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Seven

WAN Links and Backbone Protocols

Common Carrier Services for Building a WAN

1994-98 Mendes DataComm Insights

Mill Valley, California (415) 381-5500All Rights Reserved

Bridges, Switches, and Routers: Planning and Designing an Intranet

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Objectives

Participants will be able to understand and explain the choices available to a network designer when selecting a wide areanetwork backbone technology. In particular, participants will be able to compare and contrast the relative merits of

conventional T1/T3 private line networks with newer packet and cell-based switched networks such as Frame Relay, ATM,

and SMDS.





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Glue for Wide Area Networks

Communications services provided by local telephone companies and common carriers form the backbone of wide areanetworks, assuring that packets can move as easily around the globe as they do between floors of an office building. The

services include the private lines (the oldest and most commonly used service), ranging from 2400 bps to 45 mbps, ISDN,

X.25, Frame Relay, SMDS, and ATM (the newest and least used service).

From almost no choice

Until the late 1980s, choosing the right network service was pretty easy. The network designer only had two choices. For high

bandwidth and fast response time, private line networks were the only choice. On the other hand, connecting hundreds of

locations together with private lines is a pricey undertaking, leading to the other choiceX.25 packet switched networks which

provide fault tolerant, reliable communications by means of a shared network cloud which transparently delivers packets

from any host to any host.

To Let me count the ways. . .

The network designer of the mid-1990s has several new choices in selecting wide area connectivity tools. In a note of

serendipity, the user demand for faster speeds1 has been met by a telecommunications industry rushing to deliver newer, faster,

and cheaper services that can glue WANs together.

Its an interesting confluence of needs and availability. During the 1980s, the telecommunications industry was planning new,

high speed switching fabrics that would be able to switch data transmissions, video signals, and electronic images as well as

ordinary telephone calls. At about the same time, the seeds were being planted for the new applications that would demand

high bandwidth, many-to-many connectivity.

1 Driven by newer applications that use more and more bandwidth to transmit information between pairs of computers. Examples include image

files, multimedia (for instance, desktop-to-desktop video and voice through the computer), and even simple files transfers, as users discover

volumes of information stored on distant computers that were once considered inaccessible.





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Glue for Wide Area Networks

Communications services form the backbone of the WAN

Private lines and X.25the only choices up through the 1980s

Late 1990s present new choices

Frame Relay, ATM, SMDS, & ISDN





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Private Line Services

In Chapter 4, we talked about several private line service offerings, generally available from both local telephone companiesand interexchange carriers. Before considering Frame Relay, ATM, or SMDS, it may be helpful to take a closer look at some

of the underlying technology used to provide dedicated T1/E1 and T3/E3 (and fractional) services.

Getting from here to thereThe central offices of both local telephone companies and interexchange carriers have two major sub-systems, developed

primarily for routing and delivering ordinary telephone calls. Switching systems provide circuit switching (for telephone

calls), setting up temporary paths through the telephone network for the duration of a single telephone call; some switching

systems, however, provide packetor cellswitching (initially for data transmission, but soon for multimedia connections as

well), setting up temporary paths through the telephone network for each packet or cell.2

Transmission systems are the fiber optic, microwave, and satellite systems used to connect central offices to one another, andto customer sites as well.

How private lines are builtIn the example on the facing page, a router in San Francisco is connected to another in New York City. A private line provides

the connectivity, using the transmission systems of three telecommunications service providers. Note that the switching

systems, present in all central offices, do not participate in this connection. Rather, technicians for each of the providers make

permanent connections in each of the central offices, linking one transmission system to the next. The local loops, connecting

the customer sites to the nearest central office, are also hard-wired into to the transmission systems.

2 The term cell emerged in the 1980s as the unit of data which is moved through the network. The cell has a standard, fixed length of 53 bytes (or

octets), including 48 bytes for user information (called the payload), and 5 bytes for addressing and other overhead. If you think of the cell as a

small packet of fixed length, you wont be far off.





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Private Line Services





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Private Line ServicesPrivate lines provide point-to-point links between pairs of routers, affording the simplest, most direct means of providing

connection.

24 x 7 service

When you order private line services, you have the full use of the link, essentially guaranteed 24 hours per day, 7 day per

week. If you order T1 service, you can be sure that your routers can transmit to each other at the full 1.536 mbps, hour after

hour, day after day, forever. Traffic from other companies or government agencies (using the same carrier) will never impact

your throughput (and yours will never impact theirs).

Typical speeds

56/64 kbps lines, T1/E1 lines (1.536/2.048 mbps), and fractional T1/E1 lines (N x 64 kbps) are all examples of private lines.For higher speeds, most carriers offer T3/E3 lines (45/34 mbps) as well as fractional variants, or even OC3 (155 mbps) or

OC12 (622 mbps).





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Private Line Service

Point to point links

24 x 7 service

Typical speeds are 56 kbps to 1.536/2.048 mbps (T1/E1)

Higher speeds T3/E3, OC3, and OC12 are available





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Advantages of Private Lines

Traffic requirements drive network choicesThe most challenging requirement facing the design team of a wide area network is to provide enough throughput in the

network to deliver good performance for the users, without spending too much money. As the explanations and exercises in

earlier chapters have made clear, the task requires an understanding of the traffic sources and sinks that drive a network. Once

the team understands the behavior of traffic, it must select routers that will support the required level of activity, and provide

alternate routing in the event of a failure.

Zero packet lossPrivate lines do not discard packets. Ever! When congested, Frame Relay, ATM, and SMDS networks will discard packets,

relying on your routers or computers to retransmit.

Looking for predictable performanceIt is always much simpler for a designer to understand end-to-end performance of a network when all the components in the

network have fixed, perfectly predictable throughput limitations. Ethernet and Token Ring LANs offer this predictability.

Routers offer it too. Private line links between routers also offer the same, perfectly predictable performance. If a half T1 link

connects two routers, the designer knows that 768 kbps is always available.

Design process is easy (or easier)With a number of new services available and competing for the job of handling traffic between pairs of routers, designers will

continue to choose private lines in many cases because they are easy to understand and to implement. The newer services,

such as Frame Relay, ATM, and SMDS will all work, but additional layers of complexity to the design process because they

all rely on allocating a shared resource (the switching systems) among competing users (the data packets or cells).





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Advantages of Private Lines

Cardinal rule of network design (part 2)

Zero packet loss

Designers want predictable performance in every component

Private lines make the design process easier





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PPPThe Point to Point ProtocolIf the designer chooses private line links between routers, then he or she should also select a standards-based link protocol

designed to run over point to point links. The Point to Point Protocol, defined by RFC 1661, is an industry standard designedto provide logical connectivity between routing protocols in two neighboring routers.

The private line (and DS1 framing) provide the physical connectivity. PPP provides a standard method for transporting

multi-protocol datagrams over point-to-point links.3

Before PPP

In the absence of any industry standard prior to PPPs general adoption, router vendors designed their own HDLC link

protocols, making it impossible for the routers of two different companies to talk to each other over a point to point link.

What does PPP do?

