ECE124 Data Communications - Research Work No. 5 - SDLC and HDLC

ECE 124

Research Work

SDLC and HDLC

IBM developed the Synchronous Data Link Control (SDLC) protocol in the mid-1970s for use in Systems Network Architecture (SNA) environments. SDLC was the first link layer protocol based on synchronous, bit-oriented operation. This chapter provides a summary of SDLC's basic operational characteristics and outlines several derivative protocols.

After developing SDLC, IBM submitted it to various standards committees. The International Organization for Standardization (ISO) modified SDLC to create the High-Level Data Link Control (HDLC) protocol. The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T; formerly CCITT) subsequently modified HDLC to create Link Access Procedure (LAP) and then Link Access Procedure, Balanced (LAPB). The Institute of Electrical and Electronic Engineers (IEEE) modified HDLC to create IEEE 802.2. Each of these protocols has become important in its domain, but SDLC remains the primary SNA link layer protocol for WAN links.

SDLC Types and Topologies

SDLC supports a variety of link types and topologies. It can be used with point-to-point and multipoint links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and circuit-switched and packet-switched networks.

SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the operation of other stations, called secondaries. The primary polls the secondaries in a predetermined order, and secondaries can then transmit if they have outgoing data. The primary also sets up and tears down links and manages the link while it is operational. Secondary nodes are controlled by a primary, which means that secondaries can send information to the primary only if the primary grants permission.

SDLC primaries and secondaries can be connected in four basic configurations:

•Point-to-point—Involves only two nodes, one primary and one secondary.

•Multipoint—Involves one primary and multiple secondaries.

•Loop—Involves a loop topology, with the primary connected to the first and last secondaries. Intermediate secondaries pass messages through one another as they respond to the requests of the primary.

•Hub go-ahead—Involves an inbound and an outbound channel. The primary uses the outbound channel to communicate with the secondaries. The secondaries use the inbound channel to communicate with the primary. The inbound channel is daisy-chained back to the primary through each secondary.

SDLC Frame Format

The SDLC frame is shown in Figure 16-1.

Figure 16-1 Six Fields Comprise the SDLC Frame

The following descriptions summarize the fields illustrated in Figure 16-1:

•Flag—Initiates and terminates error checking.

•Address—Contains the SDLC address of the secondary station, which indicates whether the frame comes from the primary or secondary. This address can contain
a specific address, a group address, or a broadcast address. A primary is either a communication source or a destination, which eliminates the need to include the address of the primary.

•Control—Employs three different formats, depending on the type of SDLC frame used:

–Information (I) frame—Carries upper-layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs flow and error control. The send sequence number refers to the number of the frame to be sent next. The receive sequence number provides the number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers.

A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response.

–Supervisory (S) frame—Provides control information. An S frame can request and suspend transmission, report on status, and acknowledge receipt of I frames. S frames do not have an information field.

–Unnumbered (U) frame—Supports control purposes and is not sequenced. A U frame can be used to initialize secondaries. Depending on the function of the U frame, its control field is 1 or 2 bytes. Some U frames have an information field.

•Data—Contains a path information unit (PIU) or exchange identification (XID) information.

•Frame check sequence (FCS)—Precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If the result differs from the value in the original frame, an error is assumed.

A typical SDLC-based network configuration is shown in Figure 16-2. As illustrated, an IBM establishment controller (formerly called a cluster controller) in a remote site connects to dumb terminals and to a Token Ring network. In a local site, an IBM host connects (via channel-attached techniques) to an IBM front-end processor (FEP), which also can have links to local Token Ring LANs and an SNA backbone. The two sites are connected through an SDLC-based 56-kbps leased line.

Figure 16-2 An SDLC Line Links Local and Remote Sites over a Serial Line

Derivative Protocols

Despite the fact that it omits several features used in SDLC, HDLC is generally considered to be a compatible superset of SDLC. LAP is a subset of HDLC and was created to ensure ongoing compatibility with HDLC, which had been modified in the early 1980s. IEEE 802.2 is a modification of HDLC for LAN environments. Qualified Logical Link Control (QLLC) is a link layer protocol defined by IBM that enables SNA data to be transported across X.25 networks.

High-Level Data Link Control

HDLC shares the frame format of SDLC, and HDLC fields provide the same functionality as those in SDLC. Also, as in SDLC, HDLC supports synchronous, full-duplex operation.

HDLC differs from SDLC in several minor ways, however. First, HDLC has an option for a 32-bit checksum. Also, unlike SDLC, HDLC does not support the loop or hub go-ahead configurations.

The major difference between HDLC and SDLC is that SDLC supports only one transfer mode, whereas HDLC supports three:

•Normal response mode (NRM)—This transfer mode is also used by SDLC. In this mode, secondaries cannot communicate with a primary until the primary has given permission.

•Asynchronous response mode (ARM)—This transfer mode enables secondaries to initiate communication with a primary without receiving permission.

•Asynchronous balanced mode (ABM)—ABM introduces the combined node, which can act as a primary or a secondary, depending on the situation. All ABM communication occurs between multiple combined nodes. In ABM environments, any combined station can initiate data transmission without permission from any other station.

Reference:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/sdlcetc.htm