Chapter 18
Virtual-Circuit Networks:
Frame Relay, ATM,
MPLS, and VPNs
18.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
18-1 FRAME RELAY
Frame Relay is a virtual-circuit wide-area network
that was designed in response to demands for a new
type of WAN in the late 1980s and early 1990s.
Topics discussed in this section:
Architecture
Frame Relay Layers
Extended Address
FRADs
VOFR
LMI
18.2
Frame Relay
What is so great about frame relay?
1. X.25, its predecessor, had a max. data rate of 64 kbps
2. X.25 has extensive flow and error control
3. X.25 doesn’t fit well into TCP/IP protocol stack
(remember the packet layer in X.25?)
4. Frame relay can handle bandwidth on demand using
SVCs
5. Frame relay can transfer data up to 45 Mbps
6. Frame relay is a layer two protocol
7. Frame relay is reasonably priced and allows 9000 byte
payloads
18.3
Figure 18.1 Frame Relay network
18.4
Note
VCIs in Frame Relay are called DLCIs.
18.5
Figure 18.2 Frame Relay layers
Only two layers in frame relay.
Actually, only data link defined.
18.6
Figure 18.3 Frame Relay frame
Looks like HDLC, except no Control field (no flow and error control).
Flag is 01111110. FCS is CRC.
DLCI is 10-bit data link connection identifier
C/R bit tells if this is a command or a response. Not used.
EA bit tells if this is an extended address
FECN - informs destination that congestion is occurring (more later)
BECN - informs source that congestion is occurring
DE - discard eligible bit (oh-oh)
18.7
Figure 18.4 Three address formats
18.8
Figure 18.5 FRAD
Voice over Frame Relay (VoFR) possible too!
18.9
Frame Relay (continued)
Frame Relay (continued)

Permanent virtual circuit (PVC) –
connection between two endpoints
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Created by the provider of the frame relay
service
The user uses a high-speed telephone line
to connect its company to a port, which is
the entryway to the frame relay network
The high-speed line, the port, and the PVC
should all be chosen to support a desired
transmission speed
Frame Relay (continued)
Frame Relay Setup

Consider a company that has four office
locations and currently has six leased lines
interconnecting the four locations
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To install frame relay, the company would ask
for six PVCs in place of the six leased lines
The company would also need four highspeed telephone lines and four ports
connecting the four locations to the frame
relay cloud
Frame Relay Setup (continued)
Frame Relay Setup (continued)
Committed Information Rate
(CIR)
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The user and frame relay service would
agree upon a committed information rate
(CIR)
The CIR states that if the customer stays
within a specified data rate (standard rate
plus a burst rate) the frame relay provider
will guarantee delivery of 99.99% of the
frames
The burst rate cannot be exceeded for
longer than 2 seconds
Committed Information Rate
(CIR) (continued)

Example – if a company agrees to a CIR of 512 kbps
with a burst rate of 256 kbps, the company must
stay at or below 512 kbps, with an occasional burst
up to 768 kbps, as long as the burst does not last
longer than 2 seconds

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If the company maintains their end of the agreement,
the carrier will provide something like 99.99%
throughput and a network delay of no longer than 20
ms
If the customer exceeds its CIR, and the network
becomes congested, the customer’s frames may be
discarded
Frame Relay vs. the Internet


Frame relay has many advantages over
the Internet, including guaranteed
throughput and minimum delay, and
better security
Internet has the advantage of being
practically everywhere, cheaper, and
simpler to create connections (no PVCs
necessary)

And Internet tunnels (VPNs) are attractive
18-2 ATM
Asynchronous Transfer Mode (ATM) is the cell relay
protocol designed by the ATM Forum and adopted by
the ITU-T.
Topics discussed in this section:
Design Goals
Problems
Architecture
Switching
ATM Layers
18.19
Figure 18.6 Multiplexing using different frame sizes
18.20
Note
A cell network uses the cell as the basic
unit of data exchange.
A cell is defined as a small, fixed-size
block of information.
18.21
Figure 18.7 Multiplexing using cells
Cleaner. Fixed buffer sizes,
uniform time spent on
each cell.
18.22
Figure 18.8 ATM multiplexing
Notice that ATM tries to not waste cell space
18.23
Figure 18.9 Architecture of an ATM network
UNI - user network interface
NNI - network network interface
18.24
Figure 18.10 TP, VPs, and VCs
VC - virtual channel
VP - virtual path
TP - transmission path
18.25
Figure 18.11 Example of VPs and VCs
18.26
Note
Note that a virtual connection is defined
by a pair of numbers:
the VPI and the VCI.
18.27
Figure 18.12 Connection identifiers
18.28
Figure 18.13 Virtual connection identifiers in UNIs and NNIs
18.29
Figure 18.15 Routing with a switch
18.30
Protocol Architecture
18.31
Figure 18.16 ATM layers
18.32
Figure 18.17 ATM layers in endpoint devices and switches
18.33
Protocol Architecture
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User plane
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Control plane
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Provides for user information transfer along with flow
control and error control
Performs call and connection control functions
Management plane