There are three capabilities that PPP provides:

1. Manages the link. Provides logical connection between the two routers on start-up, terminates the connection if the

link should fail, and assure that the link performs properly.

2. Handles encapsulation. Multiplexes different network layer protocols simultaneously over the same link, in a

recognized manner, so that the peer router can determine which network layer packet is contained in each frame, and

deliver the contents to the appropriate routing software upon receipt.

3. Implements protocol-specific handling. Network control protocols, developed for each of the network layer protocols

being routed, manage specific details required for each protocol.

3 Quoted from the RFC.





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PPPThe Point to Point Protocol

Provides logical connectivity between two neighboring routers

Standards-based (RFC 1661)

PPP provides:

1. Link management

2. Multiprotocol encapsulation (i.e.multiplexing)

3. Protocol-specific handling





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PPP Operation

PPP Frame

The Point to Point Protocol is based on standard HDLC framing, and has the format shown on the facing page. The fields are

defined as follows:

FLAG. The standard HDLC sequence, 01111110 (Hex 7E).

ADDRESS. PPP requires that the address be 11111111 (Hex FF).

CONTROL. All frames are unnumbered information type (UI).

PROTOCOL. Identifies the layer 3 protocol which is encapsulated in the PPP frame, using the values

defined in RFC 1340 for Assigned Numbers. For example, 0021 indicates IP data

packets, while 002B indicates Novell IPX data packets.

INFORMATION. Contains the link establishment and maintenance, and disconnect commands during set-

up or disconnect. Contains the encapsulated layer 3 data packets once logical connection

is established. The RFC specifies that the maximum transmission unit (MTU) for PPP

must be 1500 octets or greater.

FCS. Can be either 16 or 32 bit CRC.

PPP State Diagram

The purpose of PPP is to allow routers to open logical links with one another, and exchange data frames. The state diagram

illustrates the process, beginning with a dead link. PPP then attempts to Establish a link with the neighboring router, thenmust Authenticate itself to its neighbor, and finally begin the process of transferring network layer information (denoted by

the box labeled Network).





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PPP Operation

PPP Frame

Flag Address Control Protocol Information FCS Flag

Number of Bytes 1 1 1 2 0 - 1500 2 or 4 1

PPP State Diagram

Link Dead Establish AuthenticateUP OPENED SUCCES

FAIL

CLOSING

FAIL

DOWN Terminate Network





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Frame or Cell-based Switching Schemes

Although private lines offer the designer an easy solution, and provide reliable, predictable performance, a number ofrelatively recent schemes based on packet or cell switching have emerged to offer viable alternatives to the fixed bandwidth

and topology offered by private lines. These newer schemes offer the potential of significant savings (for long-haul transport

of messages) and offer a strikingly high degree of fault tolerance.4 They have in common the use of new switching systems,

located in the central offices of local telephone companies and interexchange carriers.

Connecting many to many

The telecommunications industry has built its well deserved reputation for extraordinary reliability on the simple premise that

intelligent switching systems located in central offices can connect any telephone in the world to any other, by the use of a

call-setup process at the beginning of each telephone call. Whats more, the architecture of telephone networks in developed

countries have evolved with multiple alternative routes between major switching centers, offering the strong likelihood that our

calls will go through on the first attempt nearly every time.

Whats different about frame or cell-based switching?

Different switching systems, for one thing. The switch fabric that works so well for telephone calls is not particularly suitable

for handling the kind of routing decisions required. Frame Relay, ATM, and SMDS all use some form of packet switchingthe switch fabric reads the addressing information contained in the header of every packet or cell and makes instantaneous

routing decisions for each chunk.5

4 We shouldnt forget one thats been around for a whileX.25.5 Not unlike the process going on in the router, except that Frame Relay, ATM, and SMDS are all Layer 2 switching technologies, while routers

switch at Layer 3.





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Frame or Cell-Based Switching Schemes





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The users perspective

The illustration on the previous page presents the carriers view of how traffic is delivered between a single pair of routers

when using packet or cell-based switching. Its important to see the physical topology and to compare it to the private line

solution, but it doesnt really help in understanding the effect that this new telecommunications infrastructure has on the design

of your wide area network. Its important to view the network from the perspective of the user.

Ubiquitous connectionsfrom anywhere to anywhereOn the facing page we see a new, and elegant network topology. Gone are the multiplicity of T1 links between city-pairs.

Gone are the requirements for multiple WAN ports in every router. The wide area network is an intelligent cloud thataccepts packets or cells from any number of routers, and delivers each to the correct destination at the far side of the cloud.

The need to plan for adequate bandwidth and redundancy is given over to the local telephone companies and the interexchange

carriers! Or so it would seem.

How savings derive from packet or cell-based switching

A major portion of the cost in a wide area network is the provisioning of theT1/E1 (or FT1/FE1) access channels required to

connect a router to the long-haul network of the interexchange carrier. If we can recall the diagrams in Chapter 5 illustrating

the full mesh network connecting four cities, we will see that each router requires three T1 ports (in the router itselfabout

$1,000 each), three DSUs (another $1,000 each), and three access channels (about $125 to $1,000depending on the distance

spanned by the Access Channel, and on the RBOCper month each). Now consider the diagram presented here. Although

every city has links to every other city, those links are managed invisibly, inside the cloud, by the telecommunications service

provider. Each router requires only one port into the cloud, and only one access channel.

It is through this scheme that substantial economies can be derived.





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Design Concerns

Sharing the access channel

Although lower costs result from the ability to share a single access channel into the switched network cloud, obviously the

traffic to and from any one router must fit on a single access channel. The designer must now be able not only to estimate

the peak load to each of the other cities, but also to understand how all remote destinations, taken together, drive the router port

and access channel at peak load. Any margin of error (gained by using multiple access channels) formerly available to

overcome unforeseen traffic peaks in traditional point-to-point networks evaporates in this new scheme. The design team is

forced to do more careful data gathering and make more accurate estimates.

Congestion problems in switches and cell discard

The switches used in Frame Relay, ATM, and SMDS networks are engineered to carry a predicted peak cell arrival rate.Although service providers make every effort to model activity of large groups of customers accurately (and engineer their

switches accordingly), no model can be exactly correct under all conditions. Bursts will occur in data traffic that momentarily

exceed a switchs design limits, causing temporary congestion. Buffers can fill up, and cells will be discarded. Well designed

switched data networks regularly maintain packet discard rates of 1 x 10-6 or below.

If a single cell is discarded, applications, transport layer protocols, or the routers will discover the incomplete transmission, and

retransmit an entire packet (or PDU), not just the one missing cell. Of course, the retransmitted PDUs may compound the

congestion, creating the possibility of unstable performance.

Some variability of propagation time and latency

Propagation time (how long an electrical or optical signal takes getting from here to there) and latency (how long the network

elementsswitches, for examplespend processing a packet or cell) are invariant when private lines are used as links. Packet

and cell-based switching systems, such as Frame Relay, ATM, and SMDS will introduce variability in both propagation time

and latency, creating some mathematical uncertainty in the network design. The effects of this uncertainty are usually not

large, but can become sizable during periods of congestion, and should not be totally discounted.





