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Chapter 18

Virtual-Circuit Networks:Frame Relay, ATM, MPLS, and VPNs

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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18-1 FRAME RELAY18-1 FRAME RELAY

Frame Relay is a virtual-circuit wide-area network Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new that was designed in response to demands for a new type of WAN in the late 1980s and early 1990s.type of WAN in the late 1980s and early 1990s.

ArchitectureFrame Relay LayersExtended AddressFRADsVOFRLMI

Topics discussed in this section:Topics discussed in this section:

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Frame RelayFrame 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 usingSVCs

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 bytepayloads

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Figure 18.1 Frame Relay network

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VCIs in Frame Relay are called DLCIs.

Note

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Figure 18.2 Frame Relay layers

Only two layers in frame relay.Actually, only data link defined.

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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 identifierC/R bit tells if this is a command or a response. Not used.EA bit tells if this is an extended addressFECN - informs destination that congestion is occurring (more later)BECN - informs source that congestion is occurringDE - discard eligible bit (oh-oh)

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Figure 18.4 Three address formats

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Figure 18.5 FRAD

Voice over Frame Relay (VoFR) possible too!

Frame Relay (continued)

Frame Relay (continued)

Permanent virtual circuit (PVC) – connection between two endpoints 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 To install frame relay, the company would

ask for six PVCs in place of the six leased lines

The company would also need four high-speed 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) 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 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

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18-2 ATM18-2 ATM

Asynchronous Transfer Mode (ATM) is the Asynchronous Transfer Mode (ATM) is the cell relaycell relay protocol designed by the ATM Forum and adopted by protocol designed by the ATM Forum and adopted by the ITU-T. the ITU-T.

Design GoalsProblemsArchitectureSwitchingATM Layers

Topics discussed in this section:Topics discussed in this section:

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Figure 18.6 Multiplexing using different frame sizes

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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.

Note

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Figure 18.7 Multiplexing using cells

Cleaner. Fixed buffer sizes, uniform time spent oneach cell.

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Figure 18.8 ATM multiplexing

Notice that ATM tries to not waste cell space

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Figure 18.9 Architecture of an ATM network

UNI - user network interfaceNNI - network network interface

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Figure 18.10 TP, VPs, and VCs

VC - virtual channelVP - virtual pathTP - transmission path

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Figure 18.11 Example of VPs and VCs

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Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

Note

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Figure 18.12 Connection identifiers

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Figure 18.13 Virtual connection identifiers in UNIs and NNIs

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Figure 18.15 Routing with a switch

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Protocol Architecture

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Figure 18.16 ATM layers

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Figure 18.17 ATM layers in endpoint devices and switches

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Protocol Architecture User plane

Provides for user information transfer along with flow control and error control

Control plane Performs call and connection control functions

Management plane Plane management

Management functions related to system as a whole; make sure the various planes coordinate their activities properly

Layer management Provides operations, administration, and

maintenance (OAM) services thru info packets that switches exchange to keep system running effectively

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Figure 18.14 An ATM cell

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Figure 18.18 ATM layer

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Figure 18.19 ATM headers

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Header Format Generic flow control

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

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Header Format Virtual path identifier

An 8-bit (UNI) or 12-bit (NNI) path ID Virtual channel identifier

A 16-bit channel ID. Together, VPI and VCI identify a logical connection

Payload type 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

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Header Format

Cell loss priority 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.

Header error control See the following slides

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Header Error Control Provides for error checking on the header

only Payload is unprotected. Is this a good

idea? 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)

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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 singlebit or will detect that a multi-bit error has occurred. In either case, thereceiver now moves to Detection Mode (because there may be a burstof errors, a condition for which the HEC is insufficient for error correction).

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Header Error Control

HEC can also be used for providing synchronization 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.

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ATM Service Categories

An ATM network can support many types of traffic:

Real time Constant bit rate (CBR) Real time variable bit rate (rt-VBR)

Non-real time Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR)

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CBR Fixed data rate continuously available Tight upper bound on delay Can support uncompressed audio and

video 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)

$$$$

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rt-VBR Time sensitive application

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

$$$

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nrt-VBR 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

$$$

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ABR 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) $$

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UBR 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) $

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ATM Bit Rate Services

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ATM Adaptation Layer Essentially the “translation layer” between

ATM layer and other layers, such as PCM and IP

PCM (voice) Assemble bits into cells Re-assemble into constant flow

IP Map IP packets onto ATM cells Fragment IP packets Use LAPF over ATM to retain all IP infrastructure

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AAL Protocols

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Adaptation Layer Services 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

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Supported Application types 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

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AAL 1

AAL 1 is the interface between a real-time 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

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AAL 1 continued AAL 1 header consists of following:

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!)

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Figure 18.20 AAL1

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AAL 2

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

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Figure 18.21 AAL2

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AAL 3/4

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.

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Figure 18.22 AAL3/4

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AAL 5

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)

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Figure 18.23 AAL5

In Summary

Frame relay Up to 45 Mbps but usually slower Local and long distance Cloud computing Being replaced by IP and MPLS

ATM Fast! Different classes of service! Small cells

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A Transition

Use of Frame Relay is dropping off quickly

ATM is starting to die off too(?) What is replacing these protocols?

MPLS VPN

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Multiprotocol Label Switching 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

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Multiprotocol Label Switching 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

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Multiprotocol Label Switching Four fields in an MPLS header:

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

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Multiprotocol Label Switching

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Multiprotocol Label Switching 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)

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VPN (Virtual Private Network) Many types of VPNs

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

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VPN (Virtual Private Network) 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

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In Conclusion

There are a number of ways to provide a “tunnel” through a network Virtual LANs Frame relay ATM MPLS VPN