Modified by Masud-ul-Hasan and Ahmad Al-Yamani 1 Chapter 6 Wide Area Networking Concepts, Architectures, and Services.

1Modified by Masud-ul-Hasan and Ahmad Al-Yamani

Chapter 6

Wide Area Networking Concepts, Architectures, and Services


Objectives Study WAN switching: Circuit and Packet switching Study the concepts of different WAN transmissions

and services: Local Loop transmissions alternatives:

POTS ISDN ADSL (xDSL) Cable TV

WAN architecture and services: X.25 Frame Relay SMDS ATM (cell-relay ATM) Broadband ISDN

Goal: To understand the basic concepts of WAN


Basic Principles of WAN Business Issues in wide area networking, like in most

businesses, the desire to maximize the impact of any investment in technology is a central focus.

Technical concepts the two basic principles involved in sharing a single data link among multiple sessions are:Packetizing – the segmenting of data transmission

between devices into structured blocks or packets of data.

Multiplexing – takes packetized data from multiple sources and sends over a single wide area connection.


System 1A System 1B

System 2A System 2B

System 3A System 3B

System 4A System 4B

System 5A System 5B

A. Dedicated Multiple Wide Area System-to-System Connections

Dedicated point to point connections


B. Single Wide Area Link Shared to Provide Multiple System-to-System Connections

System 1A System 1B

System 2A System 2B

System 3A System 3B

System 4A System 4B

System 5A System 5B

Single shared WAN link


WAN Design Principles

Performance

Cost Reduction

Security/Auditing

Availability/Reliability

Manageability & Monitoring

Quality of Service/Class of Service

Support for Business Recovery Planning


Major Components of a WAN Architecture

Running from customer to the entry point or gateway to the carrier’s network.

The transparent interoperability of network services from different carriers.

Provides the circuit or data highways over which the information is actually delivered.

Circuit switching or packet switching, proper routing information.

Residential and business user demands are driving forces behind WAN services.


Wide Area Network Architecture

WAN Architecture = Switching Architecture + Transmission Architecture

Switching Architecture: Methods to ensure proper routing of information from source to destination.

Transmission Architecture: Circuits or data highway over which the information is actually delivered.


Broadband Transmission

T-1

SONET (Synchronous Optical NETwork)


T-1It is the standard high capacity digital

transmission service in America 1.544 Mbps

In other parts of the world the standard is E-1 2.048 Mbps

T-1 is divided into twenty four 64K channels. Each of which is known as DS-0. Some may be used for voice and some for data.

Each channel consists of group of 8-bits known as time slot. Each time slot represents one voice sample or a byte of data to be transmitted.


Figure 8-18 T-1 Frame Layout

T-1 Frame Layout

A T-1 frame consists of a framing bit & 24 DS-0 channels, each containing eight bits, for a total of 193 bits per frame.


Superframes and Extended Superframes

Framing bit marks the end of each 24-channel frame

Superframe = 12 frames

1 0 0 0 0 0 0 1 1 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0

Extended Superframe (ESF) = 24 frames

Channel 1 (8 bits)

Channel 2 (8 bits)

Channel 24 (8 bits)

bits

Frame = 24 time slots plus 1 framing bit = 193 bits

Sequence of framing bits used for management

and control information

Superframe = 12 frames

1 0 1 1 0 0 1 0 1 0 1 0 1 1 1 1 0 0 0 1 0 0 1 1

GOLDMAN & RAWLES: ADC3e FIG. 08-19

12


Digital Service Level

Number of Voice Channels

Transmission Rate Corresponding Transmission Service

DS-0

DS-1

DS-1C

DS-2

DS-3

DS-4

1

24

48

96

672

4032

64

1.544

3.152

6.312

44.736

274.176

Kbps

Mbps

Mbps

Mbps

Mbps

Mbps

DS-0 or switched 64K

T-1 or switched T-1

T-1C

T-2

T-3

T-4

Digital Service (DS) Hierarchy

T-1 and T-3 are by far the most common service levels delivered.

T-1 service is most often delivered via 4 copper wires (2 twisted pair).

T-3 service is most commonly delivered via optical fiber. Some T-1 marketing practices is to sell Fractional T-1 or FT-1.


