The retail chain has rolled out a Cable & Wireless IP-based virtual private network (VPN) to 130 of its 300 outlets and aims to link all its stores by the end of May.
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The network, which will cost £10m, has already speeded up information processing at shopping tills and in the IT operations of departments as varied as personnel management and product ordering.
The introduction of voice over IP networks next year will lead to dramatic benefit for the retailer, said C&W senior design consultant Mark Stevens.
"You won't need multiple boxes and different technologies to carry voice and data, nor employ several teams of staff to manage them," he said.
After the roll out to the stores, C&W will start work on M&S's distribution network. Then the company will develop an extranet that will enable the retailer's 3,000 suppliers around the world to use the VPN.
M&S is the first UK company to use C&W's IP-based VPN which was launched in December. An IDC survey released last week found that four in 10 UK firms expected IP-based VPNs to provide their communications in future. In the survey, 60% of companies used standard wide-area networks while one in 10 already used an IP-based VNS.
As part of the M&S network migration, Nortel has installed a digital multiplex system to carry voice traffic. The company is working with C&W on the combined voice and data network which should be ready by early next year.
M&S had already increased tenfold the capacity of its previous 64-bit X.25 network in 1999.