Frame Relay, solutions for businesses and corporations (Part 1)

What is Frame Relay?

Frame Relay is a wide area network data transmission service based on packet switching technology. This is a CCITT [1] and ANSI [2] standard that defines the process of data transmission over public data networks. Currently Frame Relay serves customers who need to connect wide area networks and use their own applications with high connection speeds (maximum bandwidth of 44,736 Mbit / s) and for applications. Complex like voice, sound and image. The feature of Frame Relay is to transmit information over a wide area network by dividing data into packets. Each packet goes through a series of switches in the Frame Relay network to reach the destination.

As we know, WAN technologies are divided such as "leased line" subscriber lines (T1, E1), circuit switching (POTS, ISDN), packet switching (X.25, Frame Relay). . Frame Relay itself is packet switching technology, operating at the physical layer and data link in the OSI model [3] but without error control mechanism, error control must be based on protocols upper layer such as TCP [4] for example and therefore the data transfer rate of Frame Relay is faster than some other WAN technologies. Today Frame Relay is an industry standard protocol, it can handle multiple transmission channels and use HDLC [5] to encapsulate data between connected devices.

How it works

Frame Relay is considered as the interface between the user and the network device. The network that provides the Frame Relay interface may be the network of a privately managed service provider or network device. A Frame Relay network can include computers and servers on the user's side; devices that access Frame Relay such as routers or modems; and Frame Relay network equipment such as switches, routers, CDU / CSU [6] or multiplexers as shown in Figure 1.

Frame Relay, solutions for businesses and corporations (Part 1) Picture 1

Figure 1: Frame Relay identifies the process of connecting the router and the local access switch of the service provider.

We deploy a public Frame Relay service by placing the Frame Relay switch in the central ofice of the telecommunications service provider. In this case, customers get benefits such as transfer rates and do not need to spend much time managing or maintaining network devices and services. There are currently no standards for linking devices inside a Frame Relay network. Therefore, support for Frame Relay interfaces is not necessarily the Frame Relay protocol that must be used between network devices. Thus, traditional channel switching, packet switching or a hybrid hybrid technology can be used as described in Figure 2.

Frame Relay, solutions for businesses and corporations (Part 1) Picture 2

Figure 2: Frame Relay is used as an interface to a network using Frame Relay switches and routers.

Lines connecting user devices to network devices can operate at selected transmission speeds from multiple data rates. The speeds between 56 Kbit / s and 1,544 Mbit / s are typical, although Frame Relay can support lower and higher speeds.

Here are some terms related to Frame Relay that are used in the article:

+ Access rate : The clock rate (port speed) of the connection to the Frame Relay network. This is the speed at which data is transmitted or received by the network.

+ DLCI (Data-link connection identifier) : DLCI [10] is a numerical value that identifies the logical channel between the source device and the target device. Frame Relay switches refer to DLCI values ​​between each pair of routers to create a fixed virtual channel (PVC [11]).

+ LMI (local management interface) : A signaling standard between CPE device [9] and Frame Relay switcher is responsible for managing connection and maintaining state between devices. This is the set of advanced Frame Relay specification. LMI includes support for "keepalive" mechanisms that check the flow of data in circulation; "multicast" mechanism provides its local DLCI network server and DLCI multicast; Global addressing for DLCI values ​​has overall meaning in Frame Relay networks; and the "status" mechanism provides a report on "on-going" status of known DLCI switches. There are three types of LMI supported in Cisco routers , ansi, cisco , and q933a .

+ Committed information rate : is the speed measured in bits per second that the service provider commits.

+ FECN (Forward explicit congestion notification) : A bit setting in a frame informing a DTE that the procedure to prevent congestion must be initiated by the receiving device. When a Frame Relay switch detects a congestion in the network, it sends an FECN packet to the target device, noting that a congestion occurs.

+ BECN (Backward explicit congestion notification) : A bit setting in the frame (frame) notifies a DTE that the procedure to prevent congestion must be initiated by the sending device. As shown in Figure 2 when a Frame Relay switch detects a congestion in the network, it sends a BECN packet to the source router, instructing the router to slow down the transmission rate where it sends packets. believe. If the router receives any BECN packet in the current time slot, it reduces the transmission rate by 25%.

+ Discard eligibility (DE) indicator : A set of frame message bits can be discarded when a congestion occurs. While the router detects congestion on the network, Frame Relay switches will remove the packet with the first bit DE setting.

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Figure 3: A Frame Relay switch sends BECN packets to prevent congestion.

DLCI and multiplexing

Acting as the interface between the user and the network device, Frame Relay provides a way to multiplex multiple logical data exchanges (virtual channels), through a physical environment shared by assigning DLCI values. (see explanation above) for each pair of DTE / DCE [7,8] of devices.

With multiplexing in Frame Relay you can use the available bandwidth more dynamically and efficiently. Unlike ISDN [14] (the bandwidth in each channel is unchanged), with Frame Relay, each channel can use the bandwidth that other channels do not use, as long as their total bandwidth remains unchanged. That's why Frame Relay allows users to share bandwidth at a reduced cost. For example, a WAN uses Frame Relay and Frame Relay is equivalent to a group of subscriber lines. Phone companies are often the owners and are responsible for preserving and maintaining these lines. You can choose a line to rent exclusively for your company or you can pay a lower cost when renting a channel on that shared road. Of course Frame Relay can also run completely on individual networks, but it is rarely used in this way.

The Frame Relay standards target PVC [11] which are configured by the administrator and managed in a Frame Relay network. A PVC (permanent virtual circuit) is understood here as a fixed virtual channel. For example, in Figure 4 we have three PVC, PVC from A to B with DLCI values ​​at 325 and DLCI values ​​in B of 44, from B to C with DLCI values ​​of 52 and 417 respectively, From C to A are 52 and 910. In the base Frame Relay specification (not extended), the DLCI has a local meaning: the DLCI values ​​themselves are not unique in WAN Frame Relay. Two DTE devices connected by a virtual channel can use another DLCI value to refer to the same connection as described in Figure 4.

Frame Relay, solutions for businesses and corporations (Part 1) Picture 4

Figure 4: Terminals at either end of a connection can use different DLCI numbers to refer to the same connection.

Finally, the service provider's switch builds a table of DLCI values ​​that refer to outbound ports. When a frame is received, the analyzer switches to identify the connection and transfer the frame to the relevant output port. A full transmission channel is set before the first frame is sent.

Frame format (frame)

The Frame Relay frame format is depicted in Figure 5. The Flag fields show the start and end points of Frame Relay frame. Next to the Flag field marks the beginning of the frame as 2 bytes of address information. The ten bits of these two bytes form the ID of the current channel (which is the DLCI value).

Here are the fields in Frame Relay frame:

Flag : Indicates the start and end points of Frame Relay frame
Address : Indicates the length of the address field. Although Frame Relay addresses are currently 2 bytes long, address bits also take into account the possibility of future address length expansion. Eight bits of each byte in the Address field are used to indicate the address. The address includes the following information:
- DLCI value : indicates DLCI value includes 10 bits of Address field
- Congestion Control : The last three bits in the Address field to control the congestion notification mechanism in Frame Relay. These bits are FECN, BECN and DE bits (see explanation of the terms above).
Data : This field can change the length containing upper layer packaging data
FCS (Frame check sequence): Used to ensure data integrity has been transmitted.

Frame Relay, solutions for businesses and corporations (Part 1) Picture 5

Figure 5: Flag fields limit the start and end points of the frame