Figure 8-20 Digital Service Hierarchy and CCITT Standards


Number of Voice Channels

Transmission Rate Corresponding Transmission Service

1

2

3

4

5

30

120

480

1920

7680

2.048

8.448

34.368

139.264

565.148

Mbps

Mbps

Mbps

Mbps

Mbps

E-1

E-2

E-3

E-4

E-5

CCITT Digital Hierarchy

CCITT Digital Hierarchy


T-1 Technology The fundamental piece of T-1 hardware is the T-1

CSU/DSU (Channel Service Unit/Data Service Unit). Two devices are packaged as a single unit.

The CSU is a device that connects a terminal to a digital line. The DSU is a device that performs protective and diagnostic functions for a telecommunications line. Can be thought of as a very high-powered and expensive modem.

Their primary job is to convert a digital data frame from a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa.

Such a device is required for both ends of a T-1 connection, and the units at both ends must be set to the same communications standard.


T-1 Technology A T-1 is commonly delivered as a 4-wire circuit (2 wires

for transmit and 2 wires for receive) physically terminated with a male RJ-48c connector.

The T-1 CSU/DSU (provide the RJ-48c female connector) will transfer the 1.544 Mbps of bandwidth to local devices like, routers, over high speed connections such as V.35, RS-530, RS-449 or Ethernet that are provided on the customer side of the CSU/DSU.

A CSU/DSU are often able to communicate status and alarm information to network management systems via the Simple Network Management Protocol (SNMP).


T-1 Technology ImplementationUsed to digitize analog voice and multiplex them into the DS-0 channels of T-1 frame. Commonly found in CO.

Inverse MUX - Able to combine multiple T-1 output lines to provide high bandwidth requirements like video conferencing.


SONET (Synchronous Optical Network) SONET is an optical transmission service

delivering multiple channels of data from various sources using periodic framing or TDM.

Much like T-1 service, but with higher capacity due to the following: 1. uses fiber optics.2. uses slightly different framing technique.

ANSI defined it in T1.105 and T1.106 standards. SONET in North America, SDH (Synchronous

Digital Hierarchy) in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.



Transmission Rate

OC-1

OC-3

OC-9

OC-12

OC-18

51.84

155.52

466.56

622.08

933.12

Mbps

Mbps

Mbps

Mbps

Mbps

SONET's OC (Optical Carrier) Standards

OC-24

OC-36

OC-48

1.244

1.866

2.488

Gbps

Gbps

Gbps

SONET/SDH card


SONET Framing

9 Frames = 1 SONET superframe

3 Octets for control information or overhead

87 Octets for data, also called as payload


SONET Architecture SONET network is based on layered hierarchy of

transport elements and associated technology. Section: Basic building block of a SONET network. It

is built by using a single fiber optic cable between two fiber optic transmitter/receivers. A transmitter/receiver is sometimes referred to as optical repeater or also as STE (section terminating equipment).

Line: Multiple sections combine to form a SONET line. It is terminated with LTE such as add/drop MUXs.

Path: Multiple lines combine to form a SONET path. A path is an end-to-end circuit terminating in SONET access MUXs that have channel interfaces to lower speed or digital electronic transmission equipment.


SONET and SDH

Section, Line and Path overhead in a SONET Frame.


Add/Drop Multiplexer

Note: the new signal being added can use the same optical channel (wavelength) as the dropped signal.


SONET Deployment

SONET services are usually available in large metropolitan areas (MANs).

Some ATM switches are equipped with SONET interfaces for direct access to either a local SONET ring.

Carrier bring the fiber ring directly to a corporate location and assign dedicated bandwidth to each SONET customer.

Fault tolerant and reliable.


SONET Architectures for Deployment Two main types of architectures:1. UPSR (Unidirectional Path-Switched Rings):

Share the Capacity.2. BLSR (Bidirectional Line-Switched Rings):

Redundant media, traffic can be re-routed in case of fiber failure.

SONET services cost 20% more than conventional digital services of the same bandwidth.


Unidirectional Path-Switched Rings (UPSR)

All users share transmission capacity around the ring rather than using dedicated segments.

Mostly used in access networks. Provides duplicate, geographically diverse paths for

each service protecting against cable cuts and node failures.

