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Frame Relay
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Frame Relay has become one of the most popular
WAN services deployed over the past decade
Primary reason is cost
Frame Relay technology frequently saves money over
alternatives, and very few network designs ignore the
cost factor
Frame Relay, by default, is classified as a nonbroadcast multi-access (NBMA) network
Which means that it does not send any broadcasts,
such as RIP updates, across the network by default
Frame Relay
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Frame Relay has at its roots a technology called X.25
Frame Relay essentially incorporates the components
of X.25 relevant to today’s reliable and relatively
“clean” telecommunications networks and leaves out
the error-correction components that aren’t needed
anymore
It’s more complex than the simple leased-line
networks such as HDLC and PPP protocols
It’s often represented as a “cloud” in networking
graphics
Introduction to Frame Relay
Technology
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Frame Relay is a
packet-switched
technology
Frame Relay
doesn’t work like a
point-to-point
leased line
(although it can be
made to look like
one
Frame Relay Technology
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The basic idea behind Frame Relay networks is
to allow users to communicate between two
DTE devices (in this case, routers) through DCE
devices
The users shouldn’t see a difference between
connecting to and gathering resources from a
local server and a server at a remote site
connected with Frame Relay other than a
potential change in speed
Figure illustrates everything that must happen
in order for two DTE devices to communicate
Introduction to Frame Relay
Technology
Frame Relay Technology
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1. The user’s network host sends a frame out
on the local area network
The hardware address of the router (default
gateway) will be in the header of the frame
2. The router picks up the frame, extracts the
packet, and discards what is left of the frame
It then looks at the destination IP address
within the packet and checks to see whether it
knows how to get to the destination network by
looking into the routing table
Frame Relay Technology
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3. The router next forwards the data out the interface
that it thinks can find the remote network
The router puts the packet onto the Frame Relay
network encapsulated within a Frame Relay frame
4. The channel service unit/data service unit
(CSU/DSU) receives the digital signal and encodes it
into the type of digital signaling that the switch at the
packet switching exchange (PSE) can understand
For example, it may alter it from the encoding used in
V.35 to the encoding of the access line, which might
be B8ZS over a T1
The PSE receives the digital signal and extracts the
ones and zeros from the line
Frame Relay Technology
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5. The CSU/DSU is connected to a demarc installed by
the service provider, and its location is the service
provider’s first point of responsibility (last point on the
receiving end)
The demarc is typically just an RJ-45 (8-pin modular)
jack installed close to the router and CSU/DSU
(sometimes called a Smart Jack)
6. The demarc is typically a twisted-pair cable that
connects to the local loop. The local loop connects to
the closest central office (CO), sometimes called a
point of presence (POP)
The local loop can connect using various physical
mediums; twisted-pair or fiber is common
Frame Relay Technology
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7. The CO receives the frame and sends it through the Frame
Relay “cloud” to its destination
This cloud can be dozens of switching offices—or more!
8. Once the frame reaches the switching office closest to the
destination office, it’s sent through the local loop
The frame is received at the demarc, and is then sent to the
CSU/DSU
Finally, the router extracts the packet, or datagram, from the
frame and puts the packet in a new LAN frame to be delivered
to the destination host
The frame on the LAN will have the final destination hardware
address in the header
This was found in the router’s ARP cache, or an ARP broadcast
was performed
Frame Relay Technology
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The user and server do not need to know, everything
that happens as the frame makes its way across the
Frame Relay network
The remote server should be as easy to use as a
locally connected resource
There are several things that make the Frame Relay
circuit different than a leased line
With a leased line, you typically specify the bandwidth
you desire (T1, fractional T1, DS3, etc.)
But with Frame Relay, you specify both an access rate
(port speed) and a CIR
Committed Information Rate
(CIR)
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Frame Relay provides a packet-switched network to many
different customers at the same time
This is a great idea because it spreads the cost of the switches,
etc., among many customers
Frame Relay is based on the assumption that all customers will
never need to transmit constant data all at the same time
Frame Relay works by providing a portion of dedicated
bandwidth to each user, and also allowing the user to exceed
their guaranteed bandwidth if resources on the telco network
are available
Frame Relay providers allow customers to buy a lower amount
of bandwidth than what they really use
There are two separate bandwidth specifications with Frame
Relay:
Committed Information Rate
(CIR)
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Access rate The maximum speed at which the Frame
Relay interface can transmit
CIR The maximum bandwidth of data guaranteed to
be delivered
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However, in reality, this is the average amount that the
service provider will allow you to transmit
If these two values are the same, the Frame Relay
connection is pretty much just like a leased line
However, they can also be set to different values
Committed Information Rate
(CIR)
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Here’s an example
Let’s say that, you buy an access rate of T1
(1.544Mbps) and a CIR of 256Kbps
By doing this, the first 256Kbps of traffic you
send is guaranteed to be delivered
Anything beyond that is called a “burst,” which
is a transmission that exceeds your guaranteed
256Kbps, and can be any amount up to the T1
access rate (if that amount is in your contract)
Virtual Circuits
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Frame Relay operates using virtual circuits, as
opposed to real circuits that leased lines use
These virtual circuits are what link together the
thousands of devices connected to the provider’s
“cloud.”
