The ADTRAN MX2820 system with a 23-inch chassis houses up to 18 MUX modules to provide nine redundant M13/STS-1 modules in 2U rack units of space. The ADTRAN MX2820 MUX modules are capable of combining independent T1s, E1s, or T1s and E1s on the same DS3/STS-1 signal. Each pair of MUX modules provides built-in 1:1redundancy on the DS1 and DS3/STS-1 circuits. The MX2820 requires just 2U of rack space to provide nine fully redundant M13s in a 23-inch chassis. Using the MX2820, a NEBS-compliant 7-foot rack will accommodate 63 redundant M13 multiplexers with passive cooling or 81 redundant M13 multiplexers with active cooling.