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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."
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 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.
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. It is the responsibility of the sending device to determine whether or not it receives a packet broadcast over the channel.
The ns3::CsmaChannel provides following Attributes:
- DataRate: The bitrate for packet transmission on connected devices;
- Delay: The speed of light transmission delay for the channel.
Model
The CSMA network device appears somewhat like an Ethernet device. The ns3::CsmaNetDevice provides following Attributes:
- Address: The ns3::Mac48Address 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.
Model
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.
