17.2.2 Satellite links

Figure 17.3: Main components of a satellite network interface

Satellite links resemble wireless links, which are described in Chapter 16. Each satellite node has one or more satellite network interface stacks, to which channels are connected to the physical layer object in the stack. Figure 17.3 illustrates the major components. Satellite links differ from ns wireless links in two major respects: i) the transmit and receive interfaces must be connected to different channels, and ii) there is no ARP implementation. Currently, the Radio Propagation Model is a placeholder for users to add more detailed error models if so desired; the current code does not use a propagation model.

Network interfaces can be added with the following instproc of Class Node/SatNode:

$node add-interface $type $ll $qtype $qlim $mac $mac_bw $phy

The add-interface instproc returns an index value that can be used to access the network interface stack later in the simulation. By convention, the first interface created on a node is attached to the uplink and downlink channels of a satellite or terminal. The following parameters must be provided:

• type:  The following link types can be indicated: geo or polar for links from a terminal to a geo or polar satellite, respectively, gsl and gsl-repeater for links from a satellite to a terminal, and intraplane, interplane, and crossseam ISLs. The type field is used internally in the simulator to identify the different types of links, but structurally they are all very similar.
• ll:  The link layer type (class LL/Sat is currently the only one defined).
• qtype:  The queue type (e.g., class Queue/DropTail). Any queue type may be used- however, if additional parameters beyond the length of the queue are needed, then this instproc may need to be modified to include more arguments.
• qlim:  The length of the interface queue, in packets.
• mac:  The MAC type. Currently, two types are defined: class Mac/Sat- a basic MAC for links with only one receiver (i.e., it does not do collision detection), and Class Mac/Sat/UnslottedAloha- an implementation of unslotted Aloha.
• mac_bw:  The bandwidth of the link is set by this parameter, which controls the transmission time how fast the MAC sends. The packet size used to calculate the transmission time is the sum of the values size() in the common packet header and LINK_HDRSIZE, which is the size of any link layer headers. The default value for LINK_HDRSIZE is 16 bytes (settable in satlink.h). The transmission time is encoded in the packet header for use at the receive MAC (to simulate waiting for a whole packet to arrive).
• phy:  The physical layer- currently two Phys (Class Phy/Sat and Class Phy/Repeater) are defined. The class Phy/Sat just pass the information up and down the stack- as in the wireless code described in Chapter 16, a radio propagation model could be attached at this point. The class Phy/Repeater pipes any packets received on a receive interface straight through to a transmit interface.

An ISL can be added between two nodes using the following instproc:

$ns add-isl $ltype $node1 $node2 $bw $qtype $qlim

This creates two channels (of type Channel/Sat), and appropriate network interfaces on both nodes, and attaches the channels to the network interfaces. The bandwidth of the link is set to bw. The linktype (ltype) must be specified as either intraplane, interplane, or crossseam.

A GSL involves adding network interfaces and a channel on board the satellite (this is typically done using the wrapper methods described in the next paragraph), and then defining the correct interfaces on the terrestrial node and attaching them to the satellite link, as follows:

$node add-gsl $type $ll $qtype $qlim $mac $bw_up $phy \bs
   [$node_satellite set downlink_] [$node_satellite set uplink_]

Here, the type must be either geo or polar, and we make use of the downlink_ and uplink_ instvars of the satellite; therefore, the satellite's uplink and downlink must be created before this instproc is called.

By default, the node generator for satellite nodes (described in Section 5.3) will create nodes of a given type, give them an uplink and downlink interface, and create and attach an (initial) uplink and downlink channel, based on the interface options specified.