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Latency and propagation delay in terrestrial-based transcontinental private line circuits here in North America is typically on

the order of 27 to 30 milliseconds (for each direction). End-to-end latency and propagation delay in switched data networks

(Frame Relay, ATM, and SMDS) can range between 45 to 120 msec in North America.





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Design Concerns

All traffic must share the single access line during peak traffic

Congestion problems in switches and cell discard 1 x 10-6

Some variability of propagation time and latency

45 to 120 msec





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Congestion, Latency, and Cell Discard

A simplified view of a cell-based switch is shown on the opposite page. Cells are arriving from several input queues, and areall destined for the same output queue.

When everything works OK

Cells are buffered momentarily at each input queue (San Francisco, St. Louis, and Dallas ports) until the processor can move

them into the switching fabric. Once inside the switching fabric, each cells addressing information is read and compared with

a routing table that associates each address with an output queue (or port).

As long as the combined arrival rate of all cells destined for the same output queue is less than or equal to the speed of the

output port, cells are moved out in an orderly fashion, although almost all of them will have to wait at least a fraction of one

cell time (the time for one cell to be transmitted onto the wire) in the buffer

until its their turn to actually hop onto thewire.

Delay is introduced

Now imagine that cells arrive at the output queue (headed for Boston) faster than the speed of the port. The buffer begins to

hold the cells, sending them as rapidly as the wire permits. Each cell is delayed in this output queue. The amount of delay

depends on the speed differencehow much faster do cells arrive than they departas well as on the duration of the burst. The

switch is momentarily congested.

The bit bucket

If the burst goes on too long, and the output buffer fills up, cells will be discarded, not just delayed.





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Congestion, Latency, and Cell Discard





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Frame Relay

Frame Relay sales have exploded since the mid-1990s, displacing many existing private line networks, and winning newbusiness as well. From a technical perspective, Frame Relay provides an excellent transport mechanism for linking LANs and

hosts across the country, or around the globe.

Whos a candidate for Frame Relay?The natural market for Frame Relay is the corporation, government agency, or institution with a number of LANs located in

different cities, states, or countries. LAN to LAN traffic and LAN to host traffic patterns are a good match for the operational

characteristics of Frame Relay. Having a number of locations (at least three or four), all of which need to exchange

information with each other, at widely separated distances is usually a good decision criterion for selecting Frame Relay.

Business Communications Review (July 1997) reported that users might find cost savings between 20% and 50%, when using

Frame Relay to build wide area networks, as compared to comparable private line solutions.Whos not a candidateFrame Relay can become expensive and is very difficult to implement and manage when the number of sites which must

communicate directly with each other gets large (perhaps 100 to 150 fully meshed locations). Furthermore, until recently,

Frame Relay switches havent included the ability to provide fractional or full T3 speeds T1 has been the highest speed

available. In these cases (very large networks or very high speeds), the implementer should consider ATM or SMDS as a

better choice.

From an operational perspective, applications which need real-time throughput (such as process control, video, or voice)

cannot be properly supported on Frame Relay, due to the inherent performance limitations of the protocol.





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Frame Relay

Accounts for as much as 1/3 of data networks

LAN to LAN connections and LAN to host connections are good applications

Frame Relay is not suitable for large, fully meshed networks, very high

speeds, or real-time applications





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Pricing Frame Relay Ports and PVCsOn a national scale, all of the major carriers offer Frame Relay service. AT&T, CompuServe, Sprint, and WorldCom all offer

ubiquitous service throughout the U.S.6 In addition, most RBOCs, GTE and many larger independent telcos offer Frame Relay

for Intra-LATA service.

Varying price strategies and technical characteristics among carriersMost providers charge for two pricing elements:

1. The physical port attaching to their Frame Relay networks (i.e.FT1, T1, FT3 or T3 access channel). Each router or

host connecting to Frame Relay will require at least one port. Ports are available for access speeds ranging from 56

kbps to 45 mbps.7 The higher the access speed, the more expensive is the port.

2. The permanent virtual circuits (PVCswhich provide the logical connections) between pairs of routers or between

hosts and routers. Carriers configure permanent logical pathsthe PVCsbetween all pairs of devices which mustcommunicate directly when Frame Relay is installed. This is the means by which full mesh topology is provided inside

the cloud.

PVCs are also priced by speed, ranging from 0 kbps to fractional T3 speeds of as high as 34 mbps (some carriers also

add a distance element to the price of a PVC). It is here that the network designer must use care in estimating the

traffic between pairs of locations. Although any one PVC can transmit bursts up to the full speed of the port, the Frame

Relay service provider may drop frames that exceed a committed information rate (CIR)the speed of the PVCif

their network is too busy to handle the momentary overload.

6 Stentor and Unitel also offer the service in Canada.7 T3 port speed is still relatively new, and may not be available from all carriers or in all areas.




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Pricing Frame Relay Ports and PVCs





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Pricing Comparison

In order to demonstrate the pricing concepts concretely, lets compute the costs for three alternatives to link a single pair oflocationsT1, fractional T1, and Frame Relay. In this example, we will use prices for AT&T services for all three

alternatives.

San Francisco to New York T1

We obtained the price for a T1 private line, including access channels on both ends, from the normal AT&T interstate tariffed

rates. Customers with special pricing plans will have different results.

T1 private line $14,334

San Francisco to New York FT1 (768 kbps)

We again obtained the price for a fractional T1 line, including access channels on both ends, from normal AT&T tariffed rates.T1 private line $11,469

San Francisco to New York Frame Relay

The prices reflect actual tariffed rates provided by AT&T; however, actual customer pricing for the Frame Relay ports and the

PVC tend to be somewhat flexible (unlike the access channels, which are largely driven by RBOC pricing policies). Look for

greater flexibility in access channel pricing in 1997, when local competition may tend to drive down these prices.

1 T1 access channel for San Francisco $525 525

1 T1 access channel for New York 886 886

2 1.544 mbps Frame Relay ports 2,468 4,9361 1.024 mbps PVC 2,227 2,227

TOTAL $8,574





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Pricing Comparison

San Francisco to New York T1 $14,334

San Francisco to New York T1 $11,469

San Francisco to New YorkFrame Relay with 1.024 mbps PVC $8,574





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Frame Relay Design Alternatives

Full Mesh

The Frame Relay network in the diagram at the left illustrates the full mesh design alternative. Each router has six PVCs, so

that it has a direct logical (not physical) connection with all other routers in the network. Although this design seems like a

reasonable choice at first glance, there are concerns in implementing it. A casual look reveals one issue. The number of PVCs

grows dramatically with the number of routers, increasing the cost and complexity of the network.

Another concern is a subtle problem having to do with to do with the way routing algorithms recognize resources in the router.

Generally, the routing algorithm sees the physical V.35 interface and Frame Relay protocol as a homogeneous resource.

Although individual packets are mapped to the correct PVC by the Frame Relay implementation in the router, the routing

algorithm itself is usually unaware of the granularity at the interface. There are work-arounds that overcome most of the

limitations, at the expense of added configuration effort by the router installation team.

