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Cable Modem...(恕刪)
Transport over HFC network[edit]
By using frequency division multiplexing, an HFC network may carry a variety of services, including analog TV, digital TV (SDTV or HDTV), video on demand, telephony, and high-speed data. Services on these systems are carried on radio frequency (RF) signals in the 5 MHz to 1000 MHz frequency band.
The HFC network is typically operated bi-directionally, meaning that signals are carried in both directions on the same network from the headend/hub office to the home, and from the home to the headend/hub office. The forward-path or downstream signals carry information from the headend/hub office to the home, such as video content, voice and Internet data. The very first HFC networks, and very old unupgraded HFC networks are only one-way systems. Equipment for one-way systems may use POTS or radio networks to communicate to the headend.
The return-path or upstream signals carry information from the home to the headend/hub office, such as control signals to order a movie or Internet data to send an email. The forward-path and the return-path are actually carried over the same coaxial cable in both directions between the optical node and the home.
To prevent interference of signals, the frequency band is divided into two sections. In countries that have traditionally used NTSC System M, the sections are 52–1000 MHz for forward-path signals, and 5–42 MHz for return-path signals. Other countries use different band sizes, but are similar in that there is much more bandwidth for downstream communication instead of upstream communication.
Traditionally, since video content was sent only to the home, the HFC network was structured to be asymmetrical: one direction has much more data-carrying capacity than the other direction. The return path was originally only used for some control signals to order movies, etc., which required very little bandwidth. As additional services have been added to the HFC network, such as Internet access and telephony, the return path is being utilised more.
Multiple-system operators[edit]
Multi-system operators (MSOs) developed methods of sending the various services over RF signals on the fiber optic and coaxial copper cables. The original method to transport video over the HFC network and, still the most widely used method, is by modulation of standard analog TV channels which is similar to the method used for transmission of over-the-air broadcast. See broadcast television system for more information.
One analog TV channel occupies a 6-MHz-wide frequency band in NTSC-based systems, or an 8-MHz-wide frequency band in PAL or SECAM-based systems. Each channel is centred on a specific frequency carrier so that there is no interference with adjacent or harmonic channels. To be able to view a digitally modulated channel, home, or customer-premises equipment (CPE), e.g. digital televisions, computers, or set-top boxes, are required to convert the RF signals to signals that are compatible with display devices such as analog televisions or computer monitors. The US Federal Communication Commission (FCC) has ruled that consumers can obtain a cable card from their local MSO to authorize viewing digital channels.
By using digital compression techniques, multiple standard and high-definition TV channels can be carried on one 6 or 8 MHz frequency carrier thus increasing the channel carrying-capacity of the HFC network by 10 times or more versus an all analog network.
交通運輸在HFC網絡[編輯]
通過使用頻分复用，將HFC網絡可以攜帶多種服務，包括模擬電視，數字電視（SDTV或HDTV），點播，電話視頻和高速數據。在這些系統服務進行射頻（RF）信號在5兆赫到1000兆赫的頻帶。
HFC網絡典型地操作雙向的，這意味著信號在兩個方向上從數據轉發器/集線器辦公室到家庭的同一網絡上進行，並從家庭到數據轉發器/集線器辦公室。轉發路徑或下行信號攜帶從數據轉發器/集線器辦公信息到家庭，諸如視頻內容，語音和互聯網數據。的第一HFC網絡，並且很老未升級的HFC網絡是單向的系統。設備的單向系統可以使用POTS或無線網絡通信的頭端。
返回路徑或上行信號進行從家庭信息到前端/集線器辦公，如控制信號訂購電影或互聯網數據發送電子郵件。轉發路徑和返回路徑中的光學節點和家庭之間的兩個方向上實際上結轉同一同軸電纜。
為了防止信號干擾，頻帶被分為兩部分。在傳統上使用NTSC器系統M的國家，部分是52-1000兆赫轉發路徑的信號，5-42兆赫返回路徑的信號。其他國家使用不同的頻帶的大小，但在類似的存在用於下行通信，而不是上游通信得多的帶寬。
傳統上，由於視頻內容才被發送到家用，HFC網絡被構造成不對稱：一個方向上具有比其他方向更多的數據，承載能力。返回路徑最初僅用於一些控制信號訂購電影等，這需要很小的帶寬。作為附加服務已被添加到HFC網絡，如互聯網接入和電話，返回路徑被利用更多。
多系統運營商[編輯]
多系統運營商（MSO）開發的光纖和同軸銅電纜上發送射頻信號的各種服務的方法。原來的方法來傳輸視頻在HFC網絡，並且仍然是最廣泛使用的方法，是由標準的模擬電視頻道的調製，它類似於用於通過空中廣播的傳輸的方法。參見廣播電視系統的詳細信息。
一個模擬電視信道佔用中基於NTSC的系統一個6兆赫寬的頻帶，或在PAL或SECAM基於系統的8兆赫寬的頻帶。每個信道的特定頻率載波上居中，使得存在與相鄰或諧波信道無干擾。為了能夠觀看數字調製信道，家庭，或客戶端設備（CPE），例如數字電視，計算機，或機頂盒，被要求的RF信號轉換為與顯示設備兼容，例如模擬電視機或計算機監視器信號。美國聯邦通信委員會（FCC）已經裁定，消費者可以從當地MSO電纜卡授權觀看數字電視頻道。
通過使用數字壓縮技術，多種標準和高清晰度電視頻道可以在一個6或8兆赫的頻率載波上進行從而通過10倍或更多與一個全模擬網絡增加HFC網絡的信道承載能力。
