Crossbar tandem is used in Seattle for interlocal, extended area and toll trunking. Figure 1 shows the location of the Seattle crossbar tandem units.
Mutual tandem was established in 1948 and is located in the 1200 3rd Avenue building (Mutual) in the "downtown" area of Seattle. This unit originally handled all the dialable toll traffic for the Seattle area as well as interlocal trunking. With the establishment of the Seattle 4A toll switching machine in 1959 Mutual tandem became a class 4 switch point and its use for toll reduced until at present only about 20% of the trunks terminated on the machine are toll.
East tandem was established in 1954 to provide relief to Mutual tandem and switching capacity for the "Greater Seattle Extended Service" network being established at that time. Located approximately 1 1/2 miles east of Mutual, East tandem is currently used for extended area and interlocal trunking only.
A limited amount of tandem switching was inaugurated at the Emerson office with the establishment of the #5XB growth unit in 1956. As tandem switching requirements continued to increase additional growth capacity was required. In 1958 Emerson crossbar tandem was established releasing switching capacity used for tandem in the Emerson #5XB unit and providing relief for both Mutual and East tandems.
Initial use of Emerson tandem was primarily to eliminate a dialing problem that existed between the Seattle offices and the West Coast Telephone Co's. offices to the north of Seattle. However, today Emerson tandem is the third tandem unit in the Seattle trunking network and switches interlocal and EAS traffic only.
Where ever practicable, direct trunks ("HU" from common control units and "Final" from Panel and SXS units) have been and will continue to be established. However, the continued growth is such that all Seattle tandem capacity is expected to exhaust by 1975.
The provision of additional tandem capacity for the Seattle area is currently under study. From preliminary results it appears a 4th tandem unit is required but it may not be within the Seattle exchange area. For example, Renton where the #5XB unit is used for a limited amount of local tandem switching may be the location for a new unit. What ever location is chosen, the unit must provide the needed relief to Mutual and East tandems.