Joseph works as a category manager for a large electricity supply company. His company is one of six electricity companies in the country. All have a high annual spend on the same specifications of cable. There are only five cable manufacturers capable of supplying the cables. Joseph has established all the suppliers use the same raw materials using the same types of manufacturing lines. Prices tendered by all suppliers are very similar. After paying for the cable, Joseph's company spends an additional 25% on moving and installing the cable. In order to enable Joseph to gain advantage over the other electricity companies, which type of supplier relationship should he adopt?