UNIT V

LAN AND ISDN

LAN

A local-area network (LAN) is a computer network that spans a relatively small area. Most

often, a LAN is confined to a single room, building or group of buildings, however, one LAN

can be connected to other LANs over any distance via telephone lines and radio waves.

There are many different types of LANs, with Ethernets being the most common for PCs. Most

Apple Macintosh networks are based on Apple's AppleTalk network system, which is built into

Macintosh computers. The following characteristics differentiate one LAN from another:

Topology: The geometric arrangement of devices on the network. For example, devices can be

arranged in a ring or in a straight line.

Protocols: The rules and encoding specifications for sending data. The protocols also determine

whether the network uses a peer-to-peer or client/server architecture.

Media: Devices can be connected by twisted-pair wire, coaxial cables, or fiber optic cables.

Some networks do without connecting media altogether, communicating instead via radio waves.

Wireless networks are relatively easy to implement these days, especially when compared to the

prospect of having to route wires when deploying a new wired network or overhauling an

existing one. The first step in planning a wireless LAN deployment should be to decide on your

wireless networking technology standard. Keep in mind that the standard you need to

accommodate your network access points and routers as well as the entire collection of

wireless network interface cards (NICs) for your computers and other network resources.

A wireless local area network (WLAN) links two or more devices using some wireless

distribution method (typically spread-spectrum or OFDM radio), and usually providing a

connection through an access point to the wider Internet. This gives users the ability to move

around within a local coverage area and still be connected to the network. Most modern WLANs

are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name.

Wireless LANs have become popular in the home due to ease of installation, and in commercial

complexes offering wireless access to their customers; often for free. New, for instance, has

begun a pilot program to provide city workers in all five boroughs of the city with wireless.

LAN STANDARDS AND PROTOCOLS

Each network has its own rules and standards. Therefore, protocols are use in network

technology to govern the communication between network and network. There are many

different kinds of protocols, the most common protocol used in the OSI data link layer in LAN

are Ethernet and Token Ring.

Ethernet

Ethernet is the most well-known type of local area network that most widely installed in offices,

home offices and companies. The standard of an Ethernet is IEEE 802.3. Ethernet normally are

used coaxial cable and sometimes different grades of twisted-pair cable as the transmission

medium.

Token Ring

Token Ring is a network that connects computers in a star or ring topology. It is originally

developed by IBM Company. There will have a token ring that passed through the network to

allow computers to access the network. The standard for Token Ring network is IEEE 802.5. A

token bit will move around the ring from computer to another computer. If the computer wants to

transmit a data, the data will attach to the token and pass to the next computer. It has to keep

passing through computers in the network until it comes to the destination. If the computer

doesn’t want to transmit any data, the token ring will just pass to the next computer.

IEEE 802 STANDARDS

IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specifications for

implementing wireless local area network (WLAN) computer communication in the 2.4, 3.6, 5

and 60 GHz frequency bands. They are created and maintained by

the IEEE LAN/MAN Standards Committee (IEEE 802). The base version of the standard was

released in 1997 and has had subsequent amendments. The standard and amendments provide the

basis for wireless network products using the Wi-Fi brand. While each amendment is officially

revoked when it is incorporated in the latest version of the standard, the corporate world tends to

market to the revisions because they concisely denote capabilities of their products. As a result,

in the market place, each revision tends to become its own standard.

The 802.11 family consists of a series of half-duplex over-the-air modulation techniques

that use the same basic protocol. 802.11-1997 was the first wireless networking standard in the

family, but 802.11b was the first widely accepted one, followed by 802.11a, 802.11g, 802.11n

and 802.11ac. Other standards in the family (c–f, h, j) are service amendments and extensions or

corrections to the previous specifications.

802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States

under Part 15 of the U.S. Federal Communications Commission Rules and Regulations. Because

of this choice of frequency band, 802.11b and g equipment may occasionally

suffer interference from microwave ovens, cordless telephones and Bluetooth devices. 802.11b

and 802.11g control their interference and susceptibility to interference by using direct-sequence

spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) signaling

methods, respectively. 802.11a uses the 5 GHz U-NII band, which, for much of the world, offers

at least 23 non-overlapping channels rather than the 2.4 GHz ISM frequency band, where

adjacent channels overlap - see list of WLAN channels. Better or worse performance with higher

or lower frequencies (channels) may be realized, depending on the environment.

The segment of the radio frequency spectrum used by 802.11 varies between countries. In the

US, 802.11a and 802.11g devices may be operated without a license, as allowed in Part 15 of the

FCC Rules and Regulations. Frequencies used by channels one through six of 802.11b and

802.11g fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may

operate 802.11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased

power output but not commercial content or encryption.

IEEE 802.11 ARCHITECTURE AND SERVICES

IEEE 802.11s is an IEEE 802.11 amendment for mesh networking, defining how wireless

devices can interconnect to create a WLAN mesh network, which may be used for static

topologies and ad hoc networks.

802.11 is a set of IEEE standards that govern wireless networking transmission methods.

They are commonly used today in their 802.11a, 802.11b, 802.11g, and 802.11n versions to

provide wireless connectivity in the home, office and some commercial establishments.

