Frame Relay

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Frame Relay is a data transmission technology that was originally a further development of X.25 . Many network operators today offer Frame Relay connections as a cheaper alternative to a dedicated line . In Europe, for example, the base stations of the GSM network, which receive the radio signals from the mobile phones and transfer them to the fixed network, are often connected via frame relay.


Like X.25 or the Datex-P network, Frame Relay multiplexes the data streams from different sending and receiving stations over one line according to statistical criteria and supports speeds between 56 kbit / s and 45 Mbit / s. Frame Relay is often offered with a guaranteed transmission speed ( CIR of Committed Information Rate ) and a short-term exceeding of the transmission speed ( EIR of Excess Information Rate ).

The biggest improvement over X.25 is the support for higher bandwidths. X.25 was originally developed for the transmission of data over telephone lines, which rarely have error rates better than 1: 10,000 (i.e., less than 1 bad bit for every 10,000 bits transmitted). That is why X.25 has complex mechanisms for correcting errors and for the repeated transmission of data blocks. Frame Relay does not have these mechanisms.

One thing in common with X.25 is that Frame Relay is usually connection-oriented . A separate virtual connection is established for each participant . However, so-called point-to-multipoint connections are also possible.

Frame Relay is an effective data transmission technology for data streams that have a constant bit rate, such as digitized speech. It is not particularly suitable for data transmissions with strongly changing traffic profiles or for multimedia. Nevertheless, because of its low cost, it is often used to connect LANs over long-haul routes . The error rate is clearly noticeable in load situations.

Frame Relay was developed to enable an efficient use of the existing technical resources. The provider can offer all customers a total of more transmission capacity than is available to him, since most customers do not always use 100 percent of their "line". In some market segments, Frame Relay got a bad reputation because of this - because some providers sold significantly more bandwidth overall than was actually available to them.

Frame relay is increasingly being replaced by ATM and IP- based products . In particular, virtual private networks are increasingly taking over the frame relay market segment as a cheaper alternative and MPLS networks as a broadband alternative.

Format of the data units

The data frame of a frame relay is structured as follows:

Flag (1 byte) Address (2–4 bytes, see below) User data (variable) FCS (2 bytes) Flag (1 byte)

In addition to the connection identifier, the Address field contains traffic parameters for handling congestion situations ( flow control ) and is structured as follows:

DLCI (6 bit) C / R (1 bit) EA (1 bit) DLCI (4 bit) FECN (1 bit) BECN (1 bit) DE (1 bit) EA (1 bit)

Explanations for the address field:

designation Explanation
DLCI Data Link Connection Identifier. Numeric value to identify the virtual connections.
C / R Command / Response.
FECN Forward Explicit Congestion Notification. Used to report traffic jams. A FECN message is sent to the recipient in the event of an overload.
BECN Backward Explicit Congestion Notification. Used to report traffic jams. A BECN message with the request to reduce the data rate is transmitted to the transmitter in the event of an overload.
DE Discard Eligibility Indicator. Data units for which this flag is set can be discarded in the event of an overload. This flag is set by the provider's frame relay switch when the purchased data volume has been reached. The flag can also be set by the customer and thus enables the data selected by the customer to be transmitted guaranteed, even in load situations.
EA Address field extension bit. If this flag is set, it means that further octets (bytes) follow in the header. This allows the head of the data unit to be expanded.
Maximum burst maximum number of bits in a certain period of time (bit / time)


  • Christoph Meinel, Harald Sack: Internetworking. Springer-Verlag, Berlin / Heidelberg 2012, ISBN 978-3-540-92940-6 .
  • Jonathan Chin: Cisco Frame Relay Solutions Guide. Cisco Systems Inc, Indianapolis 2004, ISBN 1-58705-116-8 .
  • John Cowley: Communications and Networking. Springer Verlag London Ltd, London 2007, ISBN 978-1-84628-488-5 .
  • Peter Bienert: Information and Communication. Springer-Verlag, Berlin / Heidelberg 2012, ISBN 978-3-642-63790-2 .
  • Erwin Rathgeb, Eugen Wallmeier: ATM - infrastructure for high-performance communication. Springer-Verlag, Berlin / Heidelberg 1997, ISBN 978-3-642-64373-6 .

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