在安全監控產業中，數位錄影系統（Digital Video Recorder）為相當重要的產品之
一。DVR 與傳統監控產品最主要的差別，除了儲存媒體改以硬碟為主以外，最
重要的特色為導入網路相關技術的應用，並提供了網路遠端監控的可能性。

在開發 DVR 產品時，無論採用的網路通訊協定多寡，通常都會在研發過程遭遇
一些問題。本文主要重點為描述 DDNS 的更新機制在套用至 DVR 產品時可能會
面臨的一大問題，也就是在 DVR 無法取得當地網路路由器之外部IP 位址的情況
下，可能會產生 DDNS 申請而來的 Domain Name 與實際使用的 IP 位址無法相
符，又不能正確更新的問題。除了描述此問題之外，亦提供二個可能的解決方案，
並詳加比較彼此的優缺點。若 DVR 可以自我偵測路由器外部 IP 的改變，並即
時發送 DDNS 更新封包至 DDNS 伺服器，則此自動調整功能將可以讓 DVR 在
搭配 DDNS 使用時，維持 Domain Name 與 IP Address 的對應關係。

而在說明 DDNS 與 DVR 互相搭配的問題之外，本文亦大略介紹安全監控產業相
關的產品與其特色，並說明在這些產品之中可能使用的壓縮技術或是網路相關通
訊協定。在搭配網路通訊協定進行開發的過程中，所遭遇的相關問題與實際案例
亦有提及。

DVR (Digital Video Recorder) is one of the major products in surveillance
industry. In addition to using hard disk as storage media, the major
difference between DVR and conventional recording system is the
application of network protocol. It also provides the possibility of remote
monitoring.

No matter how many network protocols have been used in DVR products,
it is for sure that some problems will occur during development. This
thesis mentions a serious problem DVR could meet if it integrates DDNS
function. When DVR is located in a LAN environment and rely on a router
to communicate with Internet, DVR may have no way to get router’s
external IP address. Since DVR needs to know the correct external IP
address to determine when it needs to send DDNS update packet, it may
not be able to send DDNS update packet at correct time if DVR does not
notice the change of router’s external IP address. This could generate
inconsistency issue between IP address and the domain name that user
retrieved from DDNS provider. If DVR could detect the change of router's
external IP, and send DDNS update packet to DDNS server at correct
timing, then this self-adaptive function can keep consistency between
domain name and external IP address.

In addition to discussing the problem with DDNS update mechanism with
DVR, the thesis also provides two possible methods to solve this issue,
and compares the advantage/disadvantage with both ways. DVR related
products are also introduced in this thesis, with brief discussion for
compress technology and network protocols that DVR related products
will use. Some of the real cases when developing DVR products with
network protocols are also mentioned and described.

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[17]RG-58, http://en.wikipedia.org/wiki/RG-58
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[28]JTAG, http://en.wikipedia.org/wiki/JTAG
[29]PTZ Camera, http://en.wikipedia.org/wiki/PTZ_camera
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[36]Transmission Control Protocol, http://en.wikipedia.org/wiki/Transmission_Control_Protocol
[37]User Datagram Protocol, http://en.wikipedia.org/wiki/User_Datagram_Protocol
[38]Datagram, http://en.wikipedia.org/wiki/Datagram
[39]BOOTP, http://en.wikipedia.org/wiki/BOOTP
[40]Dynamic Host Configuration Protocol, http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol
[41]Point-to-Point Protocol over Ethernet, http://en.wikipedia.org/wiki/PPPoE
[42]Echo Request/Reply: RFC-792, http://www.cs.jcu.edu.au/CIE/RFC/1122/46.htm