Frame Relay & ATM
Lecture 7
Paul Flynn
1
Virtual Circuit Switching
Digital Line Connection Identifier (DLCI)
Three Phases
Data Transfer Phase
Setup Phase
Teardown Phase
2
Virtual circuit wide area network
Frame Relay operates only at the physical and data link layers.
Frame Relay does not provide flow or error control; they must be provided by the
upper-layer protocols.
3
DLCI(Data-link Connection Identifier)
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
4
Frame Relay
Interface types
• UNI: User-|Network Interface
• NNI: Network-Network Interface
UNI
user
NNI
NNI
PVC segment
Frame Relay
network
Frame Relay
network
UNI
Frame Relay
network
user
Multi-network PVC
5
Frame Relay Functions
6
VCI
7
VCI phases
8
Switch and table
9
Frame Relay
Local addressing
• DLCI (Data Link Connection Identifier) - identification of a virtual
circuit
• DLCI - of local (for a given port) meaning
• there can be max. 976 VCs on an interface user-network
A
C
To A: DLCI 121
To B: DLCI 243
B
To A: DLCI 182
To C: DLCI 121
• DLCI values: 0 - LMI channel, 1-15 - reserved, 16-991 - available for
VCs, 992-1007 - layer 2 management of FR service, 1008-1022 reserved, 1023 - in channel layer management
10
Local Significance of DLCIs
The data-link connection identifier (DLCI) is stored
in the Address field of every frame transmitted.
11
Frame Relay
Architecture
Frame Relay Layers
FRAD
VOFR
LMI
12
Frame Relay network
VCIs in Frame Relay are called DLCIs.
13
Frame Relay layers
14
Frame Relay frame
15
Frame Relay
Global addressing
• Extension proposed by “Group of Four”
• Each end user access device FRAD is assigned a unique
DLCI number - a global address
Transmission to a given user goes over VC identified by a
unique DLCI
• Current DLCI format limits number of devices to less than
1000
• Another addition to the standard - extended DLCI
addresses
16
Three address formats
17
LAPF Frame – Address Field
6-bits
4-bits
18
FRAD
19
Frame Relay
Flow and congestion control
• There is no explicit flow control in FR; the network informs a user
about congestion
• Congestion: FR frames are discarded from overflowed buffers of
switching devices
• Congestion information:
– FECN - Forward Explicit Congestion Notification
– BECN - Backward Explicit Congestion Notification
Transmission direction
FRAD
BECN
FRAD
FECN
• There are recommendations for access devices what to do with FECN
and BECN (usually not implemented)
20
Frame Relay Concepts
Queue
21
Frame Relay Concepts
22
Frame Relay Concepts
23
FECN-tells receiving DTE device to
implement congestion avoidance
procedures
FRAMES
BECN-tells sending DTE device to
reduce the rate of sending data.
DLCI-identifies logical
connections on the
Frame Relay switch to
which the customer is
attached
Frame Relay
Parameters of a UNI interface
• Physical speed - just clock rate
• Guaranteed bandwidth parameters
– CIR: Committed Information Rate
– BC: Committed Burst Size
• Extended bandwidth parameters
– EIR: Extended Information Rate
– BE: Extended Burst Size
• TC: Measurement Interval
192kbps
User
traffic
EIR
256kbps
64kbps
CIR
time
25
Frame Relay
CIR and EIR - how does it work
• BC = TC * CIR
• BE = TC * EIR
Bits
Clock rate
BC+BE
CIR + EIR
BC
CIR
Time
T0
Frame 1
Within CIR
Frame 2
Within CIR
Frame 3
Marked DE
Frame 4
Marked DE
Frame 5
Discarded
T0+TC
26
Glossary
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.
27
More Terms
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.
28
Data Link Control Identifier
• The 10-bit DLCI associates the frame with its virtual
circuit
• It is of local significance only - a frame will not
generally be delivered with the same DLCI with which
it started
• Some DLCI’s are reserved
29
Frame Relay
Local Management Interface - LMI
• LMI - a signaling protocol used on an interface: end user - network
(UNI)
• Implementation optional (everybody implements it...)
• Usage:
– notification about: creation, deletion, existence of PVCs on a given port
– notification about status and availability of PVCs
– periodic checks of integrity of physical connection
• Planned extensions:
– dynamic (SVC) channel creation and deletion
– congestion notification
• Also planned: LMI for network-network interface (NNI)
30
LMI(Local Management Interface)
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.
31
LMI
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.
3.
ansi - Annex D defined by ANSI standard T1.617
q933a - ITU-T Q.933 Annex A
LMI encapsulation types:
1. IETF Encapsulation Type
2. Cisco Encapsulation Type
32
Local Management Interface (LMI)
• Three types of LMIs are supported by Cisco
routers:
Cisco — The original LMI extensions
Ansi — Corresponding to the ANSI standard
T1.617 Annex D
q933a — Corresponding to the ITU standard
Q933 Annex A
33
Frame Relay Map

