Network Technology CSE Network Technology CSE3020 Week 10

Network Technology CSE Asynchronous Transfer Mode (ATM) Initially, it was assumed that B-ISDN would continue the same fixed TDM approach of N-ISDN with channel speeds from 64k to 140Mbps. However, TDM is: inflexible in the face of the variety of speeds that needed to be switched. inefficient in handling bursty traffic. complicated since it needs to deal with switching of multiple rates.  ATM was proposed.

Network Technology CSE Asynchronous Transfer Mode (ATM) A C B t 2 t 1 Fixed Time sharing Asynchronous Transfer Mode time address C 2 B 2 A 2 C 1 B 1 A 1 A 2 C 1 A 1

Network Technology CSE ATM - Asynchronous Transfer Mode Developed as part of work on Broadband ISDN, but has found application in non-ISDN environments where high data rates are required Is a streamlined - minimal error and flow control capabilities - packet transfer interface using fixed size packets called cells of 53 octets Takes advantage of the reliability and fidelity of modern digital facilities to provide faster packet switching than X.25 Provides both real-time and non-real-time services Can support information transfer protocols not based on ATM Allows multiple logical connections over a single physical interface

Network Technology CSE ATM Protocol Architecture Refer fig 11.1 Stallings 6E, fig 19.4, 19.20, Forouzan 2E Illustrates the basic architecture for an interface between user and network The following layers relate to ATM functions: –ATM Layer, common to all services and provides packet transfer capabilities: fixed size cells logical connections –ATM Adaptation Layer (AAL), that is service dependent: supports information transfer protocols not based on ATM, by mapping the information into ATM cells

Network Technology CSE Why ATM? The shared media in LANs shows that the switching approach is the more efficient way to build a network. The use of Asynchronous (statistical) TDM to achieve a better handling of emerging multimedia traffic. A network that can cover a greater variety of services – voice, video, data (one for all). A network that is scalable (covers from LANs to WANs).

Network Technology CSE ATM Characteristics Connection-oriented operation. Minimal error and flow control capabilities. Limited header functions. Similarities between ATM and packet switching: – Transfer of data in discrete chunks. – Multiple logical connections over single physical interface. Data rates (physical layer) 25.6Mbps to Mbps. Small “packet” size for shorter delay and delay jitter. Because each packet is fixed and small, the term “cell” is used to describe a “packet” (header + payload) in ATM.

Network Technology CSE ATM Logical Connections Refer figs 11.2, 11.3 Stallings 6E, fig 19.6 Forouzan 2E A Virtual Channel Connection (VCC) is the basic unit of switching in an ATM network A VCC is set up between two end users through the network and supports the full duplex flow of data

Network Technology CSE ATM Logical Connections - Virtual Path Connection A VPC is a bundle of VCCs that have the same endpoints All the cells flowing in VCCs in a VPC are switched together Helps contain control cost, network management actions can be applied to a small number of groups of connections instead of a large number of individual connections

Network Technology CSE ATM Logical Connections virtual path transmission path virtual channels SW 1 SW 2 SW 3  UNINNI UNI User Network InterfaceNetwork-Network Interface 

Network Technology CSE Advantages of Virtual Path Connections (VPCs) Simplified network architecture: –Network transport functions can be separated into those related to : an individual logical connection (VCC) and a group of logical connections (VPC) Network deals with fewer, aggregated connections Performance - reduced processing and short VCC connection setup time: –Much of work is done when VPC is set up –By reserving capacity on a VPC in anticipation of later call arrivals, new VCCs can be established by executing simple control functions at the endpoints –No call processing is required at transit nodes

Network Technology CSE Call Establishment Using Virtual Paths Refer fig 11.3 Stallings 6E The process of setting up a VPC is decoupled from the process of setting up an individual VCC: –VP control mechanisms include calculating routes, allocating capacity and storing connection state information –To set up a VC, there must first be a VP to the required destination node with sufficient capacity to support the VC with appropriate Quality of Service –A VC is set up by storing the required state information - VC/VP mapping

Network Technology CSE Call Establishment Using VPs Make connection Request for VCC Originates VPC exists? Establish a new VPC Block VCC or request more capacity Reject VCC request Can QoS be satisfied? Request granted? yes no yes

Network Technology CSE Virtual Channel Connection Uses Between end users: –End to end user data. –Control signals. –VPC provides overall capacity. VCC organization done by users. Between an end user and a network entity: –Control signaling. Between two network entities: –Network traffic management. –Routing.