Hub Router

The diagram at the right side of the page illustrates a common solution being implemented in large Frame Relay networks. In

this alternative, a high powered router sits at the center of the network, forwarding all traffic to the correct destination router.

Each outlying router uses a single PVC to forward packets toward the hub. Both problems described above are solved. The

number of PVCs is small, and, because theres only one PVC, the routing algorithm has full knowledge of the port

configuration.

If this network architecture is used with OSPF, IS-IS, or NLSP (link state protocols), the hub router requires special

configuration to manually configure network layer addresses for each PVC.





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Frame Relay Design Alternatives

Full Mesh Hub Router

( )Number of PVCs =

N N 1

2

where N is the number of sites

How many PVCs would be required for a full mesh if the

network consisted of 25 routers? 50 routers?

Number of PVCs = N

where N is the number of router sites excluding the

hub

How many PVCs would be required for a hubbed network

of 25 routers? 50 routers?





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Router Unicasting and Frame Relay

Routers of course maintain a separate routing table and/or link state database for each protocol being routedTCP/IP,NetWare, AppleTalk, DECnet, and so on. As you will recall from Chapter 6, a key part of every routing algorithm is the

requirement for periodic update messages sent by each router to all peers to which it is directly linked. These update messages

provide automatic propagation of reachability information, and allow each router to construct a table or database, which tells

the routing protocol how to forward packets to all known networks and subnets. In addition, NetWare, AppleTalk, and VINES

networks involve regular timed broadcast packets to enable workstations to find and connect with various services.8

In the traditional point to point network, each WAN interface has only one peer router at the far end of a link. So a router with

three physical links to its peers sends regular routing update messages (or link state messages) to just one other router on each

link.

In a Frame Relay network, however,each router has many peers, and must send these updates and broadcasts to each. In thediagram, Router A sends a RIP packet (advertising all IP networks in city A) to Router B, then a second, identical packet is

sent to Router C, a third to D, and so on. The sequence is repeated every 30 seconds. But thats not allthe Novell IPX

networks are identified with Novell RIP packets which are also sent, in order, to each of the peer routers (usually at 60 second

intervals). Finally, Novell Service Advertising Protocol packets are sent to each of the peer routers (also at 60 second

intervals). Even though the contents of each RIP or SAP packet is identical, router A sends each one five times, varying only

the DLCI which uniquely identifies each router to the frame relay network. 9 The implementers of this network chose to use

the point-to-point paradigm in describing the frame relay cloud to the routers, failing to take advantage of an addressing

method known as multicasting.

8 Which are sent as router-to-router unicast packets across the WAN cloud.9 AppleTalk routers send RTMP packets (the equivalent of RIP) every 10 seconds.





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Router Unicasting and Frame Relay

F R A M E R E L A Y

N E T W O R K

N e t w o r k s e n d s e a c h p a c k e t to

a p p r o p r i a t e d e s t i n a t io n

R o u t e r A

R o u t e r B

R o u t e r C

R o u t e r D

R o u t e r ER o u t e r F

E v e r y 3 0 s e c o n d s :

R I P ( I P ) P a c k e t to R o u t e r B

R I P ( I P ) P a c k e t to R o u t e r C

R I P ( I P ) P a c k e t to R o u t e r D

R I P ( I P ) P a c k e t to R o u t e r E

R I P ( I P ) P a c k e t to R o u t e r F

E v e r y 6 0 s e c o n d s

R I P ( I P X ) P a c k e t t o R o u t e r B

R I P ( I P X ) P a c k e t t o R o u t e r C

R I P ( I P X ) P a c k e t t o R o u t e r D

R I P ( I P X ) P a c k e t t o R o u t e r ER I P ( I P X ) P a c k e t t o R o u t e r F

S A P P a c k e t ( s ) t o R o u t e r B

S A P P a c k e t ( s ) t o R o u t e r C

S A P P a c k e t ( s ) t o R o u t e r D

S A P P a c k e t ( s ) t o R o u t e r E

S A P P a c k e t ( s ) t o R o u t e r F

E v e r y 3 0 s e c o n d s , R I P ( IP ) P a c k e t f r o m R o u t e r A

a n d

E v e r y 6 0 s e c o n d s , R I P ( IP X ) a n d S A P P a c k e t s fr o m

R o u t e r A





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Router Multicasting and Frame Relay

The diagram on the opposite page shows how the use of multicasting (sometimes called group addressing) reduces the numberof routing information and SAP packets sent into the cloud from Router A. The router sends a single copy of each packet into

the cloud, using a specially assigned DLCIthe multicast address (sometimes called a group address). The carriers switch is

pre-programmed to understand the multicast address, and send copies of the original packets to each router that was defined as

part of the group.

In order to make this architecture work, the implementation team must negotiate the group addresses and group members with

the carriers implementation team when ordering the Frame Relay network.

Unfortunately, multicasting is not widely available for Frame Relay at this time. 10 Most carriers deployed their Frame Relay

switching networks before the multicasting standard (Frame Relay Forum implementation agreement 7) had been adopted by

the Frame Relay Forum. Although most frame relay implementations support some scheme for multicasting, not all of themwill interoperate. To learn whether you will be able to use it with your frame relay network, first check your routers software

to see whether and how it supports the group addressing or multicasting over Frame Relay, 11 and then ask your carrier if their

switching architecture is compatible.

10 In a market survey conducted by Distributed Network Associates in January 1998, only one Frame Relay provider (of 29 surveyed) was currently

offering multicasting. See11 In Ciscos IOS router software, you can configure Frame Relay Broadcast Queue to take advantage of multicasting routing updates over

Frame Relay.





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Router Multicasting and Frame Relay

F R A M E R E L A Y

N E T W O R K

N e t w o r k r e p l ia t e s e a c h m u l t i c a s t p a c k e t , a n d s e n d s

a c o p y t o a l l m e m b e r s o f t h e m u l ti c a s t g r o u p .

R o u t e r A

R o u t e r B

R o u t e r C

R o u t e r D

R o u t e r ER o u t e r F

E v e r y 3 0 s e c o n d s :

R I P ( IP ) P a c k e t to R o u t e r B w i th

M u l t i c a s t D L C I

E v e r y 6 0 s e c o n d s

R I P ( I P X ) P a c k e t t o R o u t e r B w i th


S A P P a c k e t ( s ) t o R o u t e r B w i t h


E v e r y 3 0 s e c o n d s , R I P ( I P ) P a c k e t fr o m R o u t e r A

a n d

E v e r y 6 0 s e c o n d s , R I P ( I P X ) a n d S A P P a c k e t s f r o m

R o u t e r A





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Frame Relay Protocol

The Frame Relay protocol is a data link layer protocol, unlike its older cousin, X.25, which is a network layer protocol.Although the basic concepts are the same, the execution of the details is much different. The essence of the differences can be

summed up in a few wordsFrame Relay is a simpler protocol, optimized for fast transmission over high quality digital

circuits which rarely introduce bit errors, whereas X.25 was designed to assure error-free transport in the face of older (and

noisy), error prone telephone circuits.

