/**

 * \ingroup devices

 * \defgroup PointToPointModel Point-to-Point Model

 *

 * \section PointToPointPointOverview Point-to-Point Model Overview

 *

 * The ns-3 point-to-point model is of a very simple point to point data link

 * connecting exactly two ns3::PointToPointNetDevice devices over an

 * ns3::PointToPointChannel.  This can be viewed as equivalent to a full

 * duplex RS-232 or RS-422 link with null modem and no handshaking.

 *

 * Data is encapsulated in the Point-to-Point Protocol (PPP -- RFC 1661),

 * however the Link Control Protocol (LCP) and associated state machine is 

 * not implemented.  The PPP link is assumed to be established and 

 * authenticated at all times.

 *

 * Data is not framed, therefore Address and Control fields will not be found.

 * Since the data is not framed, there is no need to provide Flag Sequence and

 * Control Escape octets, nor is a Frame Check Sequence appended.  All that is

 * required to implement non-framed PPP is to prepend the PPP protocol number

 * for IP Version 4 which is the sixteen-bit number 0x21 (see

 * http://www.iana.org/assignments/ppp-numbers).

 *

 * The PointToPointNetDevice provides following Attributes:

 *

 * - Address:       The ns3::Mac48Address of the device (if desired);

 * - DataRate:      The data rate (ns3::DataRate) of the device;

 * - InterframeGap: The optional ns3::Time to wait between "frames";

 * - Rx:            A trace source for received packets;

 * - Drop:          A trace source for dropped packets.

 *

 * The PointToPointNetDevice models a transmitter section that puts bits

 * on a corresponding channel "wire."  `The DataRate attribute specifies the

 * number of bits per second that the device will simulate sending over the 

 * channel.  In reality no bits are sent, but an event is scheduled for an

 * elapsed time consistent with the number of bits in each packet and the 

 * specified DataRate.  The implication here is that the receiving device

 * models a receiver section that can receive any any data rate.  Therefore

 * there is no need, nor way to set a receive data rate in this model.  By

 * setting the DataRate on the transmitter of both devices connected to a 

 * given PointToPointChannel one can model a symmetric channel; or by 

 * setting different DataRates one can model an asymmetric channel (e.g., 

 * ADSL).

 *

 * The PointToPointNetDevice supports the assignment of a "receive error 

 * model."  This is an ns3::ErrorModel object that is used to simulate data

 * corruption on the link.

 *

 * \section PointToPointChannelModel Point-to-Point Channel Model

 * The point to point net devices are connected via an 

 * ns3::PointToPointChannel.  This channel models two wires transmitting bits

 * at the data rate specified by the source net device.  There is no overhead

 * beyond the eight bits per byte of the packet sent.  That is, we do not 

 * model Flag Sequences, Frame Check Sequences nor do we "escape" any data.

 *

 * The PointToPointNetChannel provides following Attributes:

 *

 * - Delay:  An ns3::Time specifying the speed of light transmission delay for

 *   the channel.

 *

 * \section PointToPointTracingModel Point-to-Point Tracing Model

 *

 * Like all ns-3 devices, the Point-to-Point 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 PointToPointTracingModelUpperHooks 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 Point-to-Point net device for transmission it 

 * always passes through the transmit queue.  The transmit queue in the 

 * trace sources:

 *

 * - An Enqueue operation source (see Queue::m_traceEnqueue);

 * - A Dequeue operation source (see Queue::m_traceDequeue);

 * - A Drop operation source (see Queue::m_traceDrop).

 *

 * The upper-level (MAC) trace hooks for the PointToPointNetDevice 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 ns3::PointtoPointNetDevice::Send or 

 * ns3::PointToPointNetDevice::SendFrom is called by a higher layer to queue a 

 * packet for transmission.  An Enqueue trace event firing should be interpreted

 * as only indicating that a higher level protocol has sent a packet to the device.

 *

 * The m_traceDequeue event is triggered when a packet is removed from the

 * transmit queue.  Dequeues from the transmit queue can happen in two

 * situations:  1) If the underlying channel is idle when 

 * PointToPointNetDevice::Send is called, a packet is dequeued from the transmit

 * queue and immediately transmitted;  2) a packet may be dequeued and 

 * immediately transmitted in an internal TransmitCompleteEvent that functions 

 * much  like a transmit complete interrupt service routine.  An Dequeue trace 

 * event firing may be viewed as indicating that the PointToPointNetDevice has

 * begun transmitting a packet.

 *

 * \subsection CsmaTracingModelUpperHooks 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 

 * PointToPointChannel.

 *

 * The trace source m_dropTrace is called to indicate a packet that is dropped

 * by the device.  This happens when a packet is discarded as corrupt due to a 

 * receive error model indication (see ns3::ErrorModel and the associated 

 * attribute "ReceiveErrorModel").

 *

 * The other low-level trace source fires on reception of a packet (see 

 * ns3::PointToPointNetDevice::m_rxTrace) from the PointToPointChannel.

 *

 * \section PointToPointModelSummary Point-To-Point Model Summary

 *

 * The ns3 Point-to-Point model is a simplistic model of a point to point

 * serial line link.

 *

 * Ns-3 Attributes provide a mechanism for setting various parameters in the 

 * device and channel such as data rates, speed-of-light delays and error model

 * selection.  Trace hooks are provided in the usual manner with a set of 

 * upper level hooks corresponding to a transmit queue and used in ASCII 

 * tracing; and also a set of lower level hooks used in pcap tracing.

 */