As data travels in one direction duplicate travels in other direction for protection.

It automatically switches to the protection signal if there is a problem with primary data signal.


single pair fiber-optic cable

optical access and transport node

UPSR Ring




primary signal protection signal

Unidirectional Path-Switched Rings (UPSR)


Bidirectional Line-Switched Rings (BLSR)

In this, each user’s traffic is specifically rerouted in the case of fiber failure.

It employs 2 fiber rings with bidirectional traffic flow with each ring’s capacity divided equally between working and protection bandwidth.

BLSR survives in the event of electronic, node, cable failure by automatically routing traffic away from faults within 50msec.

Mostly used in carrier backbone networks.


Bidirectional Line-Switched Rings (BLSR)Each fiber has 6 STS-1a for working traffic and 6 STS-1a

for protection

OC-12 Two-Fiber Bidirectional

Line-Switched Ring

Each fiber has 6 STS-1a for working traffic and 6 STS-1a

for protection





STS-1 (Synchronous Transport Signal-level 1) bit rate is 51.84 Mbps, accommodates up to 28 T1 lines (672 multiplexed voice channels).


WaveLength Division Multiplexing (WDM)

WDM can be used only on fiber optic circuits. It works by sending multiple simultaneous bits of

information using different wavelengths of light (colors).

WDM on a single fiber can produce transmission capacity in the range of Terabits (1000 Giga) per second.

Multiplexing 8 or more wavelength is called Dense WDM (DWDM).

Individual DWDM wavelengths are called Lambdas and have a capacity of 2.4 Gbps each.


WAN Switching

Circuit SwitchingPacket switching

(Already covered in detail in Chapter 2)


Switched Network Services Hierarchy

X.25 Frame relay Cell relay

Fast packet switchingOriginal packet switching

Packet switchingCircuit switching

Leased lines Dial-up circuits

Switching

ATM MPLS


X.25 A popular standard for packet-switching networks.

The X.25 standard was approved by the CCITT

(now the ITU) in 1976 (30 yrs).

It defines the interface between Data Terminal

Equipment (DTE) and any packet-switched network.

It is a layer 3 protocol stack OSI Reference Model.

The aim is to produce packets in a standard format

acceptable by any X.25 compliant public network.

It provides transparency to other upper 4-7 layers.


High-Level Data Link Control (HDLC)

Link Access Procedure-Balanced (LAP-B)

RS-232

Packet Layer Protocol (PLP)

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Datalink

1. Physical

X.2

5

OS

I M

od

el

X.25 provides transparency to upper layers; the top

4 layers need not worry about delivery of data via a

packet switched network.

OSI Model and X.25

Applications running on one computer that wish to talk to another computer do not need to be concerned with anything having to do with the packet-switched network connecting the two computers. So X.25 is a transparent delivery service between computers.

It enables to form a logical link connection. LAPB is a bit-oriented protocol ensures that frames are error free and in the right sequence.

It describes the data transfer protocol in the PDN at the network layer level. PLP manages packet exchanges between DTE devices across virtual circuits. The PLP operates in five distinct modes: call setup, data transfer, idle, call clearing, and restarting.


X.25 Data-Link Layer Protocol: HDLC

Flag

8 bits

Address

field

Control

field

Information

field

Frame check

sequenceFlag

8 bits 8 bits Variable 16 bits 8 bits

The Flag fields indicate the start and end of the frame. The Address field contain the address of the DTE/DCE, it is

most important in multi-drop lines, where it is used to identify one of the terminals.

The Control field contains sequence numbers, commands and responses for controlling the data flow between the DTE and the DCE.

The Checksum field indicates whether or not errors occur in the transmission. It is the Cyclic Redundancy Codes (CRCs), also called as Frame Check Sequences (FCSs).


X.25 Technology Implementation

PAD

Packet switched

network

Packet assembler/

disassemblerMainframe with X.25

protocol software

Minicomputer with X.25 protocol software

X.25 Gateway(Proxy/Firewall)

Ethernet LAN

X.25

X.25

X.25

X.25

X.25


X.25 Technology Data must be properly packetized into X.25 packets

before it enters the cloud. If terminals do not possess X.25 protocol stack, they

must use PAD to generate these packets. Inside the cloud, X.25 switches are connected

together in a mesh topology most often using T-1 lines.

