Or they can be combined to provide 128Kbps access speeds. When an ISDN user dials in to an ISP's POP, each channel typically hits a rotary, or hunt, group. The hunt groups will either send the channels to the same POP equipment or to different devices, depending on which circuits are available. If POP equipment is lightly loaded, then connecting either one or two B channels is not a problem. But problems arise when ISDN users dial in to heavily loaded ISP POPs.
Channels can easily land on different servers in the POP, or even connect to separate POPs.
Although it is technically possible to establish a multilink stream across multiple chassis, success is difficult at best. There are other ways to address bonding issues apart from using Multichassis Multilink PPP, according to industry analysts. Another way in which ISPs are avoiding the problem is by investing in equipment that drastically increases port density at the POPs. Ascend Communications Inc. MAX TNT access switches, which support 672 ports per unit. With about 10 times the port count of current equipment, such high-density devices increase the likelihood that both B channels terminate on the same POP equipment and can be easily combined into a single 128Kbps stream to the Internet.