The term map means to “map” or bind a
Layer 2 address to a Layer 3 address.



An ARP table maps MACs to IPs in a LAN
In ISDN, we use the dailer-map command to
map SPIDs to IP addresses
In Frame Relay, we need to map the data
link layer’s DLCI to the IP address

We use the frame-relay map command
Frame Relay Map



The Frame Relay switch builds a table of
incoming/outgoing ports and DLCIs.
The router builds a Frame Relay Map through
Inverse ARP requests of the switch during the LMI
exchange process.
The Frame Relay Map is used by the router for nexthop address resolution.
Frame Relay
IARP
• FRADs know DLCIs of available PVCs (through LMI),
but don’t know IP addresses of other ends
• IP addresses for given DLCIs are obtained automatically;
mapping IP-DLCI is generated - dynamic mapping
• IARP can be switched of; static maps have to be generated
by FRAD user
36
Configuring Basic Frame Relay
37
Configuring a Static Frame Relay Map
38
Reachability Issues with Routing Updates
in NBMA
39
Reachability Issues with Routing Updates
in NBMA
By default, a Frame Relay network provides nonbroadcast
multiaccess (NBMA) connectivity between remote sites.
An NBMA environment is treated like other multiaccess
media environments, where all the routers are on the
same subnet.
40
Frame Relay Subinterfaces
41
Configuring Point-to-Point Subinterfaces
42
Verifying Frame Relay
• The show interfaces command displays
information regarding the encapsulation
and Layer 1 and Layer 2 status. It also
displays information about the following:
The LMI type
The LMI DLCI
The Frame Relay data terminal equipment/data
circuit-terminating equipment (DTE/DCE) type
43
The show interface Command
LMI Status
LMI DLCI
LMI Type
44
The show frame-relay lmi Command
45
The show frame-relay pvc Command
46
The show frame-relay map Command
47
Troubleshooting Frame Relay
The debug frame-relay lmi Command
PVC Status
0x2 – Active
0x0 – Inactive
0x4 – Deleted
48
ATM
Design Goals
Problems
Architecture
Switching
Layers
49
Multiplexing using different frame sizes
A cell network uses the cell as the basic unit of data exchange. A cell
is defined as a small, fixed-sized
block of information.
50
Multiplexing using cells
51
ATM multiplexing
52
Architecture of an ATM network
53
TP, VPs, and VCs
54
Example of VPs and VCs
Note that a virtual connection is defined by a pair of numbers:
the VPI and the VCI.
55
Connection identifiers
56
Virtual connection identifiers in UNIs and NNIs
57
An ATM cell
58
Routing with a switch
59
ATM layers
60
ATM layers in endpoint devices and switches
61
ATM layer
62
ATM headers
63
AAL1
64
AAL2
65
AAL3/4
66
AAL5
67