florrie_Chapter_6 - Chatt

fullgorgedcutNetworking and Communications

Oct 24, 2013 (3 years and 1 month ago)

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Semester 4


Chapter 6
-

Frame Relay


Frame Relay

-

Industry
-
standard, switched data
-
link layer protocol that handles
multiple

virtual
circuits

using
HDLC encapsulation

between
connected devices.



Frame Relay is
more efficient

than X.25, the
protocol for which it is generally considered a
replacement
.


By using a
simplified framing

with no
error
correction

mechanisms over
high
-
quality digital
facilities
, Frame Relay can
transmit data very
rapidly
, compared to other WAN protocols.


Frame Relay uses
Layer 2 identifiers

(
DLCIs
) and
Permanent Virtual Circuits

(
PVCs
).

Frame Relay WAN technology provides a flexible
method of connecting LANs over Frame Relay WAN
links:



A ITU
-
T (CCITT) & American National Standards Institute
(ANSI) standard


Defines the process for sending data over a Public Data
Network (PDN)


Uses
Virtual Circuits

(VCs) to establish connections
across the WAN


A
connection
-
oriented

service
-

the complete path to
the destination is established prior to the sending of the first
frame

Frame
-
Relay Technologies


DLCIs

are used to
identify

Virtual

Circuits



Provides a means for multiplexing many logical data conversations
(referred to as virtual circuits) by assigning each pair of DTEs
connection

identifiers



Can
divide

a
single

Physical
WAN

Interface into
multiple

Subinterfaces



Operates at the
Physical

and
Data

Link

Layers of the OSI model but
it relies on
upper
-
layer

protocols

such as
TCP

for
error

correction



A data
-
link technology that is streamlined (
little

error

checking
) to
provide high performance and efficiency


Defines the interconnection process between the Customer Premises
Equipment (
CPE
) (also known as data terminal equipment
-

DTE
),
such as a router, and the service provider's local access switching
equipment (known as data communications equipment
-

DCE
)

Technologies Continued

Packet/Cell Switched Connections

Frame Relay
(key points)
:


Simpler

and
faster

than X.25


PVC

(Permanent Virtual Circuit)
only
, SVC (Switched
Virtual Circuit) has been specified


Access is at
56

kbps,
64

kbps, or
1.544

Mbps


Typically operates up to
T1



Very streamlined (
little

error

checking
)


Has none of the reliability features/complexity of X.25


Uses:


LAN
-
to
-
LAN connectivity


Remote access


Logical
DTE

(router) to logical
DCE

(Frame Relay switch)


Virtual Circuits (
VC
) are
end
-
to
-
end




Logical circuit created

to ensure reliable communication
between two network devices.


A virtual circuit is defined by a
VPI/VCI

(Virtual Path
Identifier/Virtual Channel Identifier) pair


Can be either
permanent

(a PVC) or
switched

(an SVC).


Virtual circuits are used in Frame Relay and X.25.

VC


(Virtual circuit)




Virtual circuit that is
permanently

established
.


PVCs save bandwidth associated with circuit
establishment and tear down in situations where
certain virtual circuits must exist all the time.

PVC
(Permanent Virtual Circuit)

Virtual circuit that is:



dynamically

established on demand


torn

down

when transmission is complete.


SVCs are used in situations where data transmission
is
sporadic
.


It is not used in Frame Relay, but it has been specified.


SVC
(Switched Virtual Circuit)

Any network device that provides a connection between
a LAN and a Frame Relay WAN.


Frame Relay Service and Circuits:


Data
-
Link Connection Identifier (
DLCI
)


Committed information rate (
CIR
)


Access rate is
56

kbps,
64

kbps, or
1.544

Mbps


One

physical access
interface



One

logical
hop

to any point


Implemented mostly as a
carrier
-
provided

service



Can also be used for
private

networks


FRAD
(Frame Relay Access Device)

A
'local

identifier'

between the
DTE

and the
DCE
, it identifies the logical
connection that is multiplexed into the physical channel.


Value that specifies a
PVC

in a Frame Relay network.


In the basic Frame Relay specification, DLCIs are
'locally significant'
.


In the LMI extended specification, DLCIs are
'globally significant'

(DLCIs
specify individual end devices).


The
FR Switch

maps

the
DLCIs

between each pair of routers to create a
PVC
.