Plane management
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Layer management
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18.34
Management functions related to system as a whole; make
sure the various planes coordinate their activities properly
Provides operations, administration, and maintenance (OAM)
services thru info packets that switches exchange to keep
system running effectively
Figure 18.14 An ATM cell
18.35
Figure 18.18 ATM layer
18.36
Figure 18.19 ATM headers
18.37
Header Format

Generic flow control
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18.38
Used at user to network interface
Controls flow of data from user device into the
ATM network only
Essentially two classes of connections – controlled
and uncontrolled
Controlled – network provides info to user
regarding how many cells it can send – like a
credit mechanism for flow control
Uncontrolled – network simply enables or disables
sending of cells – like X-ON/X-OFF flow control
Header Format

Virtual path identifier

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Virtual channel identifier
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A 16-bit channel ID. Together, VPI and VCI identify
a logical connection
Payload type
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18.39
An 8-bit (UNI) or 12-bit (NNI) path ID
Various types of user info or network management
info
For example: leftmost bit identifies payload as user
data or admin info; second bit indicates whether cell
has passed thru any congested switches; third bit
might be used to indicate last cell in a sequence of
cells
Header Format

Cell loss priority
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Header error control

18.40
CLP bit indicates a cell’s priority level
If congestion occurs, ATM has option of deleting
cells to relieve congestion. Cells with CLP = 1 go
first.
See the following slides
Header Error Control


Provides for error checking on the header
only
Payload is unprotected. Is this a good idea?
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18.41
Fiber optic used – so low error rates
Some other layer can error detect the payload
Does it really make sense to error detect real-time
traffic?
ATM needs the speed!
Uses x8 + x2 + x + 1 checksum
Allows some error correction (single-bit
errors, which AT&T says happens 99.5% of
time)
HEC Operation at Receiver
(from the Stallings book)
As long as no errors are detected, receiver remains in Correction Mode.
When an error is detected, receiver will correct the error if it is a single
bit or will detect that a multi-bit error has occurred. In either case, the
receiver now moves to Detection Mode (because there may be a burst
of errors, a condition for which the HEC is insufficient for error correction).
18.42
Header Error Control

HEC can also be used for providing
synchronization

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18.43
Apply error-checking method using 40 consecutive
bits. If it does not generate a result consistent
with the last 8 bits, shift one bit and try again.
Repeat above step until a consistent result is
found. Could it be a coincidence? Try it
three more times. All four succeed? You are
in sync.
ATM Service Categories

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An ATM network can support many types of
traffic:
Real time
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Non-real time
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18.44
Constant bit rate (CBR)
Real time variable bit rate (rt-VBR)
Non-real time variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
CBR

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Fixed data rate continuously available
Tight upper bound on delay
Can support uncompressed audio and video

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18.45
Video conferencing
Interactive audio
A/V distribution and retrieval
Tightly controlled by Peak Cell Rate (PCR),
Cell Transfer Delay (CTD), and Cell Delay
Variation (CDV)
$$$$
rt-VBR

Time sensitive application
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18.46
Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that
varies with time
Examples include bursty voice and video
Can statistically multiplex connections
Parameters include Peak Cell Rate,
Sustainable Cell Rate, and Maximum Burst
Size
$$$
nrt-VBR
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18.47
Non-real time VBR
Intended for bursty traffic with no tight
constraints on delay and delay variation
Examples include airline reservations, banking
transactions
Parameters include Peak Cell Rate,
Sustainable Cell Rate, Maximum Burst Size,
Cell Loss Ratio, Cell Transfer Delay
$$$
ABR
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18.48
Application specifies Peak Cell Rate (PCR) and
Minimum Cell Rate (MCR)
Resources allocated to give at least MCR
Spare capacity shared among all ABR sources
Examples include LAN interconnection and
basic critical data transfer systems such as
banking, defense information
(flying standby)
$$
UBR
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18.49
For application that can tolerate some cell
loss or variable delays (non-critical apps)
Cells forwarded on FIFO basis
Do not specify traffic related service
guarantees
Examples include text/data/image
transfer, messaging, remote terminals
Best effort service (wear your parachute)
$
ATM Bit Rate Services
18.50
ATM Adaptation Layer


Essentially the “translation layer” between
ATM layer and other layers, such as PCM and
IP
PCM (voice)
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IP
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18.51
Assemble bits into cells
Re-assemble into constant flow
Map IP packets onto ATM cells
Fragment IP packets
Use LAPF over ATM to retain all IP infrastructure
AAL Protocols
18.52
Adaptation Layer Services