802.11s inherently depends on one of 802.11a, 802.11b, 802.11g or 802.11n carrying the actual

traffic. One or more routing protocols suitable to the actual network physical topology are

required. 802.11s requires Hybrid Wireless Mesh Protocol, or HWMP,[1] be supported as a

default. However, other mesh, ad hoc or dynamically link-state routed (OLSR,B.A.T.M.A.N.)

may be supported or even static routing (WDS, OSPF). See the more detailed description below

comparing these routing protocols.

A mesh often consists of many small nodes. When mobile users or heavy loads are

concerned, there will often be a handoff from one base station to another, and not only from

802.11 but from other (GSM, Bluetooth, PCS and other cordless phone) networks.

Accordingly IEEE 802.21, which specifies this handoff between nodes both obeying 802.11s and

otherwise, may be required. This is especially likely if a longer-range lower-bandwidth service is

deployed to minimize mesh dead zones, e.g. GSM routing based on Open BTS.

Mesh networking often involves network access by previously unknown parties, especially when

a transient visitor population is being served. Thus the accompanying IEEE 802.11u standard

will be required by most mesh networks to authenticate these users without pre-registration or

any prior offline communication. Pre-standard captive portal approaches are also common. See

the more detailed description below of mesh security.

Between peers, 802.11s defines a secure password-based authentication and key

establishment protocol called "Simultaneous Authentication of Equals" (SAE). SAE is based

on Diffie–Hellman key exchange using finity cyclic groups which can be a primary cyclic

group or an elliptic curve. The problem on using Diffie–Hellman key exchange is that it does not

have an authentication mechanism. So the resulting key is influenced by a pre-shared key and

the MAC addresses of both peers to solve the authentication problem.

When peers discover each other (and security is enabled) they take part in an SAE

exchange. If SAE completes successfully, each peer knows the other party possesses the mesh

password and, as a by-product of the SAE exchange, the two peers establish a cryptographically

strong key. This key is used with the "Authenticated Mesh Peering Exchange" (AMPE) to

establish a secure peering and derive a session key to protect mesh traffic, including routing

traffic.

ETHERNET

Ethernet is most widely used LAN Technology, which is defined under IEEE standards 802.3.

The reason behind its wide usability is Ethernet is easy to understand, implement, maintain and

allows low-cost network implementation. Also, Ethernet offers flexibility in terms of topologies

which are allowed.Ethernet generally uses Bus Topology. Ethernet operates in two layers of the

OSI model, Physical Layer, and Data Link Layer. For Ethernet, the protocol data unit is Frame

since we mainly deal with DLL. In order to handle collision, the Access control mechanism used

in Ethernet is CSMA/CD. Manchester Encoding Technique is used in Ethernet.

In IEEE 802.3 standard Ethernet, 0 is expressed by a high-to-low transition, a 1 by the low-to-

high transition. In both Manchester Encoding and Differential Manchester, Encoding Baud rate

is double of bit rate.

Baud rate = 2* Bit rate

Ethernet LANs consist of network nodes and interconnecting media or link. The network nodes

can be of two types:

LLC: LOGICAL LINK CONTROL

This Data Link Layer is divided into two sublayers:

Logical Link Control (LLC). This sublayer is responsible for the data transmission

between computers or devices on a network.

Media Access Control (MAC). On a network, the network interface card (NIC) has an

unique hardware address which identifies a computer or device. The physical address is

utilized for the MAC sublayer addressing.

The function of the Logical Link Control (LLC) is to manage and ensure the integrity of data

transmissions. The LLC provides Data Link Layer links to services for the Network Layer

protocols. This is accomplished by the LLC Service Access Points (SAPs) for the services

residing on network computers. Also, there is a LLC Control field for delivery requests or

services.

The Logical Link Control (LLC) has several service types:

Service type 1, is a connectionless service with no establishment of a connection, and an

unacknowledged delivery.

Service type 2, is a connection logical service with an acknowledgement of delivery.

Service type 3, is a connectionless service with an acknowledgement of delivery.

Service classes furthermore support sundry permutations of these LLC service types:

Class I supports only service type 1.

Class II supports both service type 1 and type 2.

Class III support both service type 1 and type 3.

Class IV support all three service types.

The SubNetwork Access Protocol (SNAP) is an augmentation of the IEEE 802.2 LLC header.

SNAP provides a method by which to utilize non-IEEE protocols on IEEE 802 networks.

MEDIUM ACCESS CONTROL

In the seven-layer OSI model of computer networking, media access control(MAC) data

communication protocol is a sub layer of the data link layer, which itself is layer 2. The MAC

sub layer provides addressing and channel access control mechanisms that make it possible for

several terminals or network nodes to communicate within a multiple access network that

incorporates a shared medium, e.g. Ethernet. The hardware that implements the MAC is referred

to as a medium access controller.

The MAC sub layer acts as an interface between the logical link control (LLC) sub layer and the

network's physical layer. The MAC layer emulates a full-duplex logical communication channel

in a multi-point network. This channel may provide uni

cast, multicast or broadcast communication service.

The local network addresses used in IEEE 802 networks and FDDI networks are

called MAC addresses; they are based on the addressing scheme used in

early Ethernet implementations. A MAC address is a unique serial number. Once a MAC address

has been assigned to a particular network interface (typically at time of manufacture), that device

should be uniquely identifiable amongst all other network devices in the world. This guarantees

that each device in a network will have a different MAC address (analogous to a street address).