When something goes wrong, throw away frames

The sliding window concept (send n number of frames and stop to await an acknowledgment from the receiver before

continuing) does not exist in Frame Relay networks. Hence, there is no on-off valve which limits the amount of traffic a

router might send toward the Frame Relay network. Instead the network is entitled to throw away frames that exceed an

agreed upon transmission ratethe CIR. If the router is transmitting faster than the CIR, the network may throw frames awayrandomly. By setting the DE (Discard Eligible) bit in selected frames, the router can tell the network which frames are

expendable (this feature is not widely used). Ciscos IOS allows network managers to determine classes of traffic in which the

DE bit can be set. Frame Relay switches will also throw away frames that have errored frame check sequences, or that are

otherwise corrupted.

DLCI An addressing schemeThe data link connection identifier is a virtual circuit number, assigned by the Frame Relay provider, that is mapped inside the

network to the PVC that connects specific pairs of routers. The DLCI only has meaning in the local connection between the

router and the nearby Frame Relay switch.

RFC1490 Identifying the network layer protocolThe Internet community has provided a standardized method of identifying to the receiving router which network layer

protocol is contained in each frame (provides a similar function to the protocol field in the PPP), and whether the Frame

Relay frame contains a bridged frame or routed packet. Unique numbers are assigned to each network layer protocol by ISO

(using NLPID) or IEEE (using SNAP).





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Frame Relay Protocol

Flag Address Information FCS Flag

DLCI CR EA DLCI FECNBECN DE EA

Bit

Byte

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

1 2

Control NLPID or SNAP

NOTES:

1. NLPID is the ISO Network Layer Protocol Identifier.2. SNAP is the IEEE Subnetwork Access Protocol.3. RFC1490 defines how the NLPID or SNAP field is encoded for bridges and routers.





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Frame Relay Local Management Interface

The Missing Piece

The initial Frame Relay specification did not include any method for the router to exchange management messages with thenetwork providers switch. Vendors and service providers quickly recognized that routing algorithms, unless made aware of a

Frame Relay network outage, would be slow to update their routing tables. A means to verify the integrity of the Frame Relay

network on a continual basis was needed. In addition, they wanted to provide an automated way for the router to learn the

DLCI numbers assigned by the carrier. Two similar schemes emerged to provide a solution. More recently, a third local

management interface scheme was introduced by the ITU-T, co-incident with the definition of switched virtual circuits.

The original scheme Ciscos LMIAn ad hoc industry group published the Frame Relay Specification with Extensions in 1990, which contains the LMI

specification.12 It provides a regular exchange of link integrity messages between the switch and the router, and a means of

updating the routers list of PVCs (and their DLCIs). In Ciscos IOS software, this method is unabashedly referred to ascisco.

ANSIANSI standard T1.617-1991 includes Annex D (an addendum to the underlying standard) which specifies link integrity

messages and a means to update the routers list of PVCs. The functionality is nearly identical with that provided in LMI.

Differences not significantLMI and Annex D interface management differ only in the bit patterns and fields assigned for similar functions. Either can be

used equally well; carrier and router vendor should be consulted for compatibility. Among the differences is the assignment of

DLCI 0 for signaling under Annex D, while LMI uses DLCI 1023 for the same purpose.

12 The group included Northern Telecom, Stratacom, Cisco, and Digital Equipment





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Frame Relay Interface Management

The Missing Piece

The Original, Ciscos LMI

ANSI T1.617, Annex D

Differences not significant





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ATMAsynchronous Transfer Mode

This newest of the high speed backbone technologies is also getting the most attention in the industry, from equipmentmanufacturers to trade journals to users. The interest stems from the growing view that ATM will, in the future, form the

infrastructure for a new type of telephone networkone which is specifically designed to provide multimedia communications.

In addition, adapter cards and premises ATM switches (that is, non-telco switches) are widely available for building an ATM

LAN infrastructure allowing ATM to be not just the backbone protocol, but the protocol linking workstation to workstation, no

matter whether theyre in the same building or around the globe.

What it does

Think of ATM as a replacement for Ethernet or Token Ring; it is an access method for allocating bandwidth among competing

users. However, where Ethernet and Token Ring are technologies suitable only for LANs, ATM can work equally well in both

the local and wide area paradigms. The ATM switching systems act as both hub and router (albeit it Layer 2 routing) device

all in one. Architecturally, it makes no difference whether the switch is in a wiring closet of an office building, or in the

central office of a local telephone company or interexchange carrier.

Why is it so important?

Carriers world-wide are busy building out their transport networks with ATM as a backbone technology for data transmission,

carrying Frame Relay, SMDS, and native ATM traffic over a common infrastructure. Consider the network diagram provided

by AT&T shown here.

Quality of ServiceUltimately, some industry pundits believe that ATM will be the common infrastructure for transport of voice, video, and data

ATM was designed with several so-called qualities of service, so that voice and video are allocated fixed portions of

bandwidth at the beginning of a call to guarantee real-time delivery, while data can be allocated bandwidth on demand,

matching its bursty nature.





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Technology for new telecommunications infrastructure, and . . . for

LANs, too

An example of a carriers ATM infrastructure map

Qualities of Service real-time delivery for voice and video, bandwidthon demand for computer networking.





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ATM Asynchronous Transfer Mode

ATM is about cell-based switchingThe information units defined for ATM are 53 byte cells. 5 bytes are allocated to the header (which contains full routing

information for each cell) and 48 bytes to the payload (higher layer protocols and the users information).

How the switching works

Cells arriving at a switching system from many different devices will wait briefly in queue, each awaiting its own time slot. 13

Some cellsthose for uncompressed video and voicewill have pre-arranged time slots to guarantee on-time delivery; these

information types generate continuous streams of bits, and require real-time performance, or the person seeing (or hearing) the

received information will be aware of motion stopping briefly, then suddenly catching up (speech would seem to be herky-

jerky, stopping briefly, and then catching up in a rush of sounds). Other cellsthose for data transmissionwill be given as

much bandwidth as needed (up to the limits of the physical speed of the port) for brief periods perhaps hundreds ofmicroseconds to tens of milliseconds.

When a time slot matching the needs of the sender becomes available, the switch reads the routing instructions in the cell, and

sends it on its way. Thats all! The switch immediately moves on to processing the next cell.

Routing information

Two layers of routing information are carried in each cella virtual path number, identifying a link between a pair of end-

points, and a virtual channel number for the specific logical connection between those two end-points.

13 Each device has its own dedicated link to the shared switch.





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Mill Valley, California

(415) 381-5500All Rights Reserved



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Designing ATMPorts and PVCsOn a national scale, most of the major carriers offer ATM service. AT&T, GTE, MCI WorldCom, and Sprint all offer service

throughout the U.S. In addition, Ameritech, Bell Atlantic, BellSouth, Pacific Bell, Southwestern Bell, SNET, and U.S. Westall offer some form of public ATM services in their market areas.

Varying price strategies and technical characteristics among carriersMost providers charge for two pricing elements:

1. The physical port attaching to their ATM networks. Each router or host connecting to ATM will require at least one

port. Ports are available for DS1 (1.5 Mbps), DS3 (45 Mbps), and OC3 (155 Mbps). GTE currently offers OC12 (622

Mbps) as a regular part of their service offering, while AT&T and MCI WorldCom offer it on an individual case basis.

