SDLC connections use a standard phone line (leased, public, point-to-point, or multidrop). An SDLC adapter within the SNA Server computer connects to a modem that uses the phone line to establish a connection with the host system. In a mainframe system, the SDLC connection uses a FEP, a communications controller, or an integrated SDLC adapter. In an AS/400 system, the connection goes directly into the AS/400.
SDLC throughput is limited by the medium used for the connection and the capabilities of the SDLC adapter in the SNA Server computer. The cost to implement SDLC grows significantly as faster line types are used. In general, SDLC connections are much slower than 802.2 connections. Table 4.2 lists the common line types and their speeds:
Table 4.2 SDLC and X.25 Line Speeds.
| Line type | Typical speed |
| Analog (conventional phone line) | 9600 to 19200 bps |
| Digital Data System (DDS) | 56 to 64 kbps |
| Integrated Services Digital Network (ISDN) | 56 to 128 kbps |
| T1 carrier system | 1.544 Mbps |
| T3 carrier system | 2.048 Mbps |
SDLC connections are useful for wide-area connections between geographically disparate locations, or when bandwidth and usage requirements are low. Because of these factors, SDLC is ideally suited for branch-type deployment strategies.
SNA Server supports SDLC connections using the link support included with SNA Server, or through an SDLC link service available through various third-party vendors. Not all supported SDLC adapters support all link speeds listed in Table 2.2.