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CSMA Model Description

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This commit is contained in:
Craig Dowell
2008-05-27 22:30:11 -07:00
parent 20b1d65d07
commit 0ef25b59c4
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src/devices/csma/csma.h
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/**
* \ingroup devices
* \defgroup Csma Csma Models
* \defgroup CSMA CSMA Model
*
* \section Csma Models
* \section CSMA Model
*
* The set of Csma models provides an abstrated shared media net device
* and channel, similar to a switched ethernet.
* 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 Channel Model
*
* The class ns3::CsmaChannel models the actual transmission medium.
* There is no fixed limit for the number of devices connected to the channel.
* The ns3::CsmaChannel models a bitrate and a speed-of-light delay which can
* be accessed via the attributes "BitRate" and "Delay" respectively.
*
* The ns3::CsmaChannel has three states, IDLE, TRANSMITTING and 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 immediately aware of the change in state.
* There is no time during which one device may see an IDLE channel while
* another device physically further away in the collision domain may have
* begun transmitting with the associated signals not propagated. Thus there
* is no need for collision detection in the ns3::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 IDLE to state TRANSMITTING) on a first-come first-served basis. The
* channel always goes through a three state process:
*
* IDLE -> TRANSMITTING -> PROPAGATING -> IDLE
*
* The TRANSMITTING state models the time during which the source net device
* is actually wiggling the signals on the wire. The 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 TRANSMITTING state is driven by a call to
* ns3::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 ns3::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 ns3::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.
*
* \subsection CSMA Net Device Model
*
* The CSMA network device appears somewhat like an Ethernet device. The
* ns3::CsmaNetDevice provides following Attributes:
*
* - Address: The ns3::Mac48Address of the device;
* - DataRate: The data rate of the device;
* - SendEnable: Enable packet transmission if true;
* - ReceiveEnable: Enable packet reception if true;
* - EncapsulationMode: Type of link layer encapsulation to use;
* - RxErrorModel: The receive error model;
* - TxQueue: The trasmit queue used by the device;
* - InterframeGap: The optional time to wait between "frames";
* - Rx: A trace source for received packets;
* - Drop: A trace source for dropped packets.
*
* The ns3::CsmaNetDevice supports the assignment of a "receive error model."
* This is an ns3::ErrorModel object that is used to simulate data corruption
* on the link.
*
* Packets sent over the ns3::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
* queueing strategies.
*
* Also configurable by attribute is the encapsulation method used by the
* device. Every packet gets an ns3::EthernetHeader that includes the
* destination and source MAC addresses, and a length/type field. Every packet
* also gets an ns3::EthernetTrailer which includes the FCS. Data in the
* packet may be encapsulated in different ways. By default, or by setting
* the "EncapsulationMode" attribute 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
* assert firing.
*
* The ns3::CsmaNetDevice implements a random exponential backoff algorithm
* that is executed if the channel is determined to be busy (TRANSMITTING or
* PROPAGATING) 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.
*
* \subsection CSMA Model Summary
*
* The ns3 CSMA model is a simplistic model of an Ethernet-like network. It
* supports a Carrier-Sense function and allows for Multiple Access to a
* shared medium. It is not physical in the sense that the state of the
* medium is instantaneously shared among all devices. This means that there
* is no collision detection required in this model and none is implemented.
* There will never be a "jam" of a packet already on the medium. Access to
* the shared channel is on a first-come first-served basis as determined by
* the simulator scheduler. If the channel is determined to be busy by looking
* at the global state, a random exponential backoff is performed and a retry
* is attempted.
*
* Ns-3 Attributes provide a mechanism for setting various parameters in the
* device and channel such as addresses, encapsulation modes and error model
* selection. Trace hooks are provided in the usual manner.
*
* Although the ns-3 CsmaChannel and CsmaNetDevice does not model any kind of
* network you could build or buy, it does provide us with some useful
* functionality. You should, however, understand that it is explicitly not
* Ethernet or IEEE 802.3 but an interesting subset.
*/