The IS-95 mobile networks, also known as cdmaOne systems, introduced the first CDMA based mobile networks in the mid-1990s. IS-95 stands for Interim Standard 1995, and it was a second-generation mobile network technology.
Interim Standard 95 (IS-95 or cdmaOne) uses two frequency bands: 824 to 894 MHz and 1850 to 1990 MHz. In the first band, 824 to 849 MHz is for reverse link and 869 to 894 MHz for the forward link. In the second band, 1850 to 1910 MHz is for reverse link and 1930 to 1990 MHz for the forward link.
The IS-95 standard has two variants, IS-95 A and IS-95 B, which later evolved into the third generation mobile networks CDMA2000. For completeness, another CDMA-based mobile network is 3G UMTS which followed a different track and was used for the 3G migration of GSM networks. In this post, we will dive into the frequencies and channels used by the IS-95 systems.
Frequencies used in the forward and reverse links of IS-95
| Frequency Band | Reverse Link (Uplink) | Forward Link (Downlink) |
|---|---|---|
| IS-95, first band: 824 to 894 MHz | 824 – 849 MHz | 869 – 894 MHz |
| IS-95, second band: 1850 to 1990 MHz. | 1850-1910 MHz | 1930-1990 MHz |
IS-95, commercially known as the cdmaOne system, uses a full-duplex scheme called Frequency Division Duplex (FDD), which means separate frequency bands for the forward and the reverse links. For clarity, in GSM, UMTS and LTE networks, we use the term downlink for communication from Base Station to Mobile Station, which is called forward link in cdmaOne (IS-95).
Similarly, in GSM, UMTS and LTE, the term uplink means communication from Mobile Station to Base Sation, which is called the reverse link in cdmaOne (IS-95). There are two bands in IS-95; the 850 MHz band and the 1900 MHz band. In the 850 MHz band, the reverse link is from 824 MHz to 849 MHz, while the forward link is from 869 MHz to 894 MHz.
Therefore, the separation between the starting frequencies for reverse and forward links is 45 MHz (i.e. 869 MHz minus 824MHz = 45 MHz). In the 1900 MHz band, this separation is 80 MHz, so the reverse link is from 1850 MHz to 1910 MHz, while the forward link is 1930 MHz to 1990 MHz. The table below summarises the forward and reverse link frequencies in IS-95 (cdmaOne).
What channels forward and reverse links use in IS-95
In IS-95 mobile networks, the forward and reverse links have a different set of channel types. The table below summarises the channels for the forward and reverse links.
In Interim Standard 95 (IS-95) the forward link (network to phone) has four physical channels: pilot channel, sync channel, paging channel and forward traffic channel. The reverse link (phone to network) has two physical channels: reverse access channel and reverse traffic channel.
| Channel Name | Forward or Reverse Link? | Abbreviation |
|---|---|---|
| Pilot Channel | Forward Link (Downlink) | F-PICH |
| Synch Channel | Forward Link (Downlink) | F-SYNCH |
| Paging Channel | Forward Link (Downlink) | F-PCH |
| Forward Traffic Channel | Forward Link (Downlink) | F-TCH |
| Reverse Access Channel | Reverse Link (Uplink) | R-ACH |
| Reverse Traffic Channel | Reverse Link (Uplink) | R-TCH |
What channel bandwidth and bit rates are possible in IS-95?
The frequency bands in Interim Standard 95 (IS-95) have a carrier bandwidth of 1.25 MHz to enable peak download data rates of up to 14.4 kbps using IS-95 A and up to 115 kbps using the enhanced version, IS-95 B. IS-95 offers capacity advantages due to its ability to accommodate more users per MHz.
The power consumption in IS-95 networks is low, allowing users to make phone calls in decent quality even when the signal strength is not great. Due to low power consumption in these networks, the coverage is a bit wider, which means the cell size is a bit bigger. It also means that a lesser number of cells can do a good enough job. IS-95 networks also have a soft handover (also sometimes called handoff) which means that the calls are less likely to be dropped.
Why Interim Standard 95 (IS-95) uses the CDMA technology
Interim Standard 95 (IS-95 or cdmaOne) uses CDMA because a CDMA-based multiple access technique ensures that the mobile users within a cell are separated by unique codes instead of unique frequency carriers. That allows all cells in an area to use the same frequency carrier without interference.
When IS-95 was introduced, there were two-second generation technologies, GSM (Global System for Mobile Communications) and D-AMPS (Digital Advanced Mobile Phone System). Both GSM and D-AMPS use a combination of FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) technologies.
With that approach, the reuse of frequency was a limiting factor which was handled differently by IS-95 due to the CDMA technology. IS-95 employs spread spectrum communication, more specifically direct sequence spreading, which is also used in the 3G CDMA2000 networks. In CDMA, multiple access is achieved by using unique codes that separate the mobile users instead of unique frequencies or timeslots.
cdmaOne frequency band vs. GSM and D-AMPS frequencies
cdmaOne was introduced in the same era as GSM (Global System for Mobile Communications) and D-AMPS (Digital-Advanced Mobile Phone System). Both GSM and D-AMPS used a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) to offer cellular services.
The original GSM networks used 890 to 915 MHz for the uplink and 935 to 960 MHz for the downlink. This frequency band was later extended to add 10 MHz to both the uplink and the downlink. The extended band is known as Extended GSM or E-GSM and ranges from 880 to 915 MHz for the uplink and 925 to 960 MHz for the downlink. On the other hand, D-AMPS uses 824 to 849 MHz for the uplink and 869 to 894 MHz for the downlink.
Is CDMA2000 different from cdmaOne?
CDMA2000 was the 3G upgrade path for 2G cdmaOne networks. cdmaOne is the commercial name for IS-95, whereas CDMA2000 is the commercial name for IS-2000. CMDA2000 uses the same carrier bandwidth of 1.25 MHz and is both circuit-switched and packet-switched.
Just like cdmaOne was launched in the same era as GSM and D-AMPS, CDMA2000 was introduced in the same era as UMTS (Universal Mobile Telecommunications System). More information on the differences between CDMA2000 and cdmaOne can be found in this dedicated post.
Conclusion – IS-95 frequencies
The frequency band used by IS-95 (cdmaOne) can either be 824 to 894 MHz or 1850 to 1990 MHz with separate frequency bands for the reverse and forward links with a channel bandwidth of 1.25 MHz. In the 850 MHz band of IS-95, the reverse link (uplink) employs 824 to 849 MHz, and the forward link (downlink) employs 869 to 894 MHz. In the 1900 MHz band, the reverse link is 1850-1910 MHz, and the forward link is 1930-1990 MHz.
Here are some helpful downloads
Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:
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