How SDLC devices are connected using DLSw

DLSw routers were installed in remote branches. The DLSw router enabled the connection of TCP/IP and SNA LAN-attached workstations to the mainframe. Many remote branches still have SDLC devices like IBM 3174 controllers, banking, or retail controllers. Using the DLSw branch router eliminates the need to connect the SDLC device using a dedicated communication link.

Figure 1 shows how an SDLC controller in a remote branch is connected using DLSw routers.

Figure 1. Connecting SDLC controller using DLSW

Figure 1 depicts the two types of DLSw: remote (A) and local (B).

Remote DLSw requires at least two routers (with DLSw feature loaded) connected to each other over an IP network. DLSw performs an encapsulation/decapsulation function: wrapping the SNA frames into IP packets for transportation across the IP network.
Local DLSw does not use TCP/IP. Instead, it enables communication between LAN-attached SNA devices and an SDLC device that is link-attached to the same DLSw router.

The Remote DLSw configuration in Figure 1 enables the workstations connected to the remote LAN workstations to communicate with either TCP mainframe-based applications using the communication link that connects the two routers, or mainframe SNA application using encapsulated LLC2 in TCP packets (DLSw). The SDLC traffic is converted by the router to LLC2 and is transported using DLSw.

Every router has an IP address assigned to its LAN interface.

Figure 2 illustrates the representation of the SDLC link inside a DLSw router.

Figure 2. SDLC-to-LLC2 conversion

In Figure 2, the SDLC link is represented inside the router as a connection to a virtual LAN. Virtual LAN is a LAN implemented inside the router and is not related to a physical LAN interface. The SDLC link is assigned a virtual MAC address, and this MAC address is carried in the encapsulated LLC2 frames.

Figure 3 shows the placement of DLSw routers in an IP backbone network. DLSw routers are located at the edge of the network adjacent to the SNA device, mainframe, and server in our example. The DLSw routers perform the encapsulation and decapsulation of the LLC2 frame that carries the SNA path information unit or LLC2 commands and responses.

Figure 3. A conceptual view of DLSw

The following points summarize some of the DLSw attributes:

DLSw is a forwarding mechanism, and supports SNA device types PU2, PU2.1 and PU4.
DLSw provides switching at the data link layer (layer 2), and encapsulates SNA data into TCP packets for transport over an IP network.
DLSw does not provide full routing capabilities. DLSw, like any other TCP connection, utilizes the dynamic and rerouting capabilities of the IP network.
DLSw is a TCP application that uses default ports 2065 and 2066.
SNA endpoints remain the same. No change is required to SNA endpoints.
Because SNA data is encapsulated in TCP packets, DLSw does not interpret the SNA headers. The consequence is that SNA class of service (COS) is not implemented in the IP network.
Some older models of the one gigabit OSA-Express cards in QDIO mode support the IP protocol only. Using DLSw rules out the use of these one gigabit OSA-Express cards and imposes the use of 100 megabit OSA-Express cards. These cards are defined in the HCD as non-QDIO.

DLSw was the first SNA over IP solution that became available. It is a mature product and you will find that many organizations have implemented this solution.