Network Technology CSE Virtual Path/Channel Characteristics Quality of service. –Specified by cell loss ratio and cell delay variations. Switched and semi-permanent virtual channel connections. –Switched VCC is an on-demand connection. –Semi-permanent VCC is of long duration and is set up by configuration. Call sequence integrity. Traffic parameter negotiation and usage monitoring. –Traffic parameters are average rate, peak rate, burstiness & peak duration. –Network deals with congestion and manages existing and requested VCCs. Virtual channel Id restriction within a VPC: –Some VC Ids may not be available to the user of a VPC and be reserved for network use eg VCCs for network management

Network Technology CSE Control Signaling: VCC Establishment and release of VPCs and VCCs. Takes place on separate connection. Four methods: –Semi-permanent VCC: User-to-user exchange and no control signaling. –Meta-signaling channel: Used as permanent control signal channel. –User-to-network signaling virtual channel: Used to set up VCCs to carry user data. –User-to-user signaling virtual channel: Within pre-established VPC Used by two end users without network intervention to establish and release user-to-user VCC to carry user data.

Network Technology CSE Control Signaling: VPC Semi-permanent: –By prior agreement. –No control signaling. Customer controlled: –Customer uses a signaling VCC to request the VPC from the network. Network controlled: –Network establishes a VPC for its own convenience. –Path may be network-to-network or user-to-network or user- to-user.

Network Technology CSE ATM Protocol Reference Model Higher Layer ATM Adaptation Layer (AAL) ATM Layer Physical Layer Management Plane Control PlaneUser Plane Layer Management Plane Management

Network Technology CSE ATM Reference Model: Layers Physical Layer: –Specification of transmission medium and signal encoding. ATM Layer: –Provide packet transfer capabilities. –Defines the transmission of data in fixed-size cells. –Defines the use of logical connections. Adaptation Layer: –Maps higher-layer information into ATM cells. –Collects information from ATM cells for delivery to higher layers.

Network Technology CSE ATM Reference Model: Planes User plane: –Provides for user information transfer. Control plane: –Call and connection control functions. Management plane: –Plane management: Whole system functions and coordination between planes. –Layer management: Relating to resources and parameters in protocol entities.

Network Technology CSE ATM Plane-Layer

Network Technology CSE ATM Cells Generic flow control Virtual path id Virtual path id Virtual channel id Payload type CLP Header error control Information field (48 octets) UNI formatNNI format Virtual path id Virtual channel id Payload type CLP Header error control Information field (48 octets)

Network Technology CSE ATM Cells Fixed size (53 octet). 5 octet header and 48 octet information field. Small cells reduce queuing delay for high priority cells. Small cells can be switched more efficiently. Easier to implement switching mechanism in hardware.

Network Technology CSE ATM Cells: Header Format Generic flow control (GFC): –Only at user to network interface. –Controls flow only at the local user-network interface. Virtual path identifier (VPI): –Constitutes a routing field for the network. –8 bits at the UNI and 12 bits at the NNI. Virtual channel identifier (VCI): –Used for routing to and from the end user. Payload type (PT): –e.g. user info or network management Cell loss priority (CLP): –Provide guidance to the network in the event of congestion. Header error control: –Error control and synchronization.

Network Technology CSE Generic Flow Control (GFC) Control traffic flow at user to network interface (UNI) to alleviate short term overload. Two sets of procedures: –Uncontrolled transmission. –Controlled transmission. Every connection is either subject to flow control or not. Subject to flow control: –May be one group (A) default –May be two groups (A and B) Flow control is from subscriber to network. –Controlled by network side.

Network Technology CSE Header Error Control figs 11.5, 11.6 Stallings 6E ATM cell includes an 8 bit HEC that is calculated based on the remaining 32 bits of the header The generator polynomial is x 8 + x 2 + x + 1 Single bit errors can be corrected

Network Technology CSE Header Error Control

Network Technology CSE HEC Operation at Receiver refer fig 11.5, 11.6, 11.7 Stallings 6E –Receiver on initialization --> correction mode –In Correction Mode when a cell is received --> HEC calculation --> comparison is performed If no errors are detected --> no action --> correction mode If an error is detected: –single bit --> correct --> detection mode –multi bit --> cell discarded --> detection mode –In detection mode If Error detected --> cell discarded --> detection mode No attempt is made to correct errors If no errors are detected --> no action --> correction mode

Network Technology CSE Transmission of ATM Cells ATM cells may be transmitted at: Mbps Mbps Mbps Mbps Two transmission structure: –Cell Based physical layer: No framing imposed. Continuous stream of 53 octet cells. Synchronization based on header error control field. –SDH based physical layer: Imposes structure on ATM cell stream. Can carry ATM or STM payloads. Specific connections can be circuit switched using SDH channel. SDH multiplexing techniques can combine several ATM streams.

