixed limit for the number of devices connected to the channel.
The CsmaChannel models a data rate and a speed-of-light delay which can
be accessed via the attributes "DataRate" and "Delay" respectively.
The data rate provided to the channel is used to set the data rates
used by the transmitter sections of the CSMA devices connected to the 
channel.  There is no way to independently set data rates in the
devices.  Since the data rate is only used to calculate a delay time, there
is no limitation (other than by the data type holding the value) on the 
speed at which CSMA channels and devices can operate; and no restriction
based on any kind of PHY characteristics.

The CsmaChannel has three states, @code{IDLE}, @code{TRANSMITTING} and 
@code{PROPAGATING}.  These three states are "seen" instantaneously by all 
devices on the channel.  By this we mean that if one device begins or ends a
simulated transmission, all devices on the channel are @emph{immediately}
aware of the change in state.  There is no time during which one device may
see an @code{IDLE} channel while another device physically further away in 
the collision domain may have begun transmitting with the associated signals 
not propagated down the channel to other devices.  Thus there is no need for
collision detection in the CsmaChannel model and it is not implemented in any
way.

We do, as the name indicates, have a Carrier Sense aspect to the model.
Since the simulator is single threaded, access to the common channel will
be serialized by the simulator.  This provides a deterministic mechanism
for contending for the channel.  The channel is allocated (transitioned from
state @code{IDLE} to state @code{TRANSMITTING}) on a first-come first-served 
basis.  The channel always goes through a three state process:

@verbatim
  IDLE -> TRANSMITTING -> PROPAGATING -> IDLE
@end verbatim

The @code{TRANSMITTING} state models the time during which the source net device
is actually wiggling the signals on the wire.  The @code{PROPAGATING} state 
models the time after the last bit was sent, when the signal is propagating down 
the wire to the "far end."  

The transition to the @code{TRANSMITTING} state is  driven by a call to 
CsmaChannel::TransmitStart which is called by the net device that 
transmits the packet.  It is the responsibility of that device to end the
transmission with a call to CsmaChannel::TransmitEnd at the appropriate
simulation time that reflects the time elapsed to put all of the packet bits
on the wire.  When TransmitEnd is called, the channel schedules an event
corresponding to a single speed-of-light delay.  This delay applies to all
net devices on the channel identically.  You can think of a symmetrical hub
in which the packet bits propagate to a central location and then back out
equal length cables to the other devices on the channel.  The single ``speed
of light'' delay then corresponds to the time it takes for: 1) a signal to 
propagate from one CsmaNetDevice through its cable to the hub; plus 2) the
time it takes for the hub to forward the packet out a port; plus 3) the time
it takes for the signal in question to propagate to the destination net 
device.

The CsmaChannel models a broadcast medium so the packet is delivered
to all of the devices on the channel (including the source) at the end of 
the propagation time.  It is the responsibility of the sending device to 
determine whether or not it receives a packet broadcast over the channel.

The CsmaChannel provides following Attributes:

@itemize @bullet
@item DataRate:  The bitrate for packet transmission on connected devices;
@item Delay: The speed of light transmission delay for the channel.
@end itemize

@section CSMA Net Device Model

 The CSMA network device appears somewhat like an Ethernet device.  The
 CsmaNetDevice provides following Attributes:

@itemize @bullet
@item Address:  The Mac48Address of the device;
@item SendEnable:  Enable packet transmission if true;
@item ReceiveEnable:  Enable packet reception if true;
@item EncapsulationMode:  Type of link layer encapsulation to use;
@item RxErrorModel:  The receive error model;
@item TxQueue:  The transmit queue used by the device;
@item InterframeGap:  The optional time to wait between "frames";
@item Rx:  A trace source for received packets;
@item Drop:  A trace source for dropped packets.
@end itemize

The CsmaNetDevice supports the assignment of a "receive error model."
This is an ErrorModel object that is used to simulate data corruption
on the link.

Packets sent over the CsmaNetDevice are always routed through the 
transmit queue to provide a trace hook for packets sent out over the 
network.  This transmit queue can be set (via attribute) to model different
queuing strategies.

Also configurable by attribute is the encapsulation method used by the
device.  Every packet gets an EthernetHeader that includes the 
destination and source MAC addresses, and a length/type field.  Every packet
also gets an EthernetTrailer which includes the FCS.  Data in the
packet may be encapsulated in different ways.  

By default, or by setting the "EncapsulationMode" attribute to "Dix", the 
encapsulation is according to the DEC, Intel, Xerox standard.  This is sometimes
called EthernetII framing and is the familiar destination MAC, source MAC, 
EtherType, Data, CRC format.

If the "EncapsulationMode" attribute is set to "Llc", the encapsulation is by 
LLC SNAP.  In this case, a SNAP header is added that contains the EtherType
(IP or ARP).  

The other implemented encapsulation modes are IP_ARP (set "EncapsulationMode" to
"IpArp") in which the length type of the Ethernet header receives the protocol 
number of the packet; or ETHERNET_V1 (set "EncapsulationMode" to "EthernetV1") 
in which the length type of the Ethernet header receives the length of the packet.
A "Raw" encapsulation mode is defined but not implemented -- use of the RAW mode 
results in an assertion.  

Note that all net devices on a channel must be set to the same encapsulation mode
for correct results.  The encapsulation mode is not sensed at the receiver.

