X.25 is ITU-T standard protocol suite for packet switching over telephone line, leased lines and ISDN. It is one of the oldest packet-switched methods. It specifies three layers of operation: physical, data link and network or packet. It has error correction built-in the last two layers. It was an attempt to utilize the circuit switched networks for packet switching. Typically, it allocates a static bandwidth (no congestion notifications). Nowadays, it is replaced by TCP/IP on a network layer or Frame Relay on a data link layer over high-quality digital-grade lines, which are almost error free.