Frame Relay is a protocol used to establish WAN connection between remote locations. Frame Relay operates at Layers 1 and 2 of the OSI Model. There are many details to Frame Relay that you will need to understand in order to be prepared for the CCNA Exam and the ICND2 Exam.
Virtual Circuits
Frame Relay is capable of running many virtual circuits (VC) over one physical link. Let’s say you have three different locations that are all spread out long distances from each other. Rather than having two links on each router, one that connects to each location, with a network running Frame Relay you only need one link on each router that will connect to a service providers frame relay switching cloud. So across this one physical link you will have two virtual circuit connections for the other two routers on the other side of the frame relay switching cloud.
DTE/DCE
The frame relay router inside the company’s network is known as the Data Terminal Equipment (DTE) device, which will have one physical link into the frame relay switching cloud which is known as the Data Circuit-Terminating Equipment (DCE) device. The frame relay switching cloud is made up of many different DCE switches, that we don’t need to worry about. All we need to worry about is the DTE routers on each of the company’s different locations.
Data-Link Connection Identifier
The routers use Data-Link Connection Identifiers (DLCI) to keep track of each of the different links. The DLCI’s only have local significance, that means that the DLCI doesn’t mean anything to another DTE device on the opposite side of the frame relay switching cloud because it never sees it or gets passed the remote routers DLCI. So technically different links on opposite sides of the frame relay switching cloud could in fact have the same DLCI and cause no harm, but typically they will all be assigned unique DLCI’s to keep the network administrator from getting them mixed up.
Inverse ARP
The DLCI’s are mapped to the routers layer 3 ip address. A remote routers ip address can be dynamically determined by using inverse ARP (Address Resolution Protocol). The ip address and DLCI can also be statically mapped.
Local Management Interface
The link between the router and the frame relay switching cloud is managed through the Local Management Interface (LMI) which is in charge of sending keepalives and establishing a connection between the two endpoints. There are several different LMI versions. The default is cisco, the other two types are ANSI and Q933A.
PVC and SVC
The virtual circuit is the logical connection from one router going through the frame relay switching cloud and out to another router in a remote location. There are two different types of virtual circuits. One is called a permanent virtual circuit (PVC) which costs more money, but you get a dedicated connection that is always on between the two locations. If you currently are not sending data between the two locations the link does not go down it simply will be put in an inactive state, but can be brought up instantly.
The other type over virtual circuit is known as a switched virtual circuit (SVC) this is a cheaper option because you will not get a dedicated connection between the two end points. a switched virtual circuit behaves much like a phone call does. For each data transfer the session much be established and as soon as you close the session that switched virtual circuit connection no longer exists. So when you establish another sessions you most likely will end up taking a different route through the frame relay switching cloud than you did previously.
Commited Information Rate
When you set up frame relay with your service provider you will be given a committed information rate (CIR) which is the minimum speed guaranteed to you through which data will be transferred through the frame relay switching cloud. Of course if there is extra bandwidth available you will be able to take advantage of it. If the network begins to become congested any extra frames being sent beyond the committed information rate will be flagged with a discard eligibility bit. Any frame with the discard eligibility bit turned on will be discarded in times of congested to be sure that frame without it continue to get bandwidth needed to meet the committed information rate.
BECN and FECN
There are two more terms that have to deal with a congested network they are Backward Explicit Congestion Notification (BECN) and Forward Explicit Congestion Notification (FECN). The BECN is sent out by the switch inside the frame relay switching cloud to the router inside the company’s network notifying it that it needs to slow down its rate of transfer. This type of notification occurs in the opposite direction that the frame are being sent which is why it uses the term ‘backward’.
FECN is used to notify the receiving device that it is experiencing some congestion so that it can notify upper layer protocols that a delay may occur.
Recap
Just as a recap, here is a list of terms that you should have learned about and that you will need to understand for the CCNA Exam and ICND2 Exam:
Frame Relay, DLCI, VC, SVC, PVC, LMI, BECN, FECN, DE, CIR, Inverse ARP, DTE, DCE
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