ATM Asynchronous Transfer Mode

ATM Networks Use optical fibre similar to that used for FDDI networks ATM runs on network hardware called SONET ATM cells (packets) are 53 octets long 5 bytes of header information 48 bytes of data ATM networks are packet-switched, but still create a (virtual) circuit through the network Before transfer can occur, the network must create a path (called a virtual circuit) between the two machines Once the virtual circuit (VC) has been established, packets can be transferred between the machines

SONET Hardware ATM/SONET network interface cards use photodetectors to read optical signals and photoreactors to generate them Instead of network hubs, ATM networks use switches to generate virtual circuits These are not the same as Ethernet switches ATM cells do not pass through routers or network nodes As a result, cells are transferred very quickly

Creating Virtual Circuits Creating a virtual circuit has been compared to making a telephone call A network node sends a request to the ATM switch specifying the destination The switch interacts with any other switches necessary to align themselves to form a complete path When communication is complete, the node sends a disconnect message to the switch The switch will then notify all switches involved to release the connection

ATM Switches In ATM, switches act as network hubs and routers They provide interconnection between network nodes They establish a path between source and destination The difference between switches and routers is that switches create this path once, not once for each message

Virtual Circuits Virtual circuits are also often called virtual paths For reduced bandwidth usage for path identification, these are numbered A virtual path identifier is used to indicate which virtual path is intended for a cell Within virtual circuits, a number of channels is available Nodes can share virtual paths Virtual paths can be used to communicate between more than one pair of nodes

ATM Messaging N1 N2 S1 S4 S3 S5 S6 S2 Virtual Circuit #15 Connect: N1 M1 M2 M1 M2 M3 M1 M2 M3 M4 M1 M2 M3 M4 M5 M1 M2 M3 M4 M5 M6 M1 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 M1 M2 M3 M4 M5 M6 M7 M8 Disconnect: N1 M2 M3 M4 M5 M6 M7 M8

ATM Messaging N1 N2 S1 S4 S3 S5 S6 S2 M4 M5 M6 M7 M8 M5 M6 M7 M8 M6 M7 M8 M7 M8 M3 M4 M5 M6 M7 M8 Disconnect: N1

ATM Cell Format: Data Cells Generic flow control: Not currently used Virtual path identifier: An identifier for the virtual path/circuit Virtual channel identifier: An identifier to identify which channel within the specified virtual path/circuit Payload Type: 3 flag bits Cell loss priority: Should the cell be discarded in the event of a congested switch? Header error check: Cyclical redundancy check for the cell header GFC VPIVCIPT CLP HECData x8

Cell Header Field: Payload Type The payload type field contains three bits of flags, which are used for different purposes Let’s focus on the bits for user data First bit: User data or control data? Control cells could include call setup cells Second bit: Is the sending switch congested? Third bit: Is this the last cell in a logical packet? Logical packets will be discussed later when we talked about AAL5

ATM Efficiency ATM switches do not suffer from router-like slowdowns where cells would be buffered and processed Also hub-like duplicates are avoided The fixed size of ATM cells allows for hardware optimizations The time to transmit a 53 octet cell is constant Assembling cells can also be done more quickly by hardware

Virtual Circuits A Detailed Discussion

Virtual Circuits Virtual circuits can be created in 2 ways: 1.Permanent Virtual Circuits 2.Switched Virtual Circuits

Permanent Virtual Circuits PVCs are circuits that are configured manually (through software) Thus they require more effort to create PVCs provide compatible service between any two ATM networks PVCs are used when connections between two computers will be required for long periods of time

Switched Virtual Circuits SVCs are created automatically (by the hardware) when they are needed SVCs are the VCs that were described in the previous class When an SVC is to be created: Each switch communicates with the next switch in the path When an SVC has been created the switches closest to the source and destination send those nodes the VC identifier (VCI: a 24 bit number) The VCI need not be the same value for each node

Virtual Circuit Creation Creation of virtual circuits (PVCs and SVCs) takes a long time While the VC is being created, the VC is not yet useable Creating a VC, sending one cell (packet), and closing the VC is very inefficient and should be avoided The cell transport layer handles circuit management If a VC already exists and is open, the cell transport layer uses the existing VC for transmission If no VC is available to the node, a VC is created, and the VC’s identifier is added to the switches circuit table

ATM Service Model Most ATM layers are implemented in hardware This means that ATM network device drivers are typically high level, where technologies such as TCP/IP tunneling can be implemented easily (discussed later)

ATM Service Model Device Driver (Data Link) Application Adaptation Layer Cell Transport Signal Software Hardware Optical Fibre

Adaptation Layer The adaptation layer provides an abstraction which hides hardware concerns from the application e.g. The 53 octet limit for cell sizes ATM networks use ATM Adaptation Layer 5 (AAL5) technology to provide this abstraction AAL5 provides a high level interface, allowing packets between 1 and 65,535 octets of data AAL5 also handles data integrity maintenance (using a CRC check inside the packets)

AAL5 As stated previously, AAL5 provides a high-level abstraction to the software AAL5 layers appear to accept larger chunks of data, which is segmented into 48 octet cells and managed transparently This process is known as Segmentation and Reassembly (SAR) AAL5 also ensures data integrity by using a checksum (CRC) AAL5 does not deal with cell delivery (as the cell transport layer does), so AAL5 is not necessary inside switches AAL5 is considered an end-to-end layer Whereas cell transport is considered a machine-to-machine layer

Segmentation Segmentation is the process of turning a chunk of data into a group of ATM cells For example, the chunk of data might be a packet from another type of network e.g. An Ethernet frame Since ATM cells travel on the same virtual circuit, they do not arrive out of order Thus sequencing information is not necessary Each sequence of 48 octets is sent in its own cell

Reassembly Reassembly is the process of recombining ATM cells into the original data chunk As the destination node receives cells, it removes the 48 octets and appends it to the end of a buffer One of the configuration parameters, called ‘Payload Type’ is used to indicate the final cell

Data Integrity As mentioned before, the AAL5 subsystