Previous Table of Contents Next


Local Management Interface (LMI) Protocol

The LMI protocol provides operational support for the User Network Interface (UNI). Originally defined by the Frame Relay Forum in 1990, it since has been adopted by ANSI and the ITU-T. The LMI is a polling protocol between the FRAD and the network that verifies the existence and availability of the PVC, as well as the integrity of the UNI link.

Congestion Management

As a shared network designed for intensive data communications, Frame Relay is subject to congestion. Indeed, it is designed for congestion as a natural occurrence which serves to reduce the overall cost of the network. Congestion management is addressed through the following parameters, specified in an addendum to ANSI T1.606 [11-3], [11-8], and [11-9]:

  Access Rate The maximum data rate (bandwidth) of the access channel. Data can also be transmitted or received over the access link at lesser rates.
  Committed Information Rate (CIR) The data rate which the network guarantees to handle under normal conditions, based on mutual agreement between the carrier and the customer. In the event that the CIR is exceeded, the network reserves the option to mark excess frames as discard eligible. The discard function takes place at the entry node, in order to obviate any issues of unnecessary congestion in the network.
  Offered Load The data rate offered to the network for delivery measured in bps.
  Committed Burst Size (Bc) Maximum amount of data that the carrier agrees to handle under normal conditions.
  Excess Burst Size (Be) Maximum amount of data that the network will attempt to deliver over a specified time (T), as required by the user. In recognition of the bursty nature of LAN-to-LAN communications, the transmitting device may burst above the CIR for a brief period of time, with the network attempting to accommodate such bursts within limits of burst size and burst interval. The network reserves the option to mark the excess data discard eligible.
  Measurement Interval (T) The time interval measuring burst rates above the CIR and the length of such bursts.
  Discard Eligibility (DE) Indicates the eligibility of the frame for discard, in the event of congestion. DE may be set by either the FRAD or the FRND. Theoretically, at least, the FRAD would be programmed to recognize when the CIR is exceeded and to volunteer frames for discard should the network suffer unreasonable levels of congestion.
  Explicit Congestion Notification (ECN) The means by which the network advises devices of network congestion. Forward Explicit Congestion Notification (FECN) advises the target device of network congestion in order that it might adjust its expectations. Backward Explicit Congestion Notification (BECN) advises the transmitting device of network congestion in order that it might reduce its rate of transmission accordingly. Theoretically, it is the responsibility of the end devices to adjust. However, the effective assumption of such responsibility is unusual. For example, the Frame Relay network might send a BECN to advise a router of congestion in the network. While the router can impose a certain amount of flow control, its buffer memory is limited. Ultimately, it is the transmitting DTE that must adjust its rate of flow, which is unlikely.
  Implicit Congestion Notification Inference by user equipment that congestion has occurred. Such inference is triggered by realization of the user device (e.g., FRAD, Mainframe, or Server) that one or more frames have been lost. Based on control mechanisms at the upper protocol layers of the end devices, the frames would be resent.

Frame Relay Costs

The costs of Frame Relay vary widely among equipment manufacturers and carriers. It is clear, however, that Frame Relay is highly cost-effective for building mesh networks at > 56 Kbps. The equipment cost is that of the FRAD, which can be standalone, or can be built under the skin of a bridge or router. A DSU/CSU also is included in the device. In addition to the cost of the FRAD, the carriers’ charges typically include some combination of port charge (bandwidth-sensitive), access line (bandwidth- and distance-sensitive), Committed Information Rate (CIR) and burst size (Bc) (time-sensitive).

Frame Relay networks traditionally have been priced on a case-by-case basis. In consideration of the widespread popularity of the service, however, the FCC in October 1995 classified Frame Relay as a basic service. This reclassification required that the RBOCs, AT&T, MCI, and Sprint file tariffs. The publishing of Frame Relay rates allows smaller users to comparison-shop, simplifying the process and likely reducing their costs. Lager users, on the other hand lose some bargaining power. Significantly, the FCC has taken a position which will likely extend to other fast packet services in the future [11-11] and [11-12].

Prices have swung up and down since the FCC decision as competitors seek to develop their strategies. AT&T 512 Kbps PVCs are priced at $1,037; with 512 Kbps ports at $1,370 [11-13] and [11-14]. In the highly-competitive Frame Relay market, mesh networking that can be accomplished at costs which can be as little as 50% of the cost of a comparable FT-1 network [11-15]. Further, costs can be mitigated to some extent through the use of compression devices such as those offered by Telephone Company Systems, Inc. These devices allow transmission at a lower port speed and CIR through the recognition of patterns within data frames intended for as multicasting, or transmission to multiple addresses [11-16].

Frame Relay via VSATs is now offered by Orion Atlantic LP through its VISN (Virtual Integrated Sky Network) to sites in Europe and the eastern two-thirds of the U.S. Priced at $2,000 to $5,000 per month, VISN claims to be 35% to 40% less costly than it cabled counterparts [11-17].


Previous Table of Contents Next