Understanding the network.pdf - Back to Home

• Flag—This functions as a frame delimiter. It's an 8-bit field contacting the
binary number 01111110 at the beginning and end of the frame.
• Address—The address frame is a 16-bit field made up of the following
information: The first 10 bits are the DCLI address. This is followed by an EA
bit that indicates where the DCLI address ends. Then comes the C/R bit,
which is unused. The C/R is followed by congestion control (CC) subfield,
which is 3 bits in size. The first bit is the FECN, the second is the BECN, and
the last bit is the DE.
• Data—This field contains the ULP data being carried in the Frame Relay
frame. It can be up to 16,000 octets in size.
• Frame Check Sequence (FCS)—This is a checksum of the frame generated
by the source DTE. The destination DTE performs the same checksum test of
the frame upon delivery and compares the two values.
Caveat Emptor (Buyer Beware)
Frame Relay is commonly used by ISPs to provide fractional primary rate and
primary rate dedicated Internet access connections. This kind of connection is quite
cost effective for them because it places the burden of infrastructure cost on the
Frame Relay providers and does not require ISPs to maintain large numbers of
dedicated circuits. It is also popular for large enterprise WAN transport networks,
because it is cheaper than using dedicated T-carrier circuits. However, before you
run out and contract some Frame Relay circuits, you should consider two things.
First, Frame Relay is a packet-switched technology. It shares a finite amount of
bandwidth with a group of customers. Frame Relay networks have real resource
limitations from time to time. In times of heavy congestion, performance problems
will be directly perceived and your data will be dropped. There have also been some
rather significant mishaps in recent years by the major Frame Relay providers
(AT&T and MCI WorldCom) which have illustrated what exactly happens when a
Frame Relay cloud collapses. The answer is nothing, because your network is
completely down until the frame cloud is restored.
ATM
Asynchronous Transfer Mode is a WAN and LAN cell relay transport technology
developed for broadband network applications. The term "broadband" here is not a
reference to the signaling mechanism, but rather defines any WAN data network
application that utilizes a transport speed beyond that of the primary rate
(T1/E1/DS1). ATM is the transport mode for the ITU-T B-ISDN standard, which
provides standards defining the signaling, transport, and management aspects of
digital transmission services beyond the primary rate interface.