The higher the access speed, the more expensive is the port.

2. The permanent virtual circuits or switched virtual circuits (PVCs or SVCs

which provide the logical connections)between pairs of routers or between hosts and routers. Carriers configure permanent logical pathsthe PVCsbetween

all pairs of devices which must communicate directly when ATM is installed. This is the means by which full mesh

topology is provided inside the cloud.

PVCs and SVCs are also priced by speed, ranging from 1.5 Mbps to 155 Mbps (some carriers also add a distance

element to the price of a VC). It is here that the network designer must use care in estimating the traffic between pairs

of locations. Although any one VC can transmit bursts up to the full speed of the port, the ATM service provider may

drop frames that exceed a committed information rate (CIR)the speed of the VCif their network is too busy to

handle the momentary overload.






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Designing ATM Ports and PVCs






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ATM Quality of Service

The key idea of QoS is pictured on page 7-41

each end-point negotiates for the type of service it requires whenever aconnection is made, and then sends its cells to the switch, where they are queued for delivery when the next time slot becomes

available. Now lets examine how each QoS is meant to work.

Quality of service

There are four service levels or qualities of service. Because the ATM adaptation layer (or AAL) is often related to the QoS,

theres a danger of confusing the two. with the QoSas the most widely used way of specifying the different levels. Another

term, Class of Service, identified by letters A - D, has also been used to describe the type of ATM service.

Constant Bit Rate (CBR) is a connection-oriented service, and provides circuit emulation, and is intended primarily

for voice telephony and carrying conventional, uncompressed video.

Real Time Variable Bit Rate (Rt-VBR) is a connection-oriented service, and provides a guaranteed timing

relationship between sender and receiver even though the bit rate is not constant. It is intended primarily for

transmitting compressed voice or compressed video. This QoS is currently offered by only BellSouth and MCI

WorldCom.

Non-Real Time Variable Bit Rate (Nrt-VBR) is also a connection-oriented service, and is the QoS most often used

for data communications applications. This QoS most closely resembles Frame Relay, with a committed

information rate, and the ability to accommodate brief bursts over the CIR.

Available Bit Rate (ABR) is a connection-oriented service that could be used for some data communicationsapplications. Among the carriers, it is currently provided only by U.S. West and MCI WorldCom.

Unspecified Bit Rate (UBR) is a connection-oriented service, and is the most risky of the various QoS that have

been defined. There are no commitments by the carrier or customer on bit rates or performance. Providing

transport at low cost, UBR might be used for forwarding email, low-priority file transfers, and other applications in

which significant delays are not particularly important.






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ATM Quality of Service

Constant Bit Rate (CBR) circuit emulation for uncompressed voiceand video

Real Time Variable Bit Rate (RtVBR) variable bit rate, guaranteedtiming, for compressed voice or video

Non-Real Time Variable Bit Rate (NrtVBR) variable bit rate, mostcommonly used for data communications

Available Bit Rate (ABR) used for data communications

Unspecified Bit Rate (UBR) used for low-priority datacommunications applications, such as email or file transfers






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ATM Adaptation Layer AAL

ATM specifies several sub-layers within the OSI data link layer

the Convergence layer, the Segmentation and Reassemblylayer, and the ATM Adaptation layer. AAL specifies how voice, video, and data output are mapped from the OSI network

layer into the ATM 53 octet cell structure.

For example, AAL1 specifies how Constant Bit Rate services such as real time voice and video are mapped into cells, while

AAL2 was specified for mapping Real Time Variable Bit Rate services, such as compressed voice and video.

The ATM adaptation layer for data users went through a number of fits and starts, before the ATM Forum adopted two

strategiesAAL3/4 and AAL5.14 Of the two, almost all vendors and carriers are choosing to offer Non-Real Time Variable

Bit Rate service over AAL5.15

14 Initially, there were separate, distinct AAL3 and AAL4 definitions. The members of the ATM Forum eventually found the differences to be

small, and the single AAL3/4 was the result.15 AAL3/4 is used for a single applicatonproviding SMDS over an ATM infrastructure. See page 7-70 for protocol details.






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ATM Adaptation Layer AAL

Specifies how output from network layer is mapped into 53 octet cells

AAL1 used for Constant Bit Rate QoS

AAL2 used for Real Time Variable Bit Rate QoS

AAL3/4 and AAL5 used for data services (Non-Real Time Variable Bit

Rate QoS)






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Connection-oriented data over ATM: AAL 5

SEAL the Simple and Efficient Adaptation LayerYou should realize that theres nothing inherent in AAL 3/4 that requires connectionless data service be carried. It turns out

that AAL 3/4 could carry connection-oriented data just as well. However, a number of engineers in the ATM Forum created

AAL 5, or the Simple and Efficient Adaptation Layer. It is 10% more efficient in carrying user data in cell payloads than AAL

3/4, and substantially reduces processing overhead at the receiving node.

AAL 3/4 nodes must read a sequence number, multiplexing identifier, length field, and CRC for each cell. If any cell is

received with an errored CRC, the entire PDU must be retransmitted.

Whats different about AAL 5

Theres no sequence numbers, multiplexing identifier, length, or CRC in the cell. Instead, the AAL 5 convergence layerappends a 32 bit CRC for the entire PDU, which is contained in the last cell of the segmentation and reassembly layer. This

requires that all cells of the entire convergence layer must transmitted contiguously, without other cells intervening. It also

requires that the entire convergence layer PDU be retransmitted in the event of a CRC error.

AAL 5 most common

Most ATM service providers have implemented AAL 5 in their offerings, to the exclusion of AAL 3/4. However, it appears

that AAL 3/4 will be implemented as a switching infrastructure by many SMDS service providers. They will receive SMDS

cells (Level 2 PDUs) from the CPE and encapsulate them as AAL 3/4 PDUs for transmission through the internal ATM

backbone. At the exit point from the network, the original SMDS cells will be restored, and sent to the destination CPE. Seepage 7-70 for an example.






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Connection-oriented data over ATM: AAL 5

Information ATM Trailer8 bytes

Up to 64 Kbytes

Protocol Data UnitAAL 5 Convergence Layer

AAL 5 SAR Layer

InformationDXI Header

4 bytes

DXI Format Across

Router-DSU Interface

NH TH SH PH AH Application Data

Protocol Data UnitPayload

HeaderCell

5 bytes

HeaderCell

PayloadHeader

Cell

NotEND

NotEND END

ATM Trailer

8 bytes

Payload40 bytes + pad

CRC: 4 byt

PDU Length: 2 b

CPI: 1 byte

UUI: 1 byte






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ATM in the Market Place

Still in infancy

Distributed Network Associates in its soon-to-be released 1998 ATM Marketing Report estimates that there are fewer than

1,000 public ATM UNI ports in service in the U.S., 16 although there are signs that activity is beginning to grow rapidly. By

comparison, the most recent study of Frame Relay ports by the same organization (January 1998) shows 350,000 in service.

Their speculation is that acceptance of ATM public network services today is roughly comparable to what we saw for Frame

Relay in 1994.

