CNN: Lecture Notes—Module 3 Computer Comm. & Networking 1

Module 3

Wide Area Networking (WAN)

When many nodes or LAN’s are connected together in a large scale, we have a WAN.

This may be world wide.

Because of the separation of the nodes, the Public Data Network (PDN) is used. There are two main approaches to the problem.

• Packet switching

• Circuit switching

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WAN -- Packet switching• Use Packet Switched PDN (PSPDN)

• Data is assembled into a packet and addressed to destination

• Packet is sent to Packet Switching Exchange (PSE)

• Packet is transmitted to destination via PSPDN

• Connection is purely logical not physical

• Nodes may have different data rates etc.

• Two services are provided

• Datagram: Like sending letter. One packet is addressed and sent at a time with each packet being independent of the others.

• Virtual call: Like a phone call. When many packets are to be sent to the same address a virtual circuit is established.

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WAN-- Circuit switching• Use Circuit Switched PDN (CSPDN)

• Physical connection is established by CSE

• Nodes must all obey standard rates etc.

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Packet switched NetworksWhole system based around the DTE to PSPDN connection. The original protocol used was X.25. Now see variations, eg Frame Relay etc.

Look at X.25 (CCITT): Interface between DTE and Data Circuit-Terminating Equipment (DCE) for terminals operation in the Packet mode on Public Data Networks

PSPDN

PSPDN

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Packet switched NetworksPSPDN

PSPDN

X.25 corresponds (although not exactly) to OSI’s first three layers.

Physical interface is the X.21 standard. This is a subset of V.24 (RS232) and the circuits are described in X.24.

The link layer is a subset of High Level Data Link Control (HDLC) which is called Link Access Procedure Ver. B (LAPB)

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Packet switched Networks

X.25 addresses the interface between user’s equipment (DTE) and the network. DTE comprises a computer, modem or line interfaces

X.25 provide a standard for accessing the WAN

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X.25’s Physical LayerX.25 physical layer specifies the interface between the DTE and DCE, such as physical and electrical characteristics of the media, the types of connectors, etc.

It references the X.21 interface standard.

Support RS-232C

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X.25’s Link Access LayerX.25 layer 2 is defined by the Link Access Protocol-Balanced (LAPB)

It provides link access specifications for

• Frame composition

• Flow-control procedures

• Error-checking method

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X.25’s Packet LayerX.25 packet layer specifies a virtual circuit service for transporting packets across the packet-switched network

Two services:

• Permanent virtual circuit – Fixed virtual circuit (similar to a leased line service). But the bandwidth is shared among multiple sites instead of dedicated between two sites.

• Virtual call – Dynamic virtual circuit (Requires a call initiation). X.25 can support 4096 simultaneous virtual circuits on a DTE-DCE link.

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Circuit Switched NetworksThis type of connection is concerned only with the X.21 protocol which is used from end to end over a physical connection.

Call set up, data transfer and call clearing is handled by this protocol. HDLC is used once again.

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Integrated Services Digital Network (ISDN)ISDN was conceived as a way to combine many of our communication needs into a system using the same standard techniques for each service.

The common usage of digitised analogue waveform paved the way for ISDN. E. g., telephone cables carrying time division multiplexed digitised speech, ie, Pulse Code Modulation (PCM)

This concept has been brought right back to the subscribers premises. If all information out of a subscribers premises could be put into a standard form, the it could be neatly packaged together and piped to its destination.

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ISDN (Cont’d)ISDN could supply a path for

•Telephone

•Fax

•Computer/Terminal connection

•Fire alarm

•Video phone

•HiFi music

•Video images

•Appliance control

•Meter reading

•LAN access …….

Let’s look at the adopted standard

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ISDN – Channel Types (basic rate interface)ISDN uses bearer or B channels for transmitting data, and a signaling or D channel for transmitting signaling and control information

B Channel – 64kbps

•Digital voice, computer data(text and Graphics), digitised video

D Channel -- 16kbps

•Signaling, or packet switched user data such as telemetry, meter reading (low speed)

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ISDN – Channel Types

A normal subscribers premises will be given a Basic Service• 2 × B channels

• 1 × D channel

• 48 bits of overhead for framing and synchronisation

• Total 192 kbps

• Normally shielded pair to premises

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ISDN – Channel TypesA B channel connection may be

• Circuit switched• Data is sent over B channels• Call setup and control on D channel

• Packet switched• X.25 connection

• Semi-permanent• Fixed connection to another user• What used to be leased line

The first anticipated utilisation of the B channel was the telephone (history has shown the internet has changed this)

• 4kHz speech BW• Sampled at 8kHz• 8 bits per sample (256 levels)• 8k × 8=64kbps same as PCM used now

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ISDN – Channel Types (primary rate interface)BRI is for home and small office, PRI is for organisations.

