Beginning in the 1980s transmission facilities migrated from analog cable and microwave to fiber optic cable. Data transmission went from an error prone environment to one largely free of errors. Prior to fiber, error checking was done as part of the transmission protocols. In the world of fiber, errors are so rare that error checking can be performed after reception at each end of a path.

Frame Relay (FR) is just another flavor of data service. It is commonly T1/E1 speed but can be T3/E3.  Customer access to the FR network is via router or a device called a Frame Relay Access Device (FRAD). The customer gets a Permanent Virtual Circuit (PVC) between points on the network. Unlike other types of service, FR costs are not based on distances. FR cost is based on:

·        Committed Information Rate (CIR) – Carrier guaranteed throughput over the PVC
·        Port Speed – The speed of the access port to the carries network in 64kb/s increments (speed determined  by customer)
·        Access circuit – The connection to the carriers FR network could be 56 kb/s, T1/E1, or T3 and could come from your LEC, a CLEC, or an AAC. The price will vary accordingly.

Frame Relay is widely available. FCC rules require LECs to connect to an IXC to extend FR beyond their LATA boundaries.  

Frame Relay Frame Format

DLCI – Data link connection identifier – Combining the DLCI from frames = the virtual circuit number.
EA – Address extension bit
C/R – Command/response bit
FECN – Forward explicit congestion notification – This is flow control
BECN – Backward explicit congestion notification – This is flow control
CRC – Cyclical redundancy check

Congestion Control 

·        FR protocol allows carriers to discard frames to avoid congestion
·        FECN and BECN used as flow control to reduce congestion. Not all carriers support it. 

Frame Relay Access

·        You can use routers or FRADS for customer access to FR network
·        FRAD has only one port and can only do FR, a lower cost than router.
·        Router can do multiple services via multiple ports, FR can be included

Most FR carriers do not have NNI. This means that your FR carrier cannot transition to another carriers FR network. Example: If you buy Sprint FR at a location, chances are you’ll have to have Sprint FR at your other location(s) also.

·      FR can be a dial up service. Appropriate for occasional uses. 
·        Major advantage of dial up FR is that you can connect in to more than one location (not simultaneously).

·        Special FRADS that can handle voice inputs
·        Voice frames cannot be marked as discard-eligible, they are given priority over data frames.
·        Committed Info Rate must be high enough to handle peak voice traffic while not discarding to many data frames (Increased CIR costs more money).

SNA is a protocol used in mainframe computer systems. It is not a routable protocol. To prevent needing a separate network to support SNA, it can be run via Frame Relay.

·        A router or FRAD encapsulates the SNA SDLC frame with a FR frame. The FR frame is stripped off at the distant end.
·        SNA via FR needs to be prioritized to minimize frame discard

·        Frame Relay accepted rapidly – more so than any other new technology
·        Since 1991 has become the default protocol for most data applications
·        All router vendors support it. Most LECs and big IXC’s offer FR service.

·        The Frame Relay Forum industry group sets FR standards
·        FR standards are recognized by the International Telecommunications Union (ITU)

When deciding to choose Frame relay there are multiple carriers to choose from. There are many issues to consider in making that choice. The most critical listed below.

·        Access circuit and Port Speed – What bandwidth do you need and what type of circuit will you need to access the carriers FR network?
·        NNI – What carrier(s) can provide you seamless NNI? Some FR features can be lost in the transition between LEC and IXC.
·        Point-of-Presence (Pop) – Where is the carries POP? Long access circuits can make FR expensive. 
·        Committed Information Rate (CIR) – maximum data throughput. Effects cost and amount of discarded frames. 

Should you use a router or a FRAD? A FRAD can send data over only one route. Choice of what device depends upon:
·        Buffers – must have enough to handle the traffic load (or you get too many discarded frames.
·        Voice – Do you need to carry voice traffic? Some FRADs cannot.
·        Protocols – What protocols do your LAN and WAN use. Not all routers can handle all protocols.