30 years ago, long distance circuits connecting X.25 packet switches were not as error free as they are today. So it was necessary to check for errors and request retransmissions on a point-to-point basis at every X.25 packet switch in the network.


X.25 Technology Implementation


Frame Relay

Two layer protocol (physical and data link)

Frame relay is similar to X.25, but removes the error detection/correction at each of the packet switches.

So, it increases performance by using only end-to-end error correction and flow control instead of point-to-point.


Error Detection and Correction

X.25 and Frame Relay use CRC for error detection on point-to-point basis.

While X.25 uses Discrete ARQ for error correction; Frame Relay does not use point-to-point error correction, it simply discards the frame.

By removing this point-to-point overhead, Frame Relay can offer speeds of T-1 and T-3 while X.25 is limited to 9.6 Kbps.


Point-to-Point vs. End-to-End Error Correction

X.25

X.25

X.25

X.25

X.25

X.25

X.25

X.25

PAD

PAD

1 3

2 4

5

6 7

X.25 Packet-switched network

Steps in X.25 Error Correction

1. Regenerate CRC-16

2. Compare with transmitted CRC-16

3. Send ACK or NAK to sending node

4. Wait for retransmitted packet and repeat

Point-to-Point error

detection and correction

Also called as store and forward switching methodology.


Point-to-Point vs. End-to-End Error Correction

FR

FR

FR

FR

FR

FR

FR

FR

FRAD

FRAD

1

Frame relay network

Steps in Frame Relay Error Correction

1. Regenerate CRC-16

2. Compare with transmitted CRC-16

3. Discard bad frames

4. Repeat process on next frame

Point-to-Point error

detection

End-to-End error correction


Frame Relay Frame Layout


Frame Relay (cont'd)A FRAD (Frame Relay Access Device) is used instead of a PAD for frame relay networks.

Frames can be variable in length, up to approx. 8000 characters.

Variable-length frames can be a problem, not good for carrying voice and video because of variable delay.

Combining these potentially large, variable-length frames with the low overhead and faster processing of the frame relay switching delivers a key characteristic of the frame relay network: High throughput with low delay.


Frame Relay (cont'd)Frame relay transmission rates can be as high as 1.544 Mbps.

bandwidth can be dynamically allocated in the mesh network of frame relay cloud.

frame relay is well suited to moving bursts of data by simply assembling and forwarding more frames per second onto the frame relay network.

Frame relay encapsulates user data and forwards it to its destination, thereby making it protocol independent.


Virtual CircuitsFrame Relay networks most often employ Permanent Virtual Circuits (PVC) to forward frames from source to destination through the frame relay cloud.

Switched Virtual Circuits (SVC) standards have been defined but are not readily available from all carriers. It is analogous to dial-up call.

SVC based frame relay networks use call set-up information to the frame relay network before sending information to or receiving information from a remote frame-relay device.


ATM

Asynchronous Transfer Mode (ATM) is a cell relay

(or switching) architecture and standard.Fast Packet Switching methodologyA fixed packet size (cell) makes fast switching

possible, and makes it different from Frame RelayATM is well suited to data, voice, and digital video

transmissions, because of predictable delivery time.ATM standards are still emerging, so many

incompatibilities currently exist.


ATM Bandwidth Management

Three kinds of bandwidth management schemes in ATM, also known as classes of service (CoS):CBR (Constant Bit Rate)VBR (Variable Bit Rate)ABR (Available Bit Rate)


Constant Bit Rate (CBR)

Guaranteed amount of bandwidth, equivalent to T-1 or T-3. This is analogous to a leased line.

Disadvantage: if the bandwidth is not required 100% of the time no other application can use the unused bandwidth


Variable Bit Rate (VBR)

Provides a minimum amount of constant bandwidth, below which the available bandwidth will not drop.

This is a popular choice for voice and video conferencing data.

If more bandwidth is required it will be dynamically assigned.


Available Bit Rate (ABR) Utilizes leftover bandwidth that is not required by

VBR traffic. This is the cheapest class of service. Assumes that VBR services will not frequently burst

to consume all of the available bandwidth. Should never be used for critical data.


