the operation of switching and signalling within core networks

The purpose of signalling is so that when the caller dials a number they hear progress tones such as the dial-tone, the ringing or busy signals etc. also the caller might hear a digital message telling them the number they have dialled in is not in service or it has been changed. Signals in public networks are used for billing, monitoring and links to advance features for carriers to exchange traffic with each other. Signalling is used also to process every switched call on the public switched network and the public cell network.

Touch-tone phones use 8 voice-frequency tones for signalling, pushing a button will result in one low-tone and one high-tone to be transmitted at the same time.

Pressing the number seven on the keypad will result in 1209Hz and 852Hz being sent at the same time.

The above shows In-channel and Common-channel Signalling.

The latest standard for Common-Channel signalling is 64 kbits/S.

The 4 phases of carrying traffic in a telephone network are the idle phase which is when the terminal of the subscriber (the phone or computer) is idle. Then there is the connection set-up phase which is when a telephone call or a data connection between two or more subscribers is being set-up, the there is the transfer phase which is when the call is in progress or data is being exchanged between the subscribers. And finally there is the release phase which is when the call or the data connection is being released.

There are two types of switching circuit switching and packet switching. In circuit switching the path is decided before the data transmission begins. The system decides on which route to follow, based on a resourse-optimzing algorithm, and transmission goes according to the path. For the whole length of the communication session between the two communicating bodies, the route is dedicated and exclusive and released only when the session ends. In packet-switching the packets of data are sent towards their destination irrespective of each other. Each packet has to find its own path to the destination. There is no predetermined path, the decision as to which node to hop to in the next step is taken only when a node is reached. Each packet finds its way using the information it carries such as the source and destination IP addresses.

Each Packet from a datagram is sent independently and some of the packets may arrive out-of-order. This means the destination node must reorder, but the packets may be lost so the destination node or exit node must detect the lost packet.

A virtual circuit has a pre-planned route determined before the packets are sent. There is not a dedicated line as in circuit switching, as others can use the same circuit, so the packets may be queued. Each of the packets has a virtual circuit identifier or a VCI.

Advantages of a datagram are that the call time setup is avoided, datagrams can be routed away from any congestion, the delivery is more reliable, suitable when only a few packets need to be transmitted and if the node fails the datagrams will find another route.

Advantages of a virtual circuit are that a virtual circuit is suitable when packets are to be transmitted between 2 computers for a period of time, services can be provided for virtual circuit such as error control which is when if a packet arrives at one node with an error code it can be requested again, and virtual circuits are faster because there is no routing decision required at each node.

Next i will include som equestions and there answers that I have completed about Voice over IP (VoIP) and Voice over ATM (VoATM)

Voice over IP

1. State what VoIP is.

2. Discuss Voice Quality with VoIP.

3. List 4 different applications of VoIP

4. Describe one of those applications via diagram or text

Voice over ATM

5. What is ATM

6. Discuss the ATM Cell

7. Discuss VoATM

8. What are the advantages and disadvantages of ATM

1. VoIP enables data networks such as the internet, local area networks and wide area networks to be used for voice communication and it reduces the cost of communication.

2. Voice quality is very important in VoIP and the quality must be as good as the PSTN. There are 3 factors that affect voice quality in ViOP, transmission delay is the first which is the time it takes a packet from source to destination, the second is Jitter delay which is the difference in arrival time between packets and the third is packet loss which is when if a packet is dropped there is a gap in the conversation eg. You won’t be able to makeout the full word or sentence you are being told, packet loss with VoIP can cause big bits of a conversation to be lost.

3. Four applications of VoIP are computer to computer, computer to multiple sites, voice or internet via ISP and voice over intranet or LAN.

4. Computer to multiple sites is a traditional analog telephone attached to an IP telephony enabled PBX and this requires an expensive adapter.

5. ATM or asynchronous transfer mode is a connection-oriented, cell based technique for the transfer of information, ATM can be used for the transfer of audio, video and data. ATM is an extremely fast broadband switching technology eg high bandwidth.

6. ATM uses a uniform 53 byte cell, 5 bytes of addressing information, 48 bytes of payload. The benefit of the small cell size is reduced latency in transmitting through the network nodes. There is a disadvantage which is because of the small cell size it means that there is increased overhead.

7. Class A provides for Voice Over ATM and class A includes constant bit rate or CBR services for connection-oriented transfer with end-to-end synchronisation. Protocol AAL 1 supports services class A and some examples of this include PCM-coded voice, circuit-emulated connections having a bit rate of n x 64kbits/s or n x 2Mbits/s and video signals coded for constant bit rate.

8. The advantages of ATM are that it provides hardware switching, allows dynamic bandwidth of bursty traffic, it provides QoS for multimedia and voice, it scales in speed and network size, its used for LAN and WAN, has good network management capabilities and is used by 80-85% of internet backbone networks. But there are some disadvantages which are it has small cell size, has high overhead, has a high service cost, requires new equipment and requires new technical expertise.