IMPROVING FTP APPLICATION PERFORMANCE OVER FRAME

Improving FTP Application Performance Over Frame Relay Network

Improving FTP Application Performance Over Frame Relay Network

Frame relay is a synchronous HDLC protocol based network. Data is sent in HDLC packets, referred to as "frames". The diagram below of an HDLC frame may be familiar, since without adding specific definitions of how the Address, Control and CRC is used, the diagram is applicable to IBM's SDLC, to X.25, to HDLC, to Frame Relay, as well as other protocols.

Address

Control

Data

CRC Error Correction

The protocol is similar to that of an X.25 network, except all circuits are permanently assigned. What is a circuit? A circuit is a link between user end points. In frame relay and X.25 networks, circuits are known as "permanent virtual circuits", or PVC's. The circuits are known as virtual because they are not electrical ciruits where there is a direct electrical connection from end to end. Rather, there is a "logical" connection, or virtual connection, where the user data moves from end to end, but without a direct electrical circuit.

X.25 circuits can be initiated and ended from the users terminals. Frame relay circuits are set up at the time of installation and are maintained 24 hours per day, 7 days per week. Frame relay circuits are not created and ended by user at their terminals or PC's. However, the user may have an application running over a frame relay circuit where computer to terminal sessions are initiated and ended by the user. These sessions are related to the application, not to the underlying frame relay network.

Frame relay relies on the customer equipment to perform end to end error correction. Each switch inside a frame relay network just relays the data (frame) to the next switch. X.25, in contrast, performs error correction from switch to switch. The networks of today are sufficiently error free to move the burden of error correction to the end points. Most modern protocols do error correction anyway, protocols such as SDLC, HDLC, TCP/IP, stat mux protocols, etc.

Network Characteristic

Frame Relay

X.25

Propagation Delay

Low

High

Error Correction

None, done by the terminal equipment at each end of the link

Node to Node

Protocol family

HDLC

HDLC

Good for interactive use?

Yes

Barely acceptable. Rather slow with one second or more round trip delay.

Good for polling protocols?

OK, sometimes, requires "spoofing"

Slow, even with spoofing

Good for LAN file transfer

Yes

Slow

Good for voice?

Good, standards developing

No

Ease of implementation

Easy

Difficult

Because frame relay passes blocks of data from switch to switch without error correction, propagation from customer end to customer end through the network is very fast. Propagation time in a DCB mux test at Wiltel (LDDS Worldcom) indicated a 70 millisecond round trip delay from Tulsa, Oklahoma to New York City and back. This is equal to or less than the propagation time through 9600 bps modems over the same distance. Similar propagation times have been measured by DCB over ...