Frame Relay is a standardized WAN (Wide Area Network) technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. It was originally designed for transport across Integrated Service Digital Network (ISDN) infrastructure.
Frame Relay is a packet-switched network. A "frame", in the context of Frame Relay, is a packet of data. When a data stream enters a Frame Relay network it is broken into frames and each frame is sent across the network to the destination point. The frames contain header information which tells the intervening network nodes where to route them, and which output port to use.
Network providers commonly implement Frame Relay for
voice (VoFR) and data as an local area encapsulation technique,
used between Local area networks (LANs) over a Wide Area Network (WAN). Each end-user gets a private line to a Frame Relay node. The Frame Relay network handles the
transmission over a frequently-changing path transparent to all end-users.
Frame Relay has become one of the most
extensively-used WAN protocols. Its cheapness (compared to leased lines)
provided one reason for its popularity. The extreme simplicity of configuring
user equipment in a Frame Relay network offers another reason for Frame Relay's
popularity.
- it offers benefits over leased lines in terms of performance
- cost-saving
- manageability
- and resilience.
Network Connection
A user will typically have a private line to a node on a Frame Relay network. Line speed is fixed, and will be somewhere in the range of 56 Kbps to 2 Mbps depending on the service that has been purchased. The network itself is composed of lines connecting nodes. The receiving location also has a private line to a Frame Relay node. A permanent virtual circuit, or PVC, is defined to link the sending and receiving end points. The circuit is bi-directional. Frames are routed across the network from sender to receiver using header information which is added to the incoming data stream. It may be possible to have switched access to Frame Relay by, for example, dialling up an access point on the Frame Relay network over an ISDN interface. Data then flows from the user across an ISDN network and then into the Frame Relay network. Each logical connection from a site via ISDN uses a single ISDN channel, they cannot be multiplexed into one ISDN channel. This may be a cost-effective way of connecting remote sites with low data traffic rates to a Frame Relay network. All the nodes have entry and exit ports and a particular route through the network involves each node know-ing which exit ports to use for frames in a message. Each frame has a data link connection identifier (DLCI ) which is used by the nodes to choose the right exit port. A DLCI is not constant across the network. It is of only local significance to a Frame Relaynode. The routing tables in each node for a PVC take care of alternately reading and assigning DLCI values in frame headers before they send the frame on to the next node. When PVCs are first defined mis-installation of DLCI numbers can be a common error that prevents proper message transmission and receipt.
Frame Relay versus X.25
X.25 provides
quality of service and error-free delivery, whereas, Frame Relay was designed
to relay data as quickly as possible over low error networks. Frame Relay
eliminates a number of the higher-level procedures and fields used in X.25.
Frame Relay was designed for use on links with error-rates far lower than
available when X.25 was designed.
X.25 prepares and sends packets, while Frame
Relay prepares and sends frames. X.25 packets contain several fields used for
error checking and flow control, most of which are not used
by Frame Relay. The frames in Frame Relay contain an expanded link layer address field that enables Frame Relay
nodes to direct frames to their destinations with minimal processing. The
elimination of functions and fields over X.25 allows Frame Relay to move data
more quickly, but leaves more room for errors and larger delays should data
need to be retransmitted.
X.25 packet switched networks typically
allocated a fixed bandwidth through the network for each X.25 access,
regardless of the current load. This resource allocation approach, while apt
for applications that require guaranteed quality of service, is inefficient for
applications that are highly dynamic in their load characteristics or which
would benefit from a more dynamic resource allocation. Frame Relay networks can
dynamically allocate bandwidth at both the physical and logical channel level.
References:
Broadband Telecommunication Handbook, 2nd edition
www.att.com/interspan/overview/att00171.html