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18.53
Handle transmission errors
Segmentation and re-assembly
To enable larger blocks of data to be
carried in the information field of ATM cells
Handle lost and misinserted cells (cells
routed the wrong way)
Perform flow control and timing control
Supported Application types
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18.54
Four AAL protocols defined:
AAL 1: CBR traffic, e.g. circuit emulation (T-1 over
ATM), voice over ATM, real-time video
AAL 2: rt-VBR traffic, e.g. MPEG voice and video
AAL 3/4: nrt-VBR traffic, e.g. general data service
(not really used by anyone)
AAL 5 (successor to AAL 3/4): e.g. nrt-VBR: voice on
demand; nrt-VBR: frame relay, ATM; UBR: IP over
ATM
AAL 1
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18.55
AAL 1 is the interface between a realtime uncompressed byte stream and
ATM
Got to be fast!
No convergence sublayer, only SAR
sublayer
AAL 1 takes 46 or 47 bytes of data and
puts a one or two byte header on front
AAL 1 continued

AAL 1 header consists of following:
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18.56
One bit pointer – tells whether this is a one byte
header or a two byte header. If second byte is
included, this byte tells where the data starts
within the payload (in case the payload does not
contain a full 46 bytes of data)
Three-bit sequence number – used to tell if a cell
is lost or mis-inserted (which may be too late
anyway for real-time)
Four bits of error checking on preceding 3-bit
sequence number (yikes!)
Figure 18.20 AAL1
18.57
AAL 2

18.58
AAL 2 format is used for compressed
data (MPEG voice and video), so ATM
needs to indicate where each frame of
compressed data ends and begins
Figure 18.21 AAL2
18.59
AAL 3/4
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18.60
AAL 3/4 format originally designed to
support connection-oriented (3) and
connectionless (4) data services.
As ATM evolved, they discovered that
the fundamental issues of the two
protocols were the same.
AAL 3/4 mostly replaced with AAL5.
Figure 18.22 AAL3/4
18.61
AAL 5
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18.62
AAL 5 packets can be very large – up to
65,535 byte payload
AAL 5 not designed for real-time traffic
SAR sublayer takes the potentially large
convergence sublayer packets and breaks
them into 48 byte chunks, ready for the ATM
layer
SAR sublayer also adds a 32-bit CRC at the
end of the packet, which is applied to the
entire packet (see next slide for example)
Figure 18.23 AAL5
18.63
In Summary
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Frame relay
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ATM
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18.64
Up to 45 Mbps but usually slower
Local and long distance
Cloud computing
Being replaced by IP and MPLS
Fast!
Different classes of service!
Small cells
A Transition

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Use of Frame Relay is dropping off quickly
ATM is starting to die off too(?)
What is replacing these protocols?

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MPLS
VPN
65
Multiprotocol Label Switching
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An additional layer “added to” IP layer
Could say it operates at layer 2.5 between
the IP layer and the data link layer
Used to move Internet packets more
quickly through routers
Works like a Zip code
66
Multiprotocol Label Switching

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By using the MPLS label, the router does
not have to “dig in” so deep to retrieve IP
address
The 20-bit Label field is the key identifier
that connects this packet with a particular
flow of packets
67
Multiprotocol Label Switching

Four fields in an MPLS header:

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Label (as we saw on previous slide)
3-bit Traffic Class for QoS and congestion
notification
1-bit bottom of stack identifier
8-bit TTL field
68
Multiprotocol Label Switching
69
Multiprotocol Label Switching

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When a packet with no MPLS header
arrives at a Label Edge Router (LER), the
LER creates a label with appropriate
address – the address chosen can be
based upon more than just an IP address
(QoS too!)
Routers that simply route based on MPLS
label number are called Label Switch
Routers (LSR)
70
VPN (Virtual Private Network)

Many types of VPNs

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Trusted VPNs use non-cryptographic tunneling
protocols over a single-provider network, such
as MPLS, L2TP (layer 2 tunneling protocol),
and Microsoft’s PPTP (point to point tunneling
protocol)
Secure VPNs use cryptographic tunneling
protocols, such as IPsec, Microsoft’s SSTP
(secure sockets tunneling protocol), and Cisco
VPN and their DTLS
71
VPN (Virtual Private Network)

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For example, IPsec provides encrypted
transmission of packets over the IP layer
(similar to SSL-Secure Sockets Layer and
SSH - Secure SHell at the transport layer)
Applies a header (Authentication Header)
directly on top of the IP layer
72
In Conclusion

There are a number of ways to provide a
“tunnel” through a network
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Virtual LANs
Frame relay
ATM
MPLS
VPN
73