Cisco Certified ExpertFrequency Allocation
Broadcast channel allocation is different in every region and country. In the U.S., frequency allocation is the responsibility of the Federal Communications Commission (FCC). The FCC decides which entities are authorized to broadcast at specific frequencies. Most regions and countries have authorities responsible for frequency allocation. Types of broadcast signals are usually restricted to specific frequencies by law. Figure 13-4 shows U.S. over-the-air frequency allocation for those frequencies used by cable modems for communication.
Because cable companies distribute information on cables shielded from over-the-air, or on-air broadcast frequencies, a cable operator can broadcast on any frequency that the cable plant can carry. However, a large cable plant can act like a massive antenna and interference with over-the-air broadcasts or other over-the-air signals is common. Specific areas of contention vary from region to region but can include both licensed and unlicensed radio frequencies.
Figure 13-4 U.S. Broadcast Frequency Allocation 5 to 150 MHz
SHORTWAVE HAM
LAND MOBILE TV CHANNELS TV CHANNELS RADIO & PAGING 2-4 5-6
AERO NAVIGATION AIRCRAFT VOICE
HAM OTHER
CB HAM
ASTRONOMY AERP NAVIGATION
FM RADIO
AIR TRAFFIC CONTROL AND RESEARCH
5.75
50-54
72-76
5 to 45 MHz Return Frequency Range Used for DOCSIS Upstream Channels
88 to 860 MHz Forward Frequency Range Used for DOCSIS Downstream Channels
Cable modem standards limit modem transmission (upstream) to frequencies compatible with the return path designs and frequency allocation of the cable network. Downstream transmission is likewise limited to ranges associated with downstream path designs of the cable network. In the U.S. cable modems transmit upstream between 5 and 40 MHz and receive downstream between 88 and 860 MHz, although downstream video signals begin around 52 MHz. The term split plan refers to a location in the spectrum where the upstream frequencies and downstream frequencies meet. For locations with different split plans, cable modem communication upstream and downstream is separated at a different point in the spectrum. In all regions the two frequency ranges are actually separated by a slight gap in the spectrum. This gap ranges in size from 12 MHz to 20 MHz as defined in the local region. Location of the split, the ranges available for DOCSIS upstream transmission, and those frequencies used for downstream transmission of DOCSIS and video signals are detailed in Table 13-1.
|
Split Plan (High Upstream Frequency/ |
|
|
Region or Country |
Low Downstream Frequency in MHz) |
|
North, Central, and South America |
40/52 |
|
China, Korea, Philippines, Thailand, |
65/85 |
|
Singapore, Australia |
|
|
Japan, New Zealand |
55/70 |
|
India, Malta, Eastern Europe |
30/48 |
|
Western Europe, Ireland, United Kingdom |
65/85 |
Those regions or countries with splits that occur at higher frequencies often have more upstream frequencies available. Because upstream modem transmission is limited to upstream frequencies, more bandwidth is theoretically available in the upstream path in regions with split plans, which occur higher at higher frequencies. In particular, regions such as Japan have experienced a strong demand for cable modem equipment capable of transmitting upstream at frequencies around 50 MHz. Often higher upstream frequencies have less interference than lower frequencies, which results in more reliable cable modem operation.
Was this article helpful?
Telescopes Mastery
Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!
Post a comment