4. "ROOT"是什么意思？
以下是“8051 IAR Assembler”中对此的解释：
NOROOT, ROOT
NOROOT means that the segment part may be discarded by the linker
if no symbols in this segment part are referred to. Normally all
segment parts except startup code and interrupt vectors should set this
flag. The default mode is ROOT which indicates that the segment part
must not be discarded.
简单翻译下：
NOROOT表示如果此部分代码从来没有被调用到，那么这部分代码会被linker抛弃。一般来讲，除了startup和中断的代码都应该设为NOROOT。默认的模式为ROOT，即linker不会抛弃。
--by outman from zigbeetech 2010.4.7
5. 到底学习zigbee应该从哪开始？
走马观花地看过一本入门书，本以为买了开发板，调试一下里面的通信程序就能快速入门了。昨天看了一天的程序才发现这种想法太急功近利了，有句古话怎么讲来着－－“欲速则不达”，古人的话还是有道理的，于是决定调整思路，放平心态。但是到底从哪开始呢？国内Zigbee方面的书寥寥无几，大多数又总感觉有种功利色彩。罢了，读英文的吧（借机感谢下我大学良师albert，亏着他当时的提点，现在读英文书还不算吃力）。近期准备从两个地方下手，一个是TI的Zstack手册，另外一个是国外的朋友推荐的一本叫Newnes.ZigBee.Wireless.Networks.and.Transceivers的电子书，不想读电子版的，只是在深圳的书店还没找到。这是一本360多页的大部头，个人比较欣赏国外这种风格的书，往往深入浅出，让一个没太有基础的人也容易读得懂，不用搞一大堆书去看同一个东西。国内的大部头也不少，但很多感觉像我小时候写作文的风格－－扯远了，抓紧时间吧~~~
--by outman from zigbeetech 2010.4.8
7. Coordinator, Router与End Device有什么区别？
好本与烂书的区别在于，好书在开头的时候就知道你在想什么，而烂书呢，到读到最后你也不知道他在想什么。。。
这个问题的答案在那本英文“大部头”的前面章节：
1.7 Device Types
There are two types of devices in an IEEE 802.15.4 wireless network: full-function devices (FFDs) and reduced-function devices (RFDs). An FFD is capable of performing all the duties described in the IEEE 802.15.4 standard and can accept any role in the network. An RFD, on the other hand, has limited capabilities. For example, an FFD can communicate with any other device in a network, but an RFD can talk only with an FFD device. RFD devices are intended for very simple applications such as turning on or off a switch. The processing power and memory size of RFD devices are normally less than those of FFD devices.
1.8 Device Roles
In an IEEE 802.15.4 network, an FFD device can take three different roles: coordinator, PAN coordinator, and device. A coordinator is an FFD device that is capable of relaying messages. If the coordinator is also the principal controller of a personal area network (PAN), it is called a PAN coordinator . If a device is not acting as a coordinator, it is simply called a device .
The ZigBee standard uses slightly different terminology (see Figure 1.5 ). A ZigBee coordinator is an IEEE 802.15.4 PAN coordinator. A ZigBee router is a device that can act as an IEEE 802.15.4 coordinator. Finally, a ZigBee end device is a device that is neither a coordinator nor a router. A ZigBee end device has the least memory size and fewest processing capabilities and features. An end device is normally the least expensive device in the network.
附英文原文：1.9 ZigBee Networking TopologiesThe network formation is managed by the ZigBee networking layer. The network must be in one of two networking topologies specified in IEEE 802.15.4: star and peer-to-peer. In the star topology , shown in Figure 1.6 , every device in the network can communicate only with the PAN coordinator. A typical scenario in a star network formation is that an FFD, programmed to be a PAN coordinator, is activated and starts establishing its network. The first thing this PAN coordinator does is select a unique PAN identifier that is not used by any other network in its radio sphere of influence —the region around the device in which its radio can successfully communicate with other radios. In other words, it ensures that the PAN identifier is not used by any other nearby network.In a peer-to-peer topology (see Figure 1.7 ), each device can communicate directly with any other device if the devices are placed close enough together to establish a successful communication link. Any FFD in a peer-to-peer network can play the role of the PAN coordinator. One way to decide which device will be the PAN coordinator is to pick the first FFD device that starts communicating as the PAN coordinator. In a peer-to-peer network, all the devices that participate in relaying the messages are FFDs because RFDs are not capable of relaying the messages. However, an RFD can be part of the network and communicate only with one particular device (a coordinator or a router) in the network.
A peer-to-peer network can take different shapes by defining restrictions on the devices that can communicate with each other. If there is no restriction, the peer-to-peer network is known as a mesh topology . Another form of peer-to-peer network ZigBee supports is a treetopology (see Figure 1.8 ). In this case, a ZigBee coordinator (PAN coordinator) establishes the initial network. ZigBee routers form the branches and relay the messages. ZigBee end devices act as leaves of the tree and do not participate in message routing. ZigBee routerscan grow the network beyond the initial network established by the ZigBee coordinator.Figure 1.8 also shows an example of how relaying a message can help extend the range of the network and even go around barriers. For example, device A needs to send a message to device B, but there is a barrier between them that is hard for the signal to penetrate. The tree topology helps by relaying the message around the barrier and reach device B. This is sometimes referred to as multihopping because a message hops from one node to another until it reaches its destination. This higher coverage comes at the expense of potential high message latency.An IEEE 802.15.4 network, regardless of its topology, is always created by a PAN coordinator. The PAN coordinator controls the network and performs the following minimum duties:
● Allocate a unique address (16-bit or 64-bit) to each device in the network.
● Initiate, terminate, and route the messages throughout the network.
● Select a unique PAN identifier for the network. This PAN identifier allows the devices within a network to use the 16-bit short-addressing method and still be able to communicate with other devices across independent networks.There is only one PAN coordinator in the entire network. A PAN coordinator may need to have long active periods; therefore, it is usually connected to a main supply rather than a battery. All other devices are normally battery powered. The smallest possible network includes two devices: a PAN coordinator and a device.
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