The CsmaNetDevice implements a random exponential backoff algorithm 
that is executed if the channel is determined to be busy (@code{TRANSMITTING} or
@code{PPROPAGATING}) when the device wants to start propagating.  This results in a
random delay of up to pow (2, retries) - 1 microseconds before a retry is
attempted.  The default maximum number of retries is 1000.

@node Using the CsmaNetDevice
@section Using the CsmaNetDevice

The CSMA net devices and channels are typically created and configured using
the associated @code{CsmaHelper} object.  The various ns3 device dhelpers 
generally work in a similar way, and their use is seen in many of our example
programs.

The conceptual model of interest is that of a bare computer ``husk'' into which 
you plug net devices.  The bare computers are created using a @code{NodeContainer}
helper.  You just ask this helper to create as many computers (we call them
@code{Nodes}) as you need on your network:

@verbatim
  NodeContainer csmaNodes;
  csmaNodes.Create (nCsmaNodes);
@end verbatim

Once you have your nodes, you need to instantiate a @code{CsmaHelper} and set
any attributes you may want to change.

@verbatim
  CsmaHelper csma;
  csma.SetChannelAttribute ("DataRate", StringValue ("100Mbps"));
  csma.SetChannelAttribute ("Delay", TimeValue (NanoSeconds (6560)));

  csma.SetDeviceAttribute ("EncapsulationMode", StringValue ("Dix"));
  csma.SetDeviceAttribute ("FrameSize", UintegerValue (2000));
@end verbatim
 
Once the attributes are set, all that remains is to create the devices
and install them on the required nodes, and to connect the devices 
together using a CSMA channel.  When we create the net devices, we add
them to a container to allow you to use them in the future.  This all
takes just one line of code.

@verbatim
  NetDeviceContainer csmaDevices = csma.Install (csmaNodes);
@end verbatim

@node CSMA Tracing
@section CSMA Tracing

Like all ns-3 devices, the CSMA Model provides a number of trace sources.
These trace sources can be hooked using your own custom trace code, or you
can use our helper functions to arrange for tracing to be enabled on devices
you specify.

@subsection Upper-Level (MAC) Hooks

From the point of view of tracing in the net device, there are several 
interesting points to insert trace hooks.  A convention inherited from other
simulators is that packets destined for transmission onto attached networks
pass through a single "transmit queue" in the net device.  We provide trace 
hooks at this point in packet flow, which corresponds (abstractly) only to a 
transition from the network to data link layer, and call them collectively
the device MAC hooks.

When a packet is sent to the CSMA net device for transmission it always 
passes through the transmit queue.  The transmit queue in the 
CsmaNetDevice inherits from Queue, and therefore inherits three 
trace sources:

@itemize @bullet
@item An Enqueue operation source (see Queue::m_traceEnqueue);
@item A Dequeue operation source (see Queue::m_traceDequeue);
@item A Drop operation source (see Queue::m_traceDrop).
@end itemize

The upper-level (MAC) trace hooks for the CsmaNetDevice are, in fact, 
exactly these three trace sources on the single transmit queue of the device.  

The m_traceEnqueue event is triggered when a packet is placed on the transmit
queue.  This happens at the time that CsmaNetDevice::Send or 
CsmaNetDevice::SendFrom is called by a higher layer to queue a packet for 
transmission.

The m_traceDequeue event is triggered when a packet is removed from the
transmit queue.  Dequeues from the transmit queue can happen in three 
situations:  1) If the underlying channel is idle when the 
CsmaNetDevice::Send or CsmaNetDevice::SendFrom is called, a packet
is dequeued from the transmit queue and immediately transmitted;  2) If the
underlying channel is idle, a packet may be dequeued and immediately 
transmitted in an internal TransmitCompleteEvent that functions much 
like a transmit complete interrupt service routine; or 3) from
the random exponential backoff handler if a timeout is detected.

Case (3) implies that a packet is dequeued from the transmit queue if it is 
unable to be transmitted according to the backoff rules.  It is important 
to understand that this will appear as a Dequeued packet and it is easy to 
incorrectly assume that the packet was transmitted since it passed through
the transmit queue.  In fact, a packet is actually dropped by the net device
in this case.  The reason for this behavior is due to the definition of the 
Queue Drop event.  The m_traceDrop event is, by definition, fired when a 
packet cannot be enqueued on the transmit queue because it is full.  This 
event only fires if the queue is full and we do not overload this event
to indicate that the CsmaChannel is "full."

@subsection Lower-Level (PHY) Hooks

Similar to the upper level trace hooks, there are trace hooks available at
the lower levels of the net device.  We call these the PHY hooks.  These 
events fire from the device methods that talk directly to the CsmaChannel.

The trace source m_dropTrace is called to indicate a packet that is dropped
by the device.  This happens in two cases:  First, if the receive side of 
the net device is not enabled (see CsmaNetDevice::m_receiveEnable and the 
associated attribute "ReceiveEnable").

The m_dropTrace is also used to indicate that a packet was discarded as 
corrupt if a receive error model is used (see 
CsmaNetDevice::m_receiveErrorModel and the associated attribute 
"ReceiveErrorModel").

The other low-level trace source fires on reception of an accepted packet
(see CsmaNetDevice::m_rxTrace).  A packet is accepted if it is destined
for the broadcast address, a multicast address, or to the MAC address 
assigned to the net device.