This makes it possible for data packets to be delivered to a destination within a sub network,

i.e. hosts interconnected by some combination of repeaters, hubs, bridges and switches, but not

by network layer routers. Thus, for example, when an IP packet reaches its destination

(sub)network, the destination IP address (a layer 3 or network layer concept) is resolved with

the Address Resolution Protocol for IPv4, or by Neighbor Discovery Protocol (IPv6) into the

MAC address (a layer 2 concept) of the destination host.

Examples of physical networks are Ethernet networks and Wi-Fi networks, both of which

IEEE 802 are networks and use IEEE 802 48-bit MAC addresses.

A MAC layer is not required in full-duplex point-to-point communication, but address

fields are included in some point-to-point protocols for compatibility reasons.

The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and

the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the

network gains access to the data and permission to transmit it. The LLC layer controls frame

synchronization, flow control and error checking.

MAC Layer is one of the sublayers that makeup the datalink layer of the OSI reference

Model. MAC layer is responsible for moving packets from one Network Interface card NIC to

another across the shared channel. The MAC sublayer uses MAC protocols to ensure that signals

sent from different stations across the same channel don't collide. Different protocols are used for

different shared networks, such as Ethernets, Token Rings, Token Buses, and WANs.

CARRIER SENSE MULTIPLE ACCESS PROTOCOLS (CSMA)

With slotted ALOHA, the best channel utilization that can be achieved is 1/e. Several protocols

are developed for improving the performance.

Protocols that listen for a carrier and act accordingly are called carrier sense protocols. Carrier

sensing allows the station to detect whether the medium is currently being used. Schemes that

use a carrier sense circuits are classed together as carrier sense multiple access or CSMA

schemes. There are two variants of CSMA. CSMA/CD and CSMA/CA

The simplest CSMA scheme is for a station to sense the medium, sending packets immediately if

the medium is idle. If the station waits for the medium to become idle it is called persistent

otherwise it is called non persistent.

a. Persistent

When a station has the data to send, it first listens the channel to check if anyone else is

transmitting data or not. If it senses the channel idle, station starts transmitting the data. If it

senses the channel busy it waits until the channel is idle. When a station detects a channel idle, it

transmits its frame with probability P. That’s why this protocol is called p-persistent CSMA.

This protocol applies to slotted channels. When a station finds the channel idle, if it transmits the

fame with probability 1, that this protocol is known as 1 -persistent. 1 -persistent protocol is the

most aggressive protocol.

b. Non-Persistent

Non persistent CSMA is less aggressive compared to P persistent protocol. In this protocol,

before sending the data, the station senses the channel and if the channel is idle it starts

transmitting the data. But if the channel is busy, the station does not continuously sense it but

instead of that it waits for random amount of time and repeats the algorithm. Here the algorithm

leads to better channel utilization but also results in longer delay compared to 1 –persistent.

CSMA/CD

Carrier Sense Multiple Access/Collision Detection a technique for multiple access protocols. If

no transmission is taking place at the time, the particular station can transmit. If two stations

attempt to transmit simultaneously, this causes a collision, which is detected by all participating

stations. After a random time interval, the stations that collided attempt to transmit again. If

another collision occurs, the time intervals from which the random waiting time is selected are

increased step by step. This is known as exponential back off.

Exponential back off Algorithm

1. Adaptor gets datagram and creates frame

2. If adapter senses channel idle (9.6 microsecond), it starts to transmit frame. If it senses

channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is

done with frame!

4. If adapter detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, adapter enters exponential backoff: after the mth collision, adapter

chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times (i.e. slot)

and returns to Step 2

6. After 10th retry, random number stops at 1023. After 16th retry, system stops retry.

TOKEN RING

Token ring (IEEE 802.5) is a communication protocol in a local area network (LAN) where all

stations are connected in a ring topology and pass one or more tokens for channel acquisition. A

token is a special frame of 3 bytes that circulates along the ring of stations. A station can send

data frames only if it holds a token. The tokens are released on successful receipt of the data

frame.

Token Passing Mechanism in Token Ring

If a station has a frame to transmit when it receives a token, it sends the frame and then passes

the token to the next station; otherwise it simply passes the token to the next station. Passing the

token means receiving the token from the preceding station and transmitting to the successor

station. The data flow is unidirectional in the direction of the token passing. In order that tokens

are not circulated infinitely, they are removed from the network once their purpose is completed.

TOKEN BUS

Token Bus (IEEE 802.4) is a standard for implementing token ring over virtual ring in LANs.

The physical media has a bus or a tree topology and uses coaxial cables. A virtual ring is created

with the nodes/stations and the token is passed from one node to the next in a sequence along this

virtual ring. Each node knows the address of its preceding station and its succeeding station. A

station can only transmit data when it has the token. The working principle of token bus is

similar to Token Ring.

Token Passing Mechanism in Token Bus

A token is a small message that circulates among the stations of a computer network providing

permission to the stations for transmission. If a station has data to transmit when it receives a

token, it sends the data and then passes the token to the next station; otherwise, it simply passes

the token to the next station. 

Differences between Token Ring and Token Bus

Token Ring Token Bus

The token is passed over the physical ring formed

by the stations and the coaxial cable network.

The token is passed along the virtual ring of stations

connected to a LAN.

