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ISDN Layers, Protocols & Components

Posted in WAN Technologies

Our previous article was an Introduction To The ISDN Protocol. This article dives a bit deeper by examining ISDN Layers, Protocols & Components.

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, see Table 1 below.

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; please see the ITU-T I.414 “Overview of Recommendations on Layer 1 for ISDN and B-ISDN customer accesses” document on the ITU's website. 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.

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.

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.

Table 1

OSI Layer

B Channel

D Channel



DSS1 (Q.931)



LAP-D (Q.921)


I.430/I.431 or ANSI T1.601

Users requiring information on how to configure a Cisco router for ISDN dialup can read our How To Configure ISDN Internet Dialup On A Cisco Router article.


The Different ISDN Components

As part of the ISDN Standards, there are several types devices that are used to connect to the ISDN network which are known as Terminal Equipments (TE) and also Network Termination (NT) equipment. You also have Reference Points which are used to define the connections between the various equipment that is used within the ISDN network.


Terminal Equipment and Network Termination Definitions;

Introduction To The ISDN Protocol

Posted in WAN Technologies

ISDN History

ISDN (Integrated Services Digital Network) is a digital telephone standard designed to replace analogue connections by utilising ordinary copper wires that are used in standard analogue telephone systems. It started as a recommendation within the ITU's (International Telecommunication Union) Red Book in 1984, although prior to 1992, the ITU was known as the CCITT (International Telegraph and Telephone Consultative Committee). The ITU is responsible for developing recommendations on International Standards within the industry.

ISDN was developed to provide digital transmission of both voice and data resulting in better quality and speeds over that of PSTN (Public Switched Telephone Network) systems.

Getting to Know the Digital Protocol

There are two types of IDSN Channels.

•  The B-Channel – This is known as the Bearer (“B”) channel which is a 64Kbps channel used for voice, video, data or multimedia transfer. These can be aggregated together to get higher bandwidth utilisation.

•  The D-Channel – This is known as the Delta (“D”) channel which can be either 16Kbps or 64Kbps used primarily for the signalling between the switching equipment. Some say that this adds to the security of ISDN because the controlling and data channels are separate.

N.B. Digital Signal 0 (DS0) is a basic digital signalling rate of 64Kbits which may be used to describe a single Bearer channel.

Users requiring information on how to configure a Cisco router for ISDN dialup can read our How To Configure ISDN Internet Dialup On A Cisco Router article.

BRI (Basic Rate Interface)

Can also be known as BA (Basic Access), this operates a single 16Kbps D channel and two 64Kbps B channels. Although it isn't usually pointed out, the BRI total speed is 192Kbps, this is because you have an additional 48Kbps overhead for framing and synchronisation on the D channel. (64 * 2) + (16 + 48) = (128 + 64) = 192Kbps.


Switches & Bridges

Posted in Network Fundamentals


By now you can see the limitations of a simple hub and when you also read about Ethernet, you start to understand that there are even more limitations. The companies who manufacter hubs saw the big picture quickly and came out with something more efficient, bridges, and then the switches came along! Bridges are analysed later on in this section.

Switching Technology

As we mentioned earlier, hubs work at the first layer of the OSI model and simply receive and transmit information without examining any of it.

Switches (Layer-2 Switching) are a lot smarter than hubs and operate on the second layer of the OSI model. What this means is that a switch won't simply receive data and transmit it throughout every port, but it will read the data and find out the packet's destination by checking the MAC address. The destination MAC address is located always at the beginning of the packet so once the switch reads it, it is forwarded to the appropriate port so no other node or computer connected to the switch will see the packet.

Switches use Application Specific Integrated Circuits (ASIC's) to build and maintain filter tables.
Layer-2 switches are alot faster than routers cause they dont look at the Network Layer (thats Layer-3) header or if you like, information. Instead all they look at is the frame's hardware address (MAC address) to determine where the frame needs to be forwarded or if it needs to be dropped. If we had to point a few features of switches we would say:

Hubs & Repeaters

Posted in Network Fundamentals


Here we will talk about hubs and explain how they work. In the next section we will move to switches and how they differ from hubs, how they work and the types of switching methods that are available; we will also compare them.

Before we start there are a few definitions which I need to speak about so you can understand the terminology we will be using.

Domain: Defined as a geographical area or logical area (in our imagination) where anything in it becomes part of the domain. In computer land, this means that when something happens in this domain (area) every computer that's part of it will see or hear everything that happens in it.

Collision Domain: Putting it simple, whenever a collision between two computers occurs, every other computer within the domain will hear and know about the collision. These computers are said to be in the same collision domain. As you're going to see later on, when computers connect together using a hub they become part of the same collision domain. This dosen't happen with switches.

Broadcast Domain: A domain where every broadcast (a broadcast is a frame or data which is sent to every comeputer) is seen by all computers within the domain. Hubs and switches do not break up broadcast domains. You need a router to achieve this.

There are different devices which can break-up collision domains and broadcast domains and make the network a lot faster and efficient. Switches create separate collision domains but not broadcast domains. Routers create separate broadcast and collision domains. Hubs are too simple to do either, can't create separate collision or broadcast domain.

Network Address Translation (NAT) Overload - Part 2

Posted in Network Address Translation - NAT

In our previous article, we explained what NAT Overload is and how it works. This page deals with the analysis of the packets that traverse a NAT Overload enabled device. We'll examine which fields of the packets are modified and how the NAT device, a router in our example, keeps track of them in its NAT Table.

In order to keep things simple, we're going to use a few simple examples and then deal with a few more complicated ones, this should help make the complex stuff much easier to understand and digest.

Time to grab something to drink or munch on, and prepare to download this information into your head!

Readers interested in learning how to configure NAT on a Cisco router can visit our Cisco Routers section

How NAT Translations Take Place

When covering Dynamic and Static NAT, we saw that it was either the Source or Destination IP Address that had to be modified by the NAT device. No matter which mode was used, the Source and Destination ports were never altered in any way.

NAT Overload on the other hand will use a single public IP Address for the routing process and change, in most cases, the Source or Destination port depending on whether it's an incoming or outgoing packet.

In the next diagram we have two computers that have each sent a packet out to the Internet and are expecting a reply. We take a look at how the router deals with these packets individually and where the information required to identify the expected replies is stored:


You've got to agree that that's a very simple setup. To make life easy, I haven't included any additional information about the generated packets because we'll deal with them individually.


So it's time to take a look at how the router deals with this first packet which belongs to Workstation 1:


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