CBR, VBR, and ABR Bandwidth Management for ATM

VBR traffic (up to 100 Mbit/sec)

ABR traffic (at least 5 Mbit/sec)

CBR traffic (51 Mbit/sec)

ATM access switch

ATM access switch

LAN for transaction processing

LAN for transaction processing

front end processor

front end processor

mainframe, cluster controller and terminals for batch processing

mainframe, cluster controller and terminals for batch processing

Videoconferencing stations

Videoconferencing stations

ABR acceptable

VBR requiredVBR required

CBR required

OC3 (155 Mbps) ATM trunk


ATM Technology

What makes ATM affordable is its architecture: Constant cell length: faster, predictable delivery time. Predictable nature of ATM allows voice, video, and data to

be transported effectively.

ATM protocols are supported on both LAN and WAN.

No need for protocol conversion across enterprise

networks.

Adaptive to traffic demands (CBR, VBR, ABR)

Scaleable Bandwidth (25, 100, 155, 622 Mbps)


Implementation of Variety of ATM Technology


ATM Cell Size

United States and Japan 64 Bytes, European Community 32 Bytes. So (64 + 32) / 2 = 53 Byte compromise.

The ATM header contains information about destination, type and priority of the cell.

ATM

Switch

ATM

Switch

Cell Data

(48 bytes)

Cell Data

(48 bytes)

Cell Data

(48 bytes)

Cell

Header

5 bytes

Cell

Header

5 bytes

Cell

Header

5 bytes

Cell

Header

5 bytes

53-byte cell (5 byte header + 48 byte payload)


ATM Market

Major Success: Large Carrier NetworksE.g., AT&T and MCI have stated that they will

convert their backbones to ATM within a few years

Moderate Success: Corporate BackbonesSome large corporations utilize ATM to

interconnect switched LANs and provide in-house video and audio services

Not much Success: DesktopATM to the desktop has not been very popular


B-ISDN

Broadband ISDN

B-ISDN = SONET + ATM

Transmission architecture (flexibility to carry multiple types of data simultaneously , i.e. voice, video and data)

Switching architecture (ability to switch multiple types of data simultaneously, i.e. voice, video and data)


B-ISDN

It is the service of the future that will deliver on-demand and affordable bandwidth

It supports existing services (T-1, T-3) and emerging services (SMDS, Frame Relay) and future services (HDTV, Medical imaging).


MPLS (Multi-Protocol Label Switching)

It is a second cell relay protocol. It has evolved with the strengths and weaknesses of

ATM in mind (ATM cell has 10% overhead). Some overhead for signaling between ATM network

devices. Additional overhead is necessary to establish and terminate SVC.

Need arise for a protocol having less overhead. Connection-Oriented nature of ATM is same in

MPLS due to the use of label switched paths (LSP). One address scheme for global communication-

establish end-to-end LSPs reduces overhead.


MPLS Three-Level Architecture


The outer layer of the architecture is the edge layer. Devices in this layer are non-MPLS devices. These non-MPLS devices are connected to label

edge routers in the second layer (the access layer). LER is responsible for encapsulating traffic from the

network edge within MPLS frames. LER also establishes, maintains, and terminates

LSPs through the network core for non-MPLS edge devices.

The third level is the MPLS network core, is formed by a mesh of label switch routers (LSR).

MPLS Three-Level Architecture


Modern WAN systems are evolving towards a network architecture of four layers: photonic switching, TDM, STDM, and routing.

Photonic Switching: most enterprise networks are fiber constrained.

Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Border Gateway Protocol (BGP) route packets from source to destination.

WAN Evolution


WAN Evolution


With the exponential growth in traffic, many enterprise networks are fiber based now.

Laying new fiber cable costs more – economical to increase the effective bandwidth by DWDM.

Current generation of switches is electronic. So optical signals must be converted to electronic signals before they can be switched makes it much slower.

This optical-electrical-optical (O-E-O) conversion can be avoided using photonic switching.

WAN Evolution

Modified by Masud-ul-Hasan and Ahmad Al-Yamani 1 Chapter 6 Wide Area Networking Concepts, Architectures, and Services.

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