DLCI values are typically
assigned

by

the Frame Relay
service

provider

DLCI
(Data
-
link Connection Identifier)


A
signalling

standard

between the
CPE device

and the
FR
Switch

that is responsible for
managing

the
connection

and
maintaining

"
status
"
between the devices.


Set of enhancements to the basic Frame Relay specification.
LMI includes support for:


'keepalive

mechanism'
, which verifies that data is flowing;


'multicast

mechanism'
, which provides the network server
with its local DLCI and the multicast DLCI;



global

addressing'
, which gives DLCIs global rather than
local significance in Frame Relay networks;


'status

mechanism'
, which provides an on
-
going status
report on the DLCIs known to the FR Switch.

LMI
(Local Management Interface)


The main purpose for the LMI process is:
(
management of the connection
)



PVC

status

-

What is the operational status of the various PVCs that the
router knows about?


Transmission of
'keepalive'

packets

-

Insure that the PVC stays up and
does not shut down due to inactivity.


Three types of LMIs are supported:

1.
cisco

-

LMI type defined jointly by Cisco, StrataCom, Northern
Telecom, and DEC (frame relay forum)

2.
ansi

-

Annex D defined by ANSI standard T1.617

3.
q933a

-

ITU
-
T Q.933 Annex A


LMI encapsulation types:

1.
IETF

Encapsulation Type

2.
Cisco

Encapsulation Type

LMI

CIR

(Committed Information Rate
-

The rate at which a Frame Relay
network agrees to transfer information under normal conditions,
averaged over a minimum increment of time. CIR, measured in bits per
second, is one of the key negotiated tariff metrics.


Local access rate

-

The clock speed (port speed) of the connection (local
loop) to the Frame Relay cloud. It is the rate at which data travels into
or out of the network.


Committed Burst

(Bc)
-

The maximum number of bits that the switch
agrees to transfer during any Committed Rate Measurement Interval
(Tc).


Excess Burst

-

The maximum number of uncommitted bits that the
Frame Relay switch will attempt to transfer beyond the CIR. Excess
Burst is dependent on the service offerings available by your vendor,
but is typically limited to the port speed of the local access loop.

Glossary

ECN

(Forward explicit congestion notification)
-

When a
Frame Relay switch recognizes congestion in the network,
it sends an FECN packet to the destination device
indicating that congestion has occurred.


BECN

(Backward explicit congestion notification)
-

When a
Frame Relay switch recognizes congestion in the network,
it sends a BECN packet to the source router instructing the
router to reduce the rate at which it is sending packets.


DE

(Discard Eligibility indicator)
-

When the router detects
network congestion, the FR switch will drop packets with
the
DE bit

set first. The
DE bit

is set on the oversubscribed
traffic; that is, the traffic that was received after the CIR
was met.

More Terms

(Three Phases)

1.
Connection

Establishment



a single path between source and destination devices is
determined


resources reserved to ensure a consistent rate of service


2.
Data

Transfer



data transmitted sequentially over the established path


packets arrive at the destination in the order sent


3.
Connection

Termination



terminate the connection between source and destination

Connection
-
Oriented Services

Frame Relay
'without'

SubInterfaces:


Early implementation of Frame Relay Technology required
that a router (DTE device) must have a WAN serial
interface for every (PVC) permanent virtual circuit.


Frame Relay SubInterfaces:


Subinterfaces

are required to configure
multiple

DLCIs

on a
single

router

interface
.


This involves logically
dividing

a
single

physical WAN serial
interface

into

multiple

virtual
subinterfaces



A single router interface can now service many remote
locations through individual unique suberinterfaces


Each
subinterface

is considered a
unique

network

and a
unique

DLCI

number

Subinterfaces

Reachability Issues with Routing Updates


Reducing

routing
loops

by:


Split

Horizon

-

Routing updates received at central router cannot be
advertised out the same physical interface to other routers.


Not a problem if there is only a single PVC on a physical interface,
because this would be more of a point
-
to
-
point connection type.


When running multiple PVCs over a single physical interface, this can
be a big issue.


A
single

physical

interface can be split into
multiple

logical

interfaces (Subinterfaces).


Subinterfaces

can
resolve

split

horizon

issues.


Routing

updates

can be sent out
subinterfaces

as if they were
separate physical interfaces.

Routing Updates







Good luck on the test !!!