Lecture on Layering

Peer Processors: A pair of layers talking directly. Sometimes called modules.

Layers: Collection of communication functions defined by standards.

Two aspects of layered communications use to exchange data streams.

1. Protocol (distributed algorithm): Peer layers use to change data streams.

2. Interface between a module and the modules at lower and higher levels.

Layers of Signaling Protocol

7 Layers of OSI Standard

Layer 1 – Physical Layer: Provides a virtual bit pipe for transmitting a sequence of bits between any pair of nodes linked by a physical communication channel.

1. Twisted pair2. Coaxial cable3. Fiber optics4. Air wave5. Modulation techniques are of top concern

Modem : Maps bits into signals or waveforms.1. Synchronous versus Asynchronous2. Types of modulation (e.g., PSK, FSK, AM)3. Implemented along with physical device standards (e.g., RS 232, X.21

protocol, IEEE482)

Layer 2 – Data Link Control (DLC) Layer: Layer which creates a virtual link for reliable transmission.1. Converts virtual bit pipes into reliable communication links.2. Deals with a single link connecting two end points at a time.3. Asynchronous communication is needed to perform error detection,

correction, and retransmission.4. Framing is needed to separate each packet (e.g,beginning of packet, end

of packet, spacer, idle, etc.5. For multiple access links, Medium Access Control (MAC) function is needed.

Layer 3 – Network Layer: Complex layer dependent on network equipment.

1. Deals with end to end communication2. Accepts packets from, and delivers to Transport Layer and DLC layer3. Performs routing and switching functions.4. Performs flow control functions (i.e., decides when to accept or reject

packets from Transport Layer)

5. Decides when to transmit packets to another node (thus requires buffering)

6. General control packets7. All intermediate Network Layers must work together8. Complex distributed algorithms are required.

Layer 4 – Transport Layer: Breaks messages into packets and reassembles them at the other end (requires buffering).

1. Concentrates low rate sessions.2. Breaks up high rate sessions.3. Adds reliability if required (or when failures occur)4. Complex layer requiring inter – working of all intermediate network

elements.

Layer 5 – Session Layer : Establish sessions by performing all initialization tasks necessary.

1. Security check2. Payment setup3. Directory assistant

Layer 6 – Presentation Layer : Convert raw data stream into a form suitable for transmission.

1. Data encryption2. Data compression3. Code conversion

Layer 7 – Application Layer : What users see.1. File Transfer Protocol2. Email3. Web Browser4. Virtual Terminal Protocol

Signaling Protocol Terms

Service Access Point (SAP) : a logical point of delivery for signaling information.

Primitive : unit of signaling information exchange across layers

- Request : request for initiation of service (higher to lower layer)- Indication : alert occurrence of event (lower to higher layer)- Response : act according to request (higher to lower layer)- Confirm : confirm the result of action (lower to higher layer)

Protocol Data Unit (PDU) : unit of signaling information exchange between the peer layers. PDU=protocol control + information.

Service Data Unit (SDU) : upper layer generated information that creates PDU at the present layer.

Generic Signaling Process

Logical Separation of Information Flow

User plane

Control plane

Management plane

These planes may or may not be physically separated

Digital Service Example - ISDN

End to end digital and low speed packet communication service. Signaling is done through “D” channel.

Figure 2.1 An ISDN network

___________________________________________________________________________________________Access Configuration----------------------------------------------------------------------------------------------------------------------------------------2B+D Two 64-kbps channels, plus a 16-kbps packet/signaling channel (also known as basic rate)23B+D Twenty-Three 64-kbps channels, plus a 64=-kbps packet/signaling channel (also known as primary rate)Nx64+D n(1 n 23) 64-kbps channels, plus a 64-kbps packet/signaling channel30B+D Thirty 64-kbps channels, plus a 64 kbps packet/signaling channel (Europe)H0+D A nonchannelized 384-kbps channel plus one 64-kbps packet/signaling channel.H11 A nonchannelized 1.536-kbps (signaling to be provided on another D-channel interface)H12 A nonchannelized 1.920-kbps (signaling to be provided on another D-channel interface)*___________________________________________________________________________________________*Europe.

Digital Service Example – Frame Relay

Connection oriented fast packet service

Simplified Data Link Layer functions for higher throughput- Framing (done)- Error detection (not done)- Error is recovered on an end to end basis.

Permanent Virtual Circuit (PVC), Switched Virtual Circuit (SVC)

Digital Service Example – Cell Relay

Connection oriented, sequence preserving transfer of ATM cells.

Access speed: DS3 (44.736 Mbps), STS-3c (155.52 Mbps), STS-12c (622.08 Mbps)

Service types- Class A: constant bit rate- Class B: variable bit rate with timing- Class C: connection oriented data service- Class D: connectionless data service- Class X: unrestricted

Connection Type- Point to point- Point to multi point- Multi point to multi point

Signaling System 7

Packet Data System

56 - 64Kb/sec

Digital

Message up to 272 Octets

Signaling for 30,000 Trunks

Versatile Transaction Capability

International Recommendation

Signaling System 7 (Continued)

Traditional telephone uses in-band signaling- Suited for human generated signaling messages

Common channel signaling is developed for high speed/performance equipment (e.g.,PBX, router, computer)

Features- Out-of-band signaling- Greater number of messages- Dedicated packet switched network for message transfer- High performance of signaling- Sophisticated services enabled.

SS7 Standards

Types of network supported

-POTS-PANS-ISDN-B-ISDN-Circuit switched data transmission system-Mobile wireless networks-Operation, administration, and maintenance

Division of functions- Message Transfer Part (MTP): ensures reliable transmission- User parts:provide criteria for service management

Message Transfer Part

Reliable transmission- Loss of message < 10-7

- Miss-sequencing of message < 10-10

- Undetected error < 10-10

- DLC has bit oriented framing.

Signaling System 7 is a packet data network linking central offices to each other and to service control points. It will be used for call control signaling and for data base access.

ASP - Application Service Part OMAP - Operation & Maintenance Part

SCCP - Signaling Connection Control Part TCAP - Transaction Capabilities Part

The SS7 protocol is also a layered model, however, it is significantly different from the OSI model. It has multiple very inclusive applications layers, and the OSI Network layer is divided into two SS7 layers.

Both the Signaling Point and the Service Switching Point are central offices with SS7 signaling capability. The SP does only call control signaling. The SSP does data base access and special call handling. Central offices may have both features.

The STP is the packet switch of the SS7 network. It receives message signal units from SS7 links, examines the routing label and sends them out on links toward their destination. A number of vendors are offering STPs.

The Service Control Point development system is representative of the machines which will be deployed in field SCPs. It is s multi-processor system with gigabytes of disk storage. It uses the Transactions Capability of SS7 to respond to data base queries.

The SCP Node is comprised of hardware, system software and generic node software. It can serve any variety of applications up to the limits of its processing capacity.

The Transaction Capability Part of the SS7 protocol makes possible such services as BOC 800 Service, Alternate Billing Service and Private Virtual Networks. The TCAP exchange shown above is typical.

Data bases used in SS7 are maintained by two specialized Operations Support Systems. The Service Management System (SMS) is used to maintain SCP data bases and the Signaling Engineering and Administration System (SEAS) is used to build and maintain routing tables and address translation tables in STPs.

The SMS facilities SCP data maintenance by presenting a user friendly interface to operations personnel, then compiling the entered data into records usable by the SCP.