Medium Access Control Layer
Hydra uses the Click modular router to implement its MAC layer. This software framework, developed at MIT, runs on a GPP and was originally created for building flexible and high performance routers. Using a programming paradigm similar to GNU Radio, in Click packet processing elements are coded in C++ and connected together using Click's own glue language. Elements can be flexibly configured to perform tasks for packet processing such as packet classification or scheduling, and then connected together in a flow graph to compose a protocol. Click not only handles memory management and scheduling for flow graph elements, but also allows users to select and modify various scheduling algorithms. As with GNU Radio, Click is also an attractive open-source development platform because of its growing community of users, which have used Click for a wide variety of applications including modular router design, ad hoc routing, and network coding.
We have implemented several random access MAC protocols for Hydra in Click, in particular CSMA/CA and the distributed coordination function (DCF) mode of IEEE 802.11. From this experience, it is clear that implementing a slotted MAC protocol for Hydra would be straight forward. As a proof of concept for Hydra's cross-layer design potential, we have also extended the current DCF MAC design to create a rate-adaptive MAC protocol based on RBAR. This cross-layer protocol is being used to investigate rate-adaptation in multihop networks.
Other Protocol Layers
In Hydra, the wireless ad hoc networking protocols used to manage network connectivity are also implemented in Click. The code for these network protocols was contributed to the Click codebase by the creators of Grid, as well as other networking researchers. Since the MAC and networking protocols are both implemented in Click, they run together in their own address space, separate from GNU Radio and the TCP/IP protocol stack (running in the Linux user and kernel address space respectively). This parallelism may provide a performance improvement in multiprocessor environments.
Click features a simple tunnelling mechanism that allows protocols to interface with the standard Linux TCP/IP stack. Thus, any application built on TCP/IP can be used with Hydra. This allows researchers using Hydra to easily test new wireless protocols with end-to-end application level experiments. Applications such as ping, ftp, and web sessions are used regularly to verify and debug the operation of Hydra.