The stations are connected by ring topology, or

sometimes star topology.

The underlying topology that connects the stations

is either bus or tree topology.

It is defined by IEEE 802.5 standard. It is defined by IEEE 802.4 standard.

The maximum time for a token to reach a station

can be calculated here.

It is not feasible to calculate the time for token

transfer.

FDDI

Fiber Distributed Data Interface (FDDI) is a set of ANSI and ISO standards for transmission of

data in local area network (LAN) over fiber optic cables. It is applicable in large LANs that can

extend up to 200 kilometers in diameter.

Features

FDDI uses optical fiber as its physical medium.

It operates in the physical and medium access control (MAC layer) of the Open Systems

Interconnection (OSI) network model.

It provides high data rate of 100 Mbps and can support thousands of users.

It is used in LANs up to 200 kilometers for long distance voice and multimedia

communication.

It uses ring based token passing mechanism and is derived from IEEE 802.4 token bus

standard.

It contains two token rings, a primary ring for data and token transmission and a

secondary ring that provides backup if the primary ring fails.

FDDI technology can also be used as a backbone for a wide area network (WAN).

The following diagram shows FDDI −

Frame Format

The frame format of FDDI is similar to that of token bus as shown in the following diagram −

The fields of an FDDI frame are −

Preamble: 1 byte for synchronization.

Start Delimiter: 1 byte that marks the beginning of the frame.

Frame Control: 1 byte that specifies whether this is a data frame or control frame.

Destination Address: 2-6 bytes that specifies address of destination station.

Source Address: 2-6 bytes that specifies address of source station.

Payload: A variable length field that carries the data from the network layer.

Checksum: 4 bytes frame check sequence for error detection.

End Delimiter: 1 byte that marks the end of the frame.

ALOHA

ALOHA is a simple communication scheme in which each source in a network sends its data

whenever there is a frame to send without checking to see if any other station is active. After

sending the frame each station waits for implicit or explicit acknowledgment. If the frame

successfully reaches the destination, next frame is sent. And if the frame fails to be received at

the destination it is sent again.

Pure ALOHA ALOHA is the simplest technique in multiple accesses. Basic idea of this

mechanism is a user can transmit the data whenever they want. If data is successfully transmitted

then there isn’t any problem. But if collision occurs than the station will transmit again. Sender

can detect the collision if it doesn’t receive the acknowledgment from the receiver.

In ALOHA Collision probability is quite high. ALOHA is suitable for the network where there is

a less traffic. Theoretically it is proved that maximum throughput for ALOHA is 18%.

P (success by given node) = P(node transmits) . P(no other node transmits in [t0-

1,t0] . P(no other node transmits in [t0,t0 +1]

= p . (1-p)N-1 . (1-p)N-1

P (success by any of N nodes) = N . p . (1-p) N-1 . (1-p)N-1

… Choosing optimum p as N --> infinity...

= 1 / (2e) = .18

=18%

Slotted ALOHA

In ALOHA a newly emitted packet can collide with a packet in progress. If all packets are of the

same length and take L time units to transmit, then it is easy to see that a packet collides with any

other packet transmitted in a time window of length 2L. If this time window is decreased

somehow, than number of collisions decreases and the throughput increase. This mechanism is

used in slotted ALOHA or S-ALOHA. Time is divided into equal slots of Length L. When a

station wants to send a packet it will wait till the beginning of the next time slot.

Advantages of slotted ALOHA:

single active node can continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Disadvantages of slotted ALOHA:

collisions, wasting slots

idle slots

clock synchronization

Efficiency of Slotted ALOHA:

Suppose there are N nodes with many frames to send. The probability of sending frames of

each node into the slot is p.

Probability that node 1 has a success in getting the slot is p.(1-p)N-1

Probability that every node has a success is N.p.(1-p)N-1

For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37

The clear advantage of slotted ALOHA is higher throughput. But introduces complexity in the

stations and bandwidth overhead because of the need for time synchronization.

SONET

Synchronous optical networking (SONET) is a standardized digital communication protocol that

synchronously transfers multiple data streams over long distances through fiber optic cables. It is

a physical layer specification that allows simultaneous transmission of voice, data, and video at

speeds as high as 1Gbps through a single fiber. In telephone networks, it is used for transmission

of a huge amount of telephone calls and data streams through fiber.

SONET was standardized by the American National Standards Institute (ANSI). It is equivalent

to Synchronous Digital Hierarchy (SDH) standardized by the International Telecommunication

Union (ITU).

SONET Frames

The basic SONET frame comprises of a block of 810 bytes. The frames are transmitted at the

rate of 8000 frames/bytes, which is the sampling rate of the telephone networks.

A SONET frame is represented as a rectangular block of bytes with 9 rows and 90 columns as

shown in the following diagram.

The first three columns of the SONET frame contains the system information and is generally

termed as system overhead, while the rest (marked in blue) contains the payload, i.e. the data to

be transmitted. In this frame, the first three rows of the system overhead (marked in yellow)

contain section overhead, and the nest six rows (marked in orange) contain line overhead.

ISDN

These are a set of communication standards for simultaneous digital transmission of

voice, video, data, and other network services over the traditional circuits of the public switched

telephone network. Before Integrated Services Digital Network (ISDN), the telephone system

was seen as a way to transmit voice, with some special services available for data. The main

feature of ISDN is that it can integrate speech and data on the same lines, which were not

available in the classic telephone system.

ISDN is a circuit-switched telephone network system, but it also provides access to

packet switched networks that allows digital transmission of voice and data. This results in

potentially better voice or data quality than an analog phone can provide. It provides a packet-

switched connection for data in increments of 64 kilobit/s. It provided a maximum of 128 kbit/s

bandwidth in both upstream and downstream directions.

ISDN Channels

A greater data rate was achieved through channel bonding. Generally ISDN B-channels

of three or four BRIs (six to eight 64 kbit/s channels) are bonded.

In the context of the OSI model, ISDN is employed as the network in data-link and

physical layers but commonly ISDN is often limited to usage to Q.931 and related protocols.

These protocols introduced in 1986 are a set of signaling protocols establishing and breaking

circuit-switched connections, and for advanced calling features for the user. ISDN provides

simultaneous voice, video, and text transmission between individual desktop videoconferencing

systems and group videoconferencing systems.

The ISDN works based on the standards defined by ITU-T (formerly CCITT). The

Telecommunication Standardization Sector (ITU-T) coordinates standards for

telecommunications on behalf of the International Telecommunication Union (ITU) and is based

in Geneva, Switzerland. The various principles of ISDN as per ITU-T recommendation are:

To support switched and non-switched applications

To support voice and non-voice applications

Reliance on 64-kbps connections

Intelligence in the network

Layered protocol architecture

Variety of configurations

USER INTERFACE

The following are the interfaces of ISDN:

1. Basic Rate Interface (BRI)

There are two data-bearing channels (‘B’ channels) and one signaling channel (‘D’ channel) in

BRI to initiate connections. The B channels operate at a maximum of 64 Kbps while the D

channel operates at a maximum of 16 Kbps. The two channels are independent of each other. For

example, one channel is used as a TCP/IP connection to a location while the other channel is

used to send a fax to a remote location. In iSeries ISDN supports basic rate interface (BRl).

The basic rate interface (BRl) specifies a digital pipe consisting two B channels of 64 Kbps each

and one D channel of 16 Kbps. This equals a speed of 144 Kbps. In addition, the BRl service

itself requires an operating overhead of 48 Kbps. Therefore a digital pipe of 192 Kbps is

required.

2. Primary Rate Interface (PRI)

Primary Rate Interface service consists of a D channel and either 23 or 30 B channels depending

on the country you are in. PRI is not supported on the iSeries. A digital pipe with 23 B channels

and one 64 Kbps D channel is present in the usual Primary Rate Interface (PRI). Twenty-three B

channels of 64 Kbps each and one D channel of 64 Kbps equals 1.536 Mbps. The PRI service

uses 8 Kbps of overhead also. Therefore PRI requires a digital pipe of 1.544 Mbps.

3. Broadband-ISDN (B-ISDN)

Narrowband ISDN has been designed to operate over the current communications infrastructure,

which is heavily dependent on the copper cable however B-ISDN relies mainly on the evolution

of fiber optics. According to CCITT B-ISDN is best described as ‘a service requiring

transmission channels capable of supporting rates greater than the primary rate.

ISDN LAYERS

ISDN uses circuit-switching to establish a physical permanent point-to-point connection

from the source to the destination. ISDN has standards defined by the ITU that encompass the

OSI bottom three layers of which are Physical, Data Link and Network.

It is difficult to apply the simple seven-layer architecture specified by the OSI to the

ISDN. One reason is that the ISDN specifies two different channels (B and D) with different

functionalities. As we saw earlier in this chapter, B channels are for user-to-user communication

(information exchange). D channels are predominantly for user-to-network signaling. The

subscriber uses the D channel to connect to the network then the B channel to send information

to another user. These two functions require different protocols from each other at many of

the OSI layers. The ISDN also differs from the OSI standard in its management needs. A primary

consideration of the ISDN is global integration. Maintaining the flexibility required to keep the

network truly integrated using public services requires a great deal of management.

For these reasons, the ITU-T has devised an expanded model for the ISDN layers. Instead

of a single seven-layer architecture like the OSI, the ISDN is defined in three separate planes: the

user plan, the control plane, and the management plane. The user plane defines the functionality

of the B channel and H channel: the user-to-user connection. The control plane defines the

functionality of the D channel when used for signaling. (When used for subscriber data, the D

channel is defined on the user plane.) The management plane encompasses both the user and

control planes and is used for managing the whole network.

ISDN layers

All three planes are divided into seven layers that correspond to the OSI model. The discussion

of the management plane is beyond the scope of this book; the other two planes are discussed

below.

The simplified version of the ISDN architecture for the user and control planes (B and D

channels) is given. In this figure we make a number of assumptions that enable us to simplify the

model for the purposes of discussion. First of all, we assume that the subscriber uses the D

channel only for signaling. When a D channel is used for data, it behaves like a B channel.

Eventually D channels will be used for services like telemetry, but the protocols that will make

those services possible are still under study. For our purposes, then, the D channel is a signaling

channel. Further, the D channel is used primarily for user-to-network signaling. Its functions are

therefore confined to the first three layers. Layers 4 to 7, which are concerned with end-to-end

user signaling, use other ISDN protocols (such as SS7).