Network Technology CSE ATM QoS – User Related Attributes Sustainable Cell Rate (SCR): the average cell rate calculated over the duration of the connection. Peak Cell Rate (PCR): defines the sender’s maximum cell rate on the connection Minimum Cell Rate (MCR): defines the minimum cell rate acceptable to the sender. eg 50,000 cells per sec Cell Variation Delay Tolerance (CVDT): is a measure of the variation in cell transmission time. eg if 5ns, this means the difference between minimum and maximum delays should no exceed 5ns.

Network Technology CSE ATM QoS – Network Related Attributes Cell Transfer Delay (CTD): the average time needed for a cell to travel from source to destination. max CTD and min CTD. Cell Delay Variation (CDV): Difference between max CTD and min CTD.Voice traffic is sensitive to this. Cell Loss Ratio (CLR): defines the fraction of cells lost during transmission. Data traffic is sensitive to this. Cell Error Ratio (CER): defines the fraction of cells delivered in error

Network Technology CSE ATM Service Categories Real time service: –Applications concern with the amount of delay and the variability of delay (jitter). –Constant bit rate (CBR). –Real time variable bit rate (rt-VBR). Non-real time service: –Bursty traffic and do not have tight constraints on delay and delay variations. –Non-real time variable bit rate (nrt-VBR). –Available bit rate (ABR). –Unspecified bit rate (UBR). Available bandwidth CBR VBR ABR or UBR

Network Technology CSE ATM Services: Real time Constant bit rate (CBR): –Fixed data rate continuously available, with tight upper bound on delay. –Used for uncompressed audio and video (video conferencing, interactive audio, audio/video distribution and retrieval). Real time variable bit rate (rt-VBR): –Time sensitive applications, requiring tightly constrained delay and delay variation. –rt-VBR applications transmit at a rate that varies with time (e.g. compressed video) –Produces varying sized image frames, so compressed data rate varies. –Can statistically multiplex connections.

Network Technology CSE ATM Services: Non-Real time Non-real time variable bit rate (nrt-VBR): –Improved QoS in the areas of loss and delay. –End system specifies peak cell rate, average rate and burstiness. –Network allocates resources to provide low delay and cell loss. –Used for data transfer with critical response time requirements. e.g. Airline reservations, banking transactions & process monitoring. Available bit rate (ABR). –Application specifies peak cell rate (PCR) and minimum cell rate (MCR). –Resources allocated to give at least MCR. –Spare capacity shared among all ABR sources. e.g. LAN interconnection.

Network Technology CSE ATM Services: Non-Real time Unspecified bit rate (UBR). –Additional capacity is available due to: Not all resources dedicated to CBR and VBR traffic. Bursty nature of VBR. –These unused capacity is used for UBR services. –For application that can tolerate some cell loss or variable delays. e.g. TCP based traffic. –Cells forwarded on FIFO basis. –Both delay and variable losses are possible. –Best effort service.

Network Technology CSE ATM Adaptation Layer (AAL) Support for information transfer protocol not based on ATM. –PCM voice: Assemble bits into cells. Re-assemble into constant flow. –IP: Map IP packets onto ATM cells. Fragment IP packets into a number of cells. Use of IP over ATM allows to retain all IP infrastructure for an ATM network.

Network Technology CSE AAL Services Handling of transmission errors. Segmentation and re-assembly. Handling of lost and misinserted cell conditions. Flow control and timing control. AAL Supported Application types Circuit emulation (e.g. T-1 over an ATM). VBR voice and video. General data service. IP over ATM. Multiprotocol encapsulation over ATM (MPOA) (e.g. IPX, AppleTalk, DECNET). LAN emulation.

Network Technology CSE AAL Protocols Organized in two logical sublayers: –Convergence sublayer (CS): Support for specific applications. AAL user attaches to AAL at Service Access Point (SAP). –Segmentation and re-assembly sublayer (SAR): Packaging information received from CS into cells. Unpacking the information at the other end. Four protocol types: –Type 1 –Type 2 –Type 3/4 –Type 5

Network Technology CSE AAL Protocols

Network Technology CSE AAL Protocols AAL Type 1: –CBR source. –SAR packs and unpacks bits. –Block accompanied by sequence number. AAL Type 2: –VBR information. –Analog applications (video and audio). AAL Type 3/4: –Connectionless or connection oriented. –Message mode or stream mode. AAL Type 5: –Streamlined transport for connection oriented higher layer protocols.

Network Technology CSE AAL Protocols AAL: Segmentation And Reassembly Protocol Data Unit 48 octets 22 octets44 octets 1 octet47 octets SNSNPSAR-PDU payload STSNMIDSAR-PDU payloadLICRC SAR-PDU payload AAL 1 AAL 3/4 AAL 5 SN = Sequence number (4 bits) SNP = Sequence number protection (4 bits) MID = Multiplexing identifier (10 bits) LI = Length indication (6 bits) ST = Segment Type CRC = Cyclic Redundancy Check (10 bits)

Network Technology CSE AAL Protocols and Services CBRrt-VBRnrt-VBRABRUBR AAL 1Circuit Emulation, ISDN, Voice over ATM AAL 2VBR voice and video AAL 3/4General data services AAL 5LAN Emulation Voice on demand, LAN emulation Frame relay, LAN emulation LAN emulation IP over ATM

Network Technology CSE Required Reading W. Stallings, Data and Computer Communications Prentice-Hall. >> Chapter 11 6E, 7E

Network Technology CSE Following slides are for interest only

Network Technology CSE ATM LANs An ATM network can be used as device to connect several local computers allowing them to exchange information. It acts as a LAN in this case. An ATM network can also be used as device to connect several LANs. It acts as a bridge or a backbone network. PROBLEM: It is expected that ATM should provide connectionless MAC service like IEEE 802 MAC sublayer. However, ATM switches are connection- oriented.

Network Technology CSE LAN Emulation (LANE) The ATM MAC sublayer emulates the LAN service by creating the appearance of a virtual shared medium from a point-to-point network. Creating new servers to simulate a broadcast services. The concept is: A LAN Emulation Client (LEC) performs a transmission to a server, the server then send the received information to all LECs. A LEC is an ATM end system.

Network Technology CSE Frame Relay Designed to be more efficient than X.25. X.25: –In band signaling used. –Multiplexing of virtual circuits at layer 3. –Layer 2 and 3 include flow and error control. –Considerable overhead. –Not appropriate for modern digital systems with high reliability. Developed before ATM and larger installed base than ATM. ATM now of more interest on high speed networks.

Network Technology CSE Frame Relay Call control carried in separate logical connection. Multiplexing and switching at layer 2. –Eliminates one layer of processing. No hop-by-hop error or flow control. –End-to-end flow and error control (if used) are done by higher layer. –Single user data frame sent from source to destination and ACK (from higher layer) sent back.

Network Technology CSE Frame Relay: Advantages & Disadvantages Lost link-by-link error and flow control. –Increased reliability makes this less of a problem. Streamlined communications process. –Lower delay. –Higher throughput. Access speed up to 2Mbps.

Network Technology CSE Frame Relay: Protocol Architecture

Network Technology CSE Frame Relay: Control Plane Between subscriber and network. Separate logical channel used. –Similar to common channel signaling for circuit switching services. At data link layer, LAPD (Link Access Procedure for Data services) protocol is used for: –Provide reliable data link control. –Error and flow control between user and network.

Network Technology CSE Frame Relay: User Plane Actual transfer of information between end users. LAPF (Link Access Procedure for Frame Mode Bearer Services) protocol is used for: –Frame delimiting, alignment and transparency. –Frame multiplexing/demultiplexing using addressing field. –Ensure frame is integral number of octets. –Ensure frame is neither too long nor short. –Detection of transmission errors. –Congestion control functions.

Network Technology CSE Frame Relay: Frame Formats

Network Technology CSE Frame Relay: User Data Transfer One frame type used for carrying user data. –No control frame. No inband signaling. No sequence numbers. –No flow control and no error control.

Network Technology CSE ATM Logical Connections Referred to as Virtual channel connections (VCC). Analogous to virtual circuit in X.25. Basic unit of switching. VCC is set up between two end users through the network. Variable-rate, full-duplex flow of fixed size cells. VCCs are used for data, user-network exchange (control) and network-network exchange (network management and routing). Virtual path connection (VPC). –Bundle of VCC with same end points.

Network Technology CSE ATM Logical Connections    Virtual Channel Connection Virtual Path Connection Transmission Path ATM cells Location A Location C Location B

Network Technology CSE Advantages of Virtual Paths Simplified network architecture. –Individual logical connection (virtual channel) and a group of logical connections (virtual paths). Increased network performance and reliability. –Network deals with fewer aggregated entities. Reduced processing and short connection setup time. –Much of the work with virtual path set up. –Reserving capacity on a virtual path for new virtual channels. –Minimal processing for a new virtual channel to an existing virtual path. Enhanced network services. –User may define closed user groups or closed network of virtual channel bundles.