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Like other networking technologies, serial connections require a protocol to transmit information from one side of a link to another. The selection of an encapsulation, in addition to the use of tunnels and the type of traffic traversing the link, can impact performance and manageability. The encapsulations for data over serial lines are:

  Cisco’s HDLC
  PPP (Point-to-Point Protocol)
  LAPB (Link Access Procedure, Balanced)

The data frame for each of these protocols is derived from SDLC, which is used in SNA. As shown in the following illustration, there are five components to the frame, excluding the variable-length data portion. The beginning frame flag is one byte in length and contains a hexadecimal pattern of 0x7F. The ending frame flag is set to 0x7E. The address field is shown as one byte, but it can be expanded to a two-byte value. The control field marks the frame as informational, supervisory, or unnumbered. The frame check sequence (FCS) provides limited error checking. Cisco’s HDLC encapsulation adds a type field between the control and data fields, and PPP places a protocol field in this location.


Cisco’s implementation of the HDLC protocol is the default serial line encapsulation on the router. It supports the AutoInstall feature, which permits remote configuration of newly installed routers; however, it is also proprietary. Regardless of this limitation, most administrators use Cisco HDLC.


The Point-to-Point Protocol provides a number of benefits over the HDLC encapsulation; however, it also includes a slight amount of overhead by comparison. The fact that PPP is an RFC standard is its greatest advantage, but the protocol also offers authentication and link-control features. Authentication is typically provided by the Password Authentication Protocol (PAP) or by the more secure Challenge Handshake Authentication Protocol (CHAP).


Link Access Procedure, Balanced is a reliable encapsulation for serial connections. It provides the data-link layer for X.25, but it may be used without that protocol. LAPB features link compression and excellent error correction, which makes it well suited to unreliable analog media. Because of this overhead, LAPB tends to be slower than other encapsulations.

One of the configuration options in LAPB is modulo, or the sequence number. Initial implementations of LAPB supported only eight sequence numbers—modulo 8, which quickly resulted in a windowing delay for higher speed connections. (Modulo 128 was developed to address this limitation.) Designers should make certain that the same value is used on both sides of the link.

Network Design with Frame Relay

Frame Relay networks offer the network designer many benefits that do not exist in point-to-point, leased-line transports. These include:

  Distance-insensitive billing
  Multiple destinations per physical interface
  The ability for data to burst above the tariffed data rate

Most vendors offer Frame Relay under a fairly simple tariff, or cost structure, based on the reserved capacity of the virtual circuit. Leased lines charge on a per-mile basis, and the bandwidth charge is equal to the total capacity of the circuit. As a result, Frame Relay connections can be significantly less expensive, especially when traversing hundreds of miles.

Circuit costs are recurring and thus can quickly overshadow any installation and capital expenditures.

Frame Relay is also considered a burstable technology. This refers to the difference between reserved bandwidth and total potential bandwidth available. Consider a point-to-point circuit—the network will transport only as much data as the circuit will provide, and unused bandwidth will remain unused because the connection is dedicated. Frame Relay circuits are typically provisioned with a bandwidth reservation lower than the capacity of the link—256Kbps on a T1, for example. Vendors combine virtual circuits so that the remaining bandwidth is utilized, but if the physical media has unused bandwidth, any of the virtual circuits can burst beyond their allocation and temporarily increase their available bandwidth.

Frame Relay circuits are typically provisioned with two distinct bandwidth parameters, unlike standard HDLC or switched-56 circuits, which are provisioned with the data rate equal to the port speed. In addition to the physical capacity of the circuit, Frame Relay incorporates a committed information rate, or CIR.

The CIR function varies with different telecommunications vendors, though most use the value to represent a guaranteed available bandwidth to the customer. This may be calculated on a per-second or per-minute basis, but the net result is that customers can reserve bandwidth at a lower level than the capacity of the local loop connection. For example, a CIR of 768Kbps on a T1 would offer at least 768Kbps to the customer and provide a burst up to 1.5Mbps for a short duration.

Different vendors implement bursting differently, including the concept of zero CIR, where no bandwidth is reserved. Designers should fully understand their vendor’s implementation before provisioning circuits.

Frame Relay connections use permanent virtual circuits (PVCs) to specify connections from one node to another. These PVCs are identified by a DLCI, or data link connection identifier. Frame Relay switches forward frames based solely on the DLCI in the header of each frame.

Switched virtual circuits (SVCs) are available in Frame Relay, yet most vendors do not support this configuration. As a result, this chapter discusses PVC-based Frame Relay connections only. PVCs and SVCs are discussed in more detail later in this chapter.

The Frame Relay switch simply takes one port/DLCI connection and forwards it to another port/DLCI connection. In this context, the term “port” refers to the physical interface, and “DLCI” refers to the logical Frame Relay interface. DLCIs only have local significance, and while vendors typically assign a single DLCI for each link in the PVC, it is possible to have different ones.

Consider the connections shown in Table 8.1:

TABLE 8.1 DLCI Connections

San Francisco to Denver
Port 1, DLCI 100 Port 7, DLCI 100
San Francisco to Chicago
Port 1, DLCI 200 Port 12, DLCI 400
Denver to Chicago
Port 7, DLCI 200 Port 12, DLCI 200

These connections are shown in Figure 8.1. Note that each physical connection in the diagram carries two user DLCIs, and that while a single Frame-Relay switch is shown for clarity, there would be more switches for such long connections. There are three PVCs in this full-mesh configuration.

FIGURE 8.1  A basic Frame Relay network

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