Frame Relay provides a virtual circuit to be established
between your two DTE devices, making them appear
to be connected via a circuit when in reality they are
dumping their frames into a large, shared
infrastructure
You never see the complexity of what is happening
inside the cloud because you have a virtual circuit
Virtual Circuits
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There are two types of virtual circuits—permanent and
switched
Permanent Virtual Circuits (PVCs) are by far the most
common type in use today
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Permanent means that the telco creates the mappings inside
their equipment, and as long as you pay the bill, they will
remain in place
Switched Virtual Circuits (SVCs) are more like a phone
call
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The virtual circuit is established when data needs to be
transmitted, then is taken down when data transfer is
complete
Data Link Connection Identifiers
(DLCIs)
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Frame Relay PVCs are identified to DTE end devices
using Data Link Connection Identifiers (DLCIs)
A Frame Relay service provider typically assigns DLCI
values, which are used on Frame Relay interfaces to
distinguish between different virtual circuits
Because many virtual circuits can be terminated on
one multipoint Frame Relay interface, many DLCIs are
often affiliated with it
Inverse ARP (IARP) is used with DLCIs in a Frame
Relay network
It maps a DLCI to an IP address (like ARP does with
MAC addresses to IP addresses)
Data Link Connection Identifiers
(DLCIs)
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When RouterA wants to send a frame to RouterB, it looks up the
IARP or manual mapping of the DLCI to the IP address it’s trying to
get to
Then it sends the frame out with the DLCI value it found in the DLCI
field of the FR header
The provider’s switch gets this frame and does a lookup on the
DLCI/physical-port combination it observes
Associated with that combination, it finds a new “locally significant”
(between it and the next-hop switch) DLCI to use in the header, and
in the same entry in its table, it finds an outgoing physical port
This happens all the way to RouterB
Therefore, you actually can say that the DLCI that RouterA knows
identifies the entire virtual circuit to RouterB, even though every DLCI
between every pair of devices could be completely different
The point is that RouterA is unaware of these differences
That’s what makes the DLCI locally significant
Data Link Connection Identifiers
(DLCIs)
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DLCI 100 is considered locally significant to RouterA
and identifies the circuit between RouterA and Frame
Relay switch
DLCI 200 would identify the circuit between RouterB
and its connected Frame Relay switch
DLCI numbers, used to identify a PVC, are typically
assigned by the provider and start at 16
Local Management Interface
(LMI)
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Local Management Interface (LMI) is a signaling standard
used between your router and the first Frame Relay switch
it’s connected to
It allows for passing information about the operation and
status of the virtual circuit between the provider’s network
and the DTE (your router)
It communicates information about the following:
Keepalives These verify that data is flowing
Multicasting This is an optional extension of the LMI that
allows, for example, the efficient distribution of routing
information and ARP requests over a Frame Relay network
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Multicasting uses the reserved DLCIs from 1019 through
1022
Local Management Interface
(LMI)
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Global addressing This provides global significance
to DLCIs, allowing the Frame Relay cloud to work
exactly like a LAN
Status of virtual circuits This provides DLCI status
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These status inquiries and status messages are used as
keepalives when there is no regular LMI traffic to send
LMI is not communication between your routers—it’s
communication between router and the nearest Frame Relay
switch
It’s entirely possible that the router on oneend of a PVC is
actively receiving LMI, while the router on the other end of
the PVC is not
Local Management Interface
(LMI)
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Routers receive LMI information from the service
provider’s Frame Relay switch on a frame
encapsulated interface and update the virtual circuit
status to one of three different states:
Active state Everything is up, and routers can
exchange information
Inactive state The router’s interface is up and
working with a connection to the switching office, but
the remote router is not working
Deleted state No LMI information is being received
on the interface from the switch. It could be a
mapping problem or a line failure
Frame Relay Congestion
Control
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Is there any way for us to find out when our telco’s shared
infrastructure is free and clear and when it’s not?
And if there is, how do we go about it?
Here are the three congestion bits and their meanings:
Discard Eligibility (DE) when you burst (transmit packets
beyond the CIR of a PVC), any packets exceeding the CIR are
eligible to be discarded if the provider’s network is congested
Because of this, the excessive bits are marked with a Discard
Eligibility (DE) bit in the Frame Relay header
If the provider’s network is congested, the Frame Relay switch
will discard the packets with the first DE bit set
So if your bandwidth is configured with a CIR of zero, the DE
will always be on
Frame Relay Congestion
Control
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Forward Explicit Congestion Notification (FECN)
When the Frame Relay network recognizes congestion in
the cloud, the switch will set the Forward Explicit
Congestion Notification (FECN) bit to 1 in a Frame Relay
packet header
This will indicate to the destination DTE that the path the
frame just traversed is congested
Backward Explicit Congestion Notification (BECN)
When the switch detects congestion in the Frame Relay
network, it’ll set the (BECN) bit in a Frame Relay frame
that’s destined for the source router
This notifies the router that congestion is being
encountered ahead
Frame Relay Congestion
Control
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Backward Explicit Congestion Notification
(BECN) When the switch detects congestion in
the Frame Relay network, it’ll set the (BECN)
bit in a Frame Relay frame that’s destined for
the source router
This notifies the router that congestion is being
encountered ahead