Telcos and Interexchange Carriers

Most of the regional telephone companies and the interexchange carriers in the U.S. are offering ATM service. AT&T, MCI

WorldCom, Sprint, GTE, Ameritech, Bell Atlantic, BellSouth, Pacific Bell, Southwestern Bell, SNET, and U.S. West all havesubstantial ATM infrastructure, and are aggressively marketing it to high-end business and government accounts.

Routers, DSUs, and other hardware

Most major router vendors already have available software and interface cards for ATM with either T3 or OC3 speeds. OC12

interface cards are available for the Cisco 12000 router.

Curiously, the T1 ATM is the most recent offering. Both switch vendors and carriers had been reluctant to order

With both T1 and T3 ATM access, a special ATM DSU must be used. It receives the ATM DXI frame from the router

interface, and performs the segmentation. ADC Kentrox and Digital Link have both offered T3 ADSUs (the monicker for the

ATM DSU) for three years or more. T1 ADSUs should be available before the year end.

Waiting for multimedia applications

16 UNI is the User-Network Interface, specifying how most users will connect ATM CPE to a provider network. The study did not track private

ATM growth, or NNI connections (Network-Network Interface, used between providers).






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Most industry watchers and service providers agree that commercial ATM service wont take off until a compelling killer

applicationsuch as multimedia networkingarrives on the scene.






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ATM in the Market Place

Less than 1,000 ATM UNI in the U.S. so says Distributed NetworkAssociates

Telcos and interexchange carriers are all offering public ATM UNI

service, from T1 to OC12

Routers are ATM-ready and DSUs are available

Waiting for multimedia applications






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SMDSSwitched Multimegabit Data Service

What is it?

Conceived as a data networking strategy from the beginning,17 SMDS looks and feels a lot like a LAN protocol. Its

connectionless (just like Ethernet and Token Ring), supports multicasting (just like TCP/IP), and provides a variety of speeds,

ranging from 56 kbps to 34 mbps.

SMDS doesnt require individually configured PVCs to connect one router to another. Each SMDS frame (more correctly

termed a Level 3 Protocol Data Unit) contains a unique SMDS destination address for the target router. SMDS switches read

the destination address, and make packet-by-packet switching decisions (almost like routers) to forward each PDU through the

network.

Off to a slow startAlthough there are many loyal SMDS adherents, the technology has been a slow starter in the marketplace, perhaps for three

main reasons.

1. Limited geographic scope. Until 1995, SMDS networks in the U.S. could not cross local telephone companies

LATA boundaries.18

2. Expensive. Until 1994, SMDS was available only at T1 and T3 access speeds, and required pricey DSUs (which

converted SMDS packets into 53 byte cells).

3. The largest telecommunications service provider, AT&T, chose to ignore SMDS, and marketed its Frame Relay

network.

17 Unlike Frame Relay, which was conceived as an outgrowth of ISDN, and which is based on the telephony concept of circuits between two

devices exchanging information.18 MCI has offered interexchange SMDS service since 1995.






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SMDSSwitched Multimegabit Data Service

WAN networking protocol with the look and feel of a LAN protocol

Slow start in the market

a) Until 1995, intraLATA only

b) Until 1994, T1 and T3 speeds only

c) AT&T sat out






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Who Should Use SMDS

Whos a candidate for SMDS?Since the technology allows for connectionless (i.e.no virtual circuits) service between routers, and requires no

predetermined paths (virtual circuits) through the SMDS network, it lends itself to organizations with a very large number of

locations and to those who occasionally need to send or receive large volumes of information from outside organizations (such

as customers or vendors).

1. Full mesh networks of 20 locations or more

2. T1 speed is inadequate

3. Need to network with suppliers and customers

Full mesh is no sweatJust like Ethernet and Token Ring, SMDS easily permits any node to communicate with any other node. There is no need to

size and order individual virtual circuits. Full mesh connectivity is always provided.

High speeds

SMDS is available at 56 kbps, as well as 1.5, 4, 10, 16, 25, and 34 mbps.

Split the cost with suppliers and customers

Suppliers and customers who have SMDS access can easily internetwork with your site. Since the pricing model for SMDS

requires that each site pay only for its own service, suppliers and customers pay for their access, you pay for yours, and thereare no other charges.






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Who Should Use SMDS

Full mesh, 20 sites or more

High speedfaster than T1

Connect with suppliers and customers





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RBOC Pricing Model for SMDS

SMDS service is available in service areas of Ameritech, Bell Atlantic, BellSouth, GTE, Pacific Bell, and SNET. MCI is theonly interexchange carrier currently providing (or committed to providing) long haul service. Other interexchange carriers

seem content to wait for ATM, or to sit on the sidelines.

Pricing model for SMDS

At the local exchange level, pricing of SMDS is based on port speed into the SMDS switch plus an access channel (very much

like Frame Relay, but without the additional charge for a PVC). Access channels are defined for DS0at 56 and 64 kbps, for

DS1at 1.17 mbps, and for five classes of DS3 access (with higher prices for faster speeds).

DS3 defined classes are:

1. 4 mbps

2. 10 mbps3. 16 mbps

4. 25 mbps

5. 34 mbps

Flat rate per access line

Each SMDS access lines is priced at a flat monthly rate, with no charge for usage. Communications can be established with

any number of sites.

Approximate prices in the U.S. are $120 per month for DS0 access lines, $650 for DS1, and $2,000 to $6,000 for DS3

(depending on speed).





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RBOC Pricing Model for SMDS





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RBOC/GTE Availability

Available

Although SMDS is not universally available, it is available in most metropolitan areas served by four of the RBOCs, plus GTE

and SNET.

Ameritech

Bell Atlantic

Bell South

Pacific Bell

Not available

Two of the RBOCs, NYNEX and Southwestern Bell, have never offered SMDS. In addition, U.S. West announced it was

withdrawing its SMDS service in April 1996.





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RBOC/GTE Availability

Ameritech

Bell Atlantic

Bell South

GTE

Pacific Bell





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RBOC SMDS Pricing

SMDS is priced by the RBOCs as if it were a premium DS0, DS1, or DS3 access channel. The prices presented on the facing

page include both the access line and the SMDS service with unlimited usage. Variations are due to individual RBOC pricing

strategies, and differing regulatory environments.

Flat rate

SMDS service, in the local exchange environment, is a good deal! You get an access line and a full mesh network for a flat

monthly rate for each site. Unlike the MCI interexchange service, the RBOCs do not charge on the basis of traffic volume or

usage.

Month to month vs term contract

Most RBOCs (and carriers, for that matter) offer more attractive pricing for customers willing to commit to 12 month (orlonger) contract terms.

Access line choices

Standard speeds are 56/64 kbps, 1.17 mbps (T1); and 4, 10, 16, 25, and 34 mbps (T3). Some RBOCs will provide SONET

OC3 access at 155 mbps under special terms and conditions.