ISDN DeviceB Channel 64 kbps

B Channel 64 kbps

D Channel 64 kbps

.

.

.

B Channel 64 kbps

D Channel 64 kbps

Data and Voice

Signaling

Signaling

Data and Voice

oice

U.S. = 23 Europe = 30

Data and V

U.S. = 1 Europe = 2

North American DS-1 format:

• Rate: 1.544Mbps

• 23B + D

European E-1 format:

• Rate: 2.048Mbps

• 30B + 2D

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How is control of the channels performedAll control procedures are performed on the D channel, A B channel will then be assigned to a device, thus no contention will occur.

Contention may occur on the D channel as all of the devices may try to access this channel. Before transmitting, a device must listen for a long string of consecutive 1’s (idle channel). When busy, 0’s are inserted into long 1 strings. Collision may still occur so a protocol similar to CSMA is adopted.

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Connection overviewThe interface protocol is broken into 2 planes

• C-plane Control and signalling (D channel)

• U-plane User, voice & data (B channel)

The amount of intelligence required in the NTE depends on the type of connection. C frame complexity is the same for all connections but U frame alters.

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(a) Circuit switched

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(b) Frame relay/frame switching

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(c) Packet switching

X.25

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With X.25, multiplexing of multiple virtual circuits is handled by the packet layer and the link layer handles only error correction. This makes these combined layers complex which limits the throughput.

With frame relay, multiplexing and routing are done at the link layer. This simplifies the protocol which allows greater throughput.

Using this technique many ‘calls’ may be in progress at any time. Each virtual connection is allocated an identifier which is included in the header.

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Example connection – digital telephone call

Circuit mode connection control

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ISDN FaxAt present our FAX system operates predominantly around the Group 3 standard. This allows up to 9600bps of modulated data over a PSTN line. This group will transmit a page in approximately 30 secs

Group 4 FAX is intended for ISDN usage and is purely a digital system operating at 64kbps. The source is encoded with Huffman codes

Resolution is 200dpi with options for 240, 300 and 400 dpi. These are laser printer type resolutions. Group 4 can send a page in approximate 3-4 secs.

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Broadband ISDN (B-ISDN)Data hungry users require more and more bps for many applications

To serve this need a faster service called B-ISDN was introduced.

Ordinary ISDN is now often called Narrowband ISDN (N-ISDN)

Services are broken into two types, Interactive and Distributive.

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B-ISDNInteractive

Conversational: Real time services such as telephone calls, video telephony etc.

Messaging: Non real time services such as voicemail, data mail, video mail etc.

Retrieval: Services requiring action from both end, eg. Retrieving video from a videotex provider.

DistributiveServices from providers to subscribers without subscribers issuing a

request.

User control: Service where subscriber needs to enable acceptance of service eg. Pay TV where programs are sent in defined time slots.

No user control: Data is supplied at providers discretion without control of subscriber, eg. TV broadcast to premises.

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B-ISDN (Physical Specifications)Typically 3 types of service are available

Symmetrical 155.529 Mbps: For businesses requiring normal traffic in and out including video where a N-ISDN service is not suitable

155.520Mbps/622.080 Mbps: For businesses requiring access to multiple services but who are not service providers.

Symmetrical 622.080 Mbps: For businesses who provide and receive services.

B-ISDN is the basis for ATM

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Asynchronous Transfer Mode (ATM)This is a further advancement over X.25 and frame relay especially designed for very high speed network segments. It is commonly used for high capacity backbones.

Because of the way it operates, it is commonly called Cell Relay

Like frame relay and X.25, data is sent in packets. The fundamental difference is that ATM uses fixed size packets called cells, which contains exactly 53 bytes –- 48 bytes for user data and 5 bytes for overhead.