ISDN layers for B and D channels

At the physical layer, the B and D channels are alike. They use either the BRI or PRI

interfaces and devices discussed earlier in this chapter. At the data link layer, the B channel uses

link access procedure, balanced (LAPB, a simplified subset of HDLC used only for connecting a

station to a network) or some version of it. At the network layer, the B channel has many

options. B channels (and D channels acting like B channels) can connect to circuit-switched

networks, packet-switched networks (X.25), frame relay networks, and ATM networks, among

others. The user-plane options for layers 4 to 7 are left to the user and are not defined specifically

in the ISDN. In summary, we need only discuss the physical layer shared by the B and D

channels, and the second and third layers of the D channel standard.

BROAD BAND ISDN

Broadband Integrated Service Digital Network (B-ISDN) is a standard for transmitting

voice data and video at the same time over fiber optic telephone lines. Broadband ISDN can

support data rates up to 2 Mbps which is an improvement on the original ISDN bandwidth rate of

64Kbps or 128Kbps when using both connections. The B-ISDN was envisaged to run over ATM

carrying both the synchronous voice and the asynchronous data on the same transport bearer.

One of the underlying motives to develop ISDN was to provide subscribers with a wide variety

of services direct to their home. These included video telephony, video surveillance, high speed

Internet, High Definition TV, and these services would require delivery at different bit rates and

with different time constraints. For example video and TV have greater time constraints’ than

data. B-ISDN over ATM was considered a good fit as ATM could handle various contracts based

on the service required. The only problem was that ATM had a small payload size of only 48KB

and an overhead of 5 Bytes making it an extensive transport protocol. I.E. 5/53 x 100/1 = 9.9%

overhead. There were plans to use B-ISDN over fiber optic cables in a Fiber in the loop scenario

whereby the fiber replaced the aging copper loop. This would have mitigated the high overhead

due to the vast increases in speeds that could be leveraged from a fiber implementation.

Frame Relay

Frame Relay is a standardized wide area network technology that specifies

the physical and data link layers of digital telecommunications channels using a packet

switching methodology. Originally designed for transport across Integrated Services Digital

Network (ISDN) infrastructure, it may be used today in the context of many other network

interfaces.

Network providers commonly implement Frame Relay for voice (VoFR) and data as

an encapsulation technique used between local area networks (LANs) over a wide area

network (WAN). Each end-user gets a private line (or leased line) to a Frame Relay node. The

Frame Relay network handles the transmission over a frequently changing path transparent to all

end-user extensively used WAN protocols. It is less expensive than leased lines and that is one

reason for its popularity. The extreme simplicity of configuring user equipment in a Frame Relay

network offers another reason for Frame Relay's popularity.

Frame Relay has its technical base in the older X.25 packet-switching technology,

designed for transmitting data on analog voice lines. Unlike X.25, whose designers

expected analog signals with a relatively high chance of transmission errors, Frame Relay is

a fast packet switching technology operating over links with a low chance of transmission errors

(usually practically lossless like PDH), which means that the protocol does not attempt to correct

errors. When a Frame Relay network detects an error in a frame, it simply drops that frame. The

end points have the responsibility for detecting and retransmitting dropped frames.

(However, digital networks offer an incidence of error extraordinarily small relative to that of

analog networks.)

Frame Relay often serves to connect local area networks (LANs) with major backbones,

as well as on public wide-area networks (WANs) and also in private network environments with

leased lines over T-1 lines. It requires a dedicated connection during the transmission period.

Frame Relay does not provide an ideal path for voice or video transmission, both of which

require a steady flow of transmissions. However, under certain circumstances, voice and video

transmission do use Frame Relay.

Frame Relay originated as an extension of integrated services digital network (ISDN). Its

designers aimed to enable a packet-switched network to transport over circuit-switched

technology. The technology has become a stand-alone and cost-effective means of creating a

WAN.

Frame Relay switches create virtual circuits to connect remote LANs to a WAN. The

Frame Relay network exists between a LAN border device, usually a router, and the carrier

switch. The technology used by the carrier to transport data between the switches is variable and

may differ among carriers (i.e., to function, a practical Frame Relay implementation need not

rely solely on its own transportation mechanism).

ATM CONCEPT AND ARCHITECTURE

The Asynchronous Transfer Mode (ATM) protocols and architecture have managed to gather an

impressive amount of market and media attention over the last several years. Intended as a

technique to achieve a working compromise between the rigidity of the telecommunication

synchronous architecture and packet network's unpredictable load behavior, ATM products are

appearing for everything from high-speed switching to local area networking. ATM has caught

the interest of both the telecommunications community as a broadband carrier for Integrated

Services Digital Network (ISDN) networks as well as the computer industry, who view ATM as

a strong candidate for high-speed Local Area Networking. This article covers the basic concepts

involved in the ATM architecture.

At the core of the ATM architecture is a fixed length "cell." An ATM cell is a short, fixed length

block of data that contains a short header with addressing information, followed by the upper

layer traffic, or "payload." The cell structure, shown in Figure 1, is 53 octets long, with a 5 octet

header, followed by 48 bytes of payload. While the short packet may seem to be somewhat

inefficient in its ratio of overhead to actual data, it does have some distinct advantages over the

alternatives. By fixing the length of each cell, the timing characteristics of the links and the

corresponding network are regular and relatively easy to predict; predicting the dynamics of

variable length packet switched networks isn't always easy. By using short cells, hardware based

switching can be accomplished. Finally, the use of short cells provides an ability to transfer

isochronous information with a short delay.

The information contained in the header of each cell is used to identify the circuit (in the context)

of the local link, carries local flow control information, and includes error detection to prevent

cells from being mis-routed. The remaining 48 octets are routed through the network to the

destination using the circuit.

ATM has evolved over the last 5-10 years to include a wide range of support protocols.

Routing and congestion management, have been particular areas of research. The early concepts

of cell transfer networks revolved around the thought that users could "reserve" a pre-specified

amount of traffic through a circuit on the network. Some amount of guaranteed throughput

would be provided with an additional amount only as needed. Then, through this contract, traffic

in excess of the pre-allocated bandwidth could be arbitrarily dropped if congestion problems

occurred. However, the complexities of implementation have proven these techniques to be far

too difficult. Several vendors have proposed flow control architectures that involve more active

windowing protocols between the switches for data traffic.

ATM ARCHITECTURE

As in the case of many large systems, there are a range of components and connections involved

in the ATM networks. All connections in the ATM network are point-to-point, with traffic being

switched through the network by the switching nodes. Two types of networks are included in the

ATM architecture, Public Networks and Private Networks. Private Networks, often referred to as

Customer Premises Networks, are typically concerned with end-user connections, or bridging

services to other types of networks including circuit switched services, frame relay, and voice

subsystems. The interface between the components in the Private Networks is referred to as the

Private User Network Interface (UNI). ATM also extends into the wider area Public Networks.

Interfaces between the Public and Private network switches conform to the Public UNI.

Interfaces between the switches within the Public network are the Network Node Interface

(NNI). Specifications for both the Public and Private UNI can be found in the ATM Forum's

publication "ATM User-Network Interface (UNI) Specification." The private networks often

permit the use of lower speed short haul interconnects that are useful in LAN environments, but

not of great use in wider area public networks. Three types of NNI have been developed, NNI-

ISSI that connects switches in the same Local Area Transport Area (LATA), the NNI-ICI, that

connects ATM networks of different carriers (InterCarrier), and finally, a Private NNI that

permits the connection of different switches in a private network.

ATM Cell Format

As information is transmitted in ATM in the form of fixed size units called cells. As known

already each cell is 53 bytes long which consists of 5 bytes header and 48 bytes payload.

Asynchronous Transfer Mode can be of two format types which are as follows:

1. UNI Header: which is used within private networks of ATM for communication between

ATM endpoints and ATM switches. It includes the Generic Flow Control (GFC) field.

2. NNI Header: is used for communication between ATM switches, and it does not include

the Generic Flow Control(GFC) instead it includes a Virtual Path Identifier (VPI) which

occupies the first 12 bits.

Working of ATM

ATM standard uses two types of connections. i.e., Virtual path connections (VPCs) which

consists of Virtual channel connections (VCCs) bundled together which is a basic unit carrying

single stream of cells from user to user. A virtual path can be created end-to-end across an ATM

network, as it does not routs the cells to a particular virtual circuit. In case of major failure all

cells belonging to a particular virtual path are routed the same way through ATM network, thus

helping in faster recovery.

Switches connected to subscribers uses both VPIs and VCIs to switch the cells which are Virtual

Path and Virtual Connection switches that can have different virtual channel connections

between them, serving the purpose of creating a virtual trunk between the switches which can be

handled as a single entity. It’s basic operation is straightforward by looking up the connection

value in the local translation table determining the outgoing port of the connection and the new

VPI/VCI value of connection on that link.

ISDN PROTOCOL

ISDN standards specify several reference points that functionally separate the ISDN

network. The ISDN devices need to comply with applicable reference point specifications. For

example, a TE1 device such as an ISDN phone or a computer need to comply with reference

point 'S' specifications. Various reference points specified in ISDN are given in the figure below:

R: This is the reference point between non-ISDN equipment and a Terminal Adapter (TA).

S: This is the reference point between user terminals and Network Termination Type2 (NT2).

T: This is the reference point between NT1 and NT2 devices.

U: This is the reference point between NT1 devices and line termination equipment of the Telco

PHYSICAL LAYER PROTOCOL

At the physical layer the ITU has defined the user network interface standard as I.430 for

Basic Rate Access and I.431 for Primary Rate Access. ANSI has defined the user network

interface standard as T1.601. As already stated above, the physical layer uses the normal

telephone cabling as its physical cabling structure.

ISDN physical layer: For users, the ISDN physical layer is on S reference point or T

reference point. This layer has the following major functions: coding, full-duplex transmission,

channel multiplexing, port activation and depolarization, feeding, and termination identification.

This layer can multiplex multiple links at the data link layer and use AMI, 4B3T and 2B1Q for

coding.

D-CHANNEL DATALINK LAYER

ISDN data link layer: ISDN does not define Layer 2 protocols dedicated to B channels.

Any Layer 2 protocols can be used between two communicating devices after negotiation as long

as they can transparently transmit data on B channels. The link access procedure on the D

channel (LAPD) protocol defined in Q.921 (a reliable transport protocol) is used for D channels,

which is mainly used to carry messages and data generated by Layer 3 entities.

The ISDN D channel will utilise different signalling protocols at Layer 3 and Layer 2 of

the OSI Model. Typically at Layer 2, LAP-D (Link Access Procedure – D Channel) is the Q.921

signalling used and DSS1 (Digital Subscriber Signalling System No.1) is the Q.931 signalling

that is used at Layer 3. It is easy to remember which one is used at which layer by simply

remembering that the middle number corresponds to the layer it operates at.

LAYER 3 PROTOCOLS

ISDN does not define Layer 3 protocols dedicated to B channels. Layer 3 protocol Q.931 for

D channels is mainly used to control and manage connection setup and release on B channels.

Mapping relationship between the ISDN protocol stack model and OSI model

The ISDN B channels will typically utilise a Point-to-Point protocol such as HDLC (High-Level

Data Link Control) or PPP frames at Layer 2 however you can sometimes see other

encapsulation such as Frame relay. As you would expect, at layer 3 you typically see IP packets.

ISDN operates in Full-Duplex which means that traffic can be received and transmitted at the

same time.

NETWORK SIGNALING SYSTEMS

Network Signaling System is a set of telephony signaling protocols developed in 1975,

which is used to set up and tear down telephone calls in most parts of the world-wide public

switched telephone network (PSTN). The protocol also performs number translation, local

number portability, prepaid billing, Short Message Service (SMS), and other services.

In North America SS7 is often referred to as Common Channel Signaling System

7 (CCSS7). In the United Kingdom, it is called C7 (CCITT number 7), number 7 and Common

Channel Interoffice Signaling 7 (CCIS7). In Germany, it is often called Zentraler

Zeichengabekanal Nummer 7 (ZZK-7).

The SS7 protocol is defined for international use by the Q.700-series recommendations of

1988 by the ITU-T.[1] Of the many national variants of the SS7 protocols, most are based on

variants standardized by the American National Standards Institute (ANSI) and the European

Telecommunications Standards Institute (ETSI). National variants with striking characteristics

are the Chinese and Japanese Telecommunication Technology Committee (TTC) national

variants.

The Internet Engineering Task Force (IETF) has defined the SIGTRAN protocol suite

that implements levels 2, 3, and 4 protocols compatible with SS7. Sometimes also called Pseudo

SS7, it is layered on the Stream Control Transmission Protocol (SCTP) transport mechanism for

use on Internet Protocol networks, such as the Internet.

SS7 PROTOCOL

The SS7 protocol stack may be partially mapped to the OSI Model of a packetized digital

protocol stack. OSI layers 1 to 3 are provided by the Message Transfer Part (MTP) and

the Signalling Connection Control Part (SCCP) of the SS7 protocol (together referred to as the

Network Service Part (NSP)); for circuit related signaling, such as the BT IUP, Telephone User

Part (TUP), or the ISDN User Part (ISUP), the User Part provides layer 7. Currently there are no

protocol components that provide OSI layers 4 through 6. The Transaction Capabilities

Application Part (TCAP) is the primary SCCP User in the Core Network, using SCCP in

connectionless mode. SCCP in connection oriented mode provides transport layer for air

interface protocols such as BSSAP and RANAP. TCAP provides transaction capabilities to its

Users (TC-Users), such as the Mobile Application Part, the Intelligent Network Application

Part and the CAMEL Application Part.

The Message Transfer Part (MTP) covers a portion of the functions of the OSI network

layer including: network interface, information transfer, message handling and routing to the

higher levels. Signaling Connection Control Part (SCCP) is at functional Level 4. Together with

MTP Level 3 it is called the Network Service Part (NSP). SCCP completes the functions of the

OSI network layer: end-to-end addressing and routing, connectionless messages (UDTs), and

management services for users of the Network Service Part (NSP). Telephone User Part (TUP) is

a link-by-link signaling system used to connect calls. ISUP is the key user part, providing a

circuit-based protocol to establish, maintain, and end the connections for calls. Transaction

Capabilities Application Part (TCAP) is used to create database queries and invoke advanced

network functionality, or links to Intelligent Network Application Part (INAP) for intelligent

networks, or Mobile Application Part (MAP) for mobile services.

SS7 consists of a set of reserved or dedicated channels known as signaling links. There

are three kinds of network points signaling points: Service Switching Points (SSPs), Signal

Transfer Points (STPs), and Service Control Points (SCPs). SSPs originate or terminate a call

and communicate on the SS7 network with SCPs to determine how to route a call or set up and

manage some special feature. Traffic on the SS7 network is routed by packet switches called

STPs. SCPs and STPs are usually mated so that service can continue if one network point fails.

SS7 uses out-of-band signaling, which means that signaling (control) information travels

on a separate, dedicated 56 or 64 Kbps channel rather than within the same channel as the

telephone call. Historically, the signaling for a telephone call has used the same voice circuit that

the telephone call traveled on (this is known as in-band signaling). Using SS7, telephone calls

can be set up more efficiently and special services such as call forwarding and wireless roaming

service are easier to add and manage.

SS7 is used for these and other services:

Setting up and managing the connection for a call

Tearing down the connection when the call is complete

Billing

Managing call forwarding, calling party name and number display, three-way calling, and

other Intelligent Network (IN) services

Toll-free (800 and 888) and toll (900) calls

Wireless as well as wireline call service including mobile telephone subscriber

authentication, personal communication service (PCS), and roaming.