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RBOC SMDS Pricing

DS0 SMDS line $120 to 225 per month per location

T1 SMDS lines $500 to 800 per month per location

T3 SMDS lines $2,000 to $6,000 (speed dependent)





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Interexchange AvailabilityAlone among the interexchange carriers, MCI began offering SMDS nationwide in 1995. It also offers international SMDS

networkingto the U.K. (with British Telecomm), and to Ireland (with Ireland Telecomm).SMDS users can access the MCI network by either direct access (i.e.bypassing local telco SMDS switching) or through

exchange access, in which MCI connects to your local telephone company, providing only long-haul connection between sites

in different Local Access and Transport Areas (LATAs).

Direct access

The SMDS customer purchases a 56 kbps, T1, or T3 local access channel from the RBOC, connecting to the nearest MCI Point

of Presence, and selects one of the following speeds.

56/64 kbps

128 kbps

256 kbps 384 kbps

512 kbps

678 kbps

1.024 mbps

1.536 mbps

4.5 mbps

10.5 mbps

Exchange access

The local telephone company connects to MCIs SMDS network, and hands off SMDS cells bound for destinations outside the

LATA. Speeds are the same as those offered by local telcos.





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Interexchange SMDS

Direct access

a) Wide range of speeds56 kbps to 10 mbps

b) Available in 48 statesnot RBOC-dependent

Exchange accesssame speeds as RBOC/GTE access





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MCI Pricing Model for SMDS

Pricing for both direct access and exchange access includes a fixed monthly fee for each site, plus a charge based on thevolume of data transmitted4.1 per megabyte for access speeds up to T1, and 3.0 per megabyte for 4 and 10 mbps access.

The volume charges have price caps, so that maximum montly charges are predictable.

Pricing matrix

Speed, Kbps Port Charge with

Exchange Access

Port Charge with

Direct Access

Max Volume Charge with

Exchange Access

Max Volume Charge with

Direct Access

64 $ 50 $ 180 $ 299 $ 164

128 NA 336 NA 327

256 NA 394 NA 654384 NA 578 NA 980

512 NA 735 NA 1,307

768 NA 946 NA 1,547

1,024 NA 1,178 NA 1,620

1,536 250 1,470 3,037 1,740

4,000 750 3,000 3,940 3,500

10,000 1,500 5,500 5,833 8,600

16,000 2,000 NA 8,813 NA25,000 2,500 NA 12,960 NA

34,000 3,250 NA 16,524 NA





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MCI Pricing Model for SMDS

Port charge

a) Speed dependent

b) Access dependent

Volume charge

a) Speed dependentcap is also speed dependent, but why?

b) Access dependent





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SMDS Design Issues

Background

Because SMDS is a connectionless service, and needs no PVCs, the network integrates much more easily with routers androuting algorithms than either Frame Relay or ATM (which are both connection-oriented networks). Routing algorithms have

no construct for an end-to-end connection through the WAN. Superimposing a connection-oriented protocol such as Frame

Relay or ATM on the inherently connectionless routing algorithm creates additional complexity and the opportunity for new

problems to arise. SMDS avoids most of the complexity and problems by mating a connectionless link layer to the

connectionless routing layer.

Each router (or a directly attached host) has a unique E.164 address (this looks like an international telephone number with

country code) assigned by the carrier or local telco. The SMDS network providers switches read the destination E.164

address in the SMDS header (see pages 68 and 69), and find the best path for delivery of each packet, in a manner not unlike

that used by routers.

Multicasting in SMDS

The ability to send multicast packetssimultaneously sending the same packet to all nodes on the networkis fully supported

in all implementations of SMDS, through the use ofgroup addresses. A number of groups can be definedone might be all

routers on the network (for OSPF link state update messages), while another group could include all intra-company locations,

and a third group, outside vendors. Group members are negotiated with the carriers implementation team during installation.

Address screening for authentication and security

Authentication and security mechanisms are implemented in the carriers switches. Each port will accept only packets with the

correct source addressauthentication. In addition, each port is configured with address screens, which only allow receptionfrom, or transmission to, a pre-determined list of source and/or destination addresses. The list is administered by the SMDS

service provider, based on the connectivity needs of each customer.





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SMDS Design Issues

Connectionless vs. connection-oriented service

Multicasting through group addresses

Address screening for authentication and security





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SMDS Protocol

OverviewSMDS borrowed a little from here, and a little from there in order to build a specification for the technology. They borrowed

the 53 octet (byte) cell format from the Broadband ISDN and ATM development communities. And, from the IEEE 802.6

working group, they borrowed a MAC method known as distributed queue, dual busDQDB(actually invented by an

Australian firm, QPSX).

The most important concept in understanding the protocol is that, unlike PPP and Frame Relay, it is a connectionless

servicemeaning that there is no pre-established connection between pairs of sites. The SMDS switch reads each protocol

data unit (another name for a packet), and makes an instantaneous switching decision for that PDU. And it does the same for

each subsequent PDU, perhaps making several million switching decisions every second.19

How it works

The SMDS Interface Protocol is actually two protocols, defined as sub-layers within the OSI layer 2 definitions. The upper

sub-layer, called Level 3 Protocol Data Unit, can carry up to 9,188 bytes of user information; it is this packet format that

emerges from the routers serial port. The lower sub-layer is known as Level 2 Protocol Data Unit, and is responsible for

dividing the larger packets into the 53 byte cells. This part of the protocol is usually performed by a special DSUthe SMDS

DSUavailable from vendors, such as ADC Kentrox and Digital Link.

The SMDS switch receives the individual cells, reads the destination address (contained in the first few cells), and forwards

them to the next SMDS switch or directly to another user locally attached.

If one or more cells are lost, the entire Level 3 PDU must be retransmitted.

19 MCI estimates that latency encountered from New York City to Los Angeles is of the order of 40 msec.





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SMDS Protocol

InformationTrailer

38 - 50 bytes

4 bytes

Up to 9188 bytes

Level 3 Protocol Data UnitSMDS Interface Protocol

PayloadHeader T PayloadHeader T PayloadHeader TLevel 2 Protocol Data UnitSMDS Interface Protocol

InformationTrailer4 bytes

DXI Header4 bytes

DXI Format AcrossRouter-DSU Interface

NH TH SH PH AH Application Data

SMDS SMDSDA SA

SMDS SMDSDA SA

SMDS Header

AddressType

SMDS Address

4 bits 60 bits

Address type can indicate eitherindividual or group address

NOTES:

1. Cells are 53 bytes each7 bytes for the header, 44 bytes for the payload, and 2

bytes for the trailer.

2. The header of all cells from the same Level 3 PDU carry a unique message ID

number, as well as sequence numbers to enable the SMDS switch to reassemble

the Level 3 format.





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SMDS over ATM: AAL 3/4

Conundrum of connectionless service over ATMBy definition, two end points such as hosts or routers in an ATM network are always connected by a virtual circuit,

composed of a concatenated set of virtual paths and/or virtual channels. The set may be pre-engineeredthe PVC (permanent

virtual circuit), or it may be established dynamically by the networkthe SVC (switched virtual circuit). However, a router

or host using a connectionless protocol, such as SMDS, is not prepared to request a virtual circuit when transmitting packets to

a peer; the individual packets are transmitted with an expectation of best effort delivery by the network (datagram service).

Connectionless

backbone 1

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