This allows ATM switches to operate very quickly.

A common implementation is 155 Mbps ATM over SONET (Synchronous Optical Network)

ATM is often implemented on various physical layers

SONET, FDDI, 155 MB fibre

ATM does not map neatly into the ISO reference.

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ATM

• End nodes communicate with an ATM device via a user-to-network interface (UNI)

• ATM devices communicate with each other via a network-to-network interface (NNI).

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ATM-- example of how ATM facilitates multiservice networking

ATM is the only network that was built from the ground up to support data, voice, and video at the same time.

Therefore, an ATM network can be used for almost any type of network environment in use today and in the future.

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ATM—Protocol Architecture

User Plane -- Info exchange, and flow and error control.

Control plane -- Control and connection functions

Management plane -- Management functions to provide coordination between layers.

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ATM—Protocol Architecture

Physical layer -- involves the specification of a transmission medium and a signal encoding scheme. The data rates specified in this layer range from 25.6Mbps to 622.08Mbps. Higher or lower rates are possible.

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ATM—Protocol Architecture

ATM layer -- defines the transmission of data in fixed-size cells and defines the use of logical connections

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ATM—Protocol Architecture

AAL layer -- maps higher layer information into ATM cells to be transported over an ATM network, and collects information from cells for delivery to higher layer.

The AAL is required to provide interface to transfer protocols that don’t support fixed cell size transfer.

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ATM layer—Logical ConnectionATM uses Virtual Channel Connections (VCC) and Virtual Path Connections (VPC)

VCC -- Like a virtual circuit in X.25. It is the basic unit of switching in an ATM network. The data is transferred over this circuit in fixed size cells at a variable rate.

VPC -- a VPC is a bundle of VCCs that go between the same source and destination. Thus all of the cells flowing over all of the VCCs in a single VPC are switched together. This allows ATM to operate faster as it may switch all the cells together to solve the control costs problem in high-speed networking.

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ATM layer—VPCUse of VPCs gives

• Network simplification – can switch individual logical connections (virtual channel) or a group of logical connections (virtual path)

• Improved performance & reliability – switching paths is quicker

• Improved connection setup and processing times (shorter) -- new connections can be added to paths very quickly

• Improved network services – users can define structure of virtual channels and virtual paths.

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ATM layer—cellsGFC: Controls the data flow of UNI. Become VPI bits at the NNI

VPI: Virtual Path Identifier. Part of network address used to identify a group of channels between network entities.

VCI: Virtual Channel Identifier. A pointer that identifies the virtual channel the system is using on a particular path.

PT: Payload Type indicates the type of information contained in the cell.

0 in the MSB– the cell is carrying user data

1 in the MSB—control of resource management data

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ATM layer—cells

CLP: Cell Loss Payload specify whether to discard the cell in the presence of congestion

1 -- can discard

0 –- should not discarded. High priority

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ATM layer—cellsHEC: Header Error Control provides error correction for 1 bit errors and error detection for multiple errors in the cell header

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ATM Adaptation layer — AALThe AAL is the layer which takes care of translating from another protocol to the fixed cells required by ATM.

Historically there have been a number of AAL types developed

AAL1 AAL2 AAL3/4 AAL5

AAL5 is now the version of choice for LANs and most other applications.

The AAL is divided into 2 sub-layers

Common Part Convergence Sublayer (CPCS) -- Talk to applications requiring the AAL

Segmentation And Reassembly sublayer (SAR) – Performs the breakup and assembly of frames to 48 octet cells and vice versa.

A protocol data unit (PDU) is passed to the CPCS. Certain functions may be performed here before passing this CPCS-PDU to the SAR.

The SAR breaks the CPCS-PDU into SAR-PDUs each of 48 octets which fit into an ATM cell

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ATM—SAR Format of the AAL5 SAR

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ATM—advantagesATM advantages

• Uses fixed length cells

• Gives much higher throughput

• Short cells have less chance of error

• Usually single bits errors (networks are now quite reliable)

• CRC can repair cells

• Cells can be handled mainly by hardware (ATM switches)

• Network protocol does not rely on retransmission

• Less work for network and more for nodes

• Can interface to most higher level protocols and most physical layers

At present ATM is expected to be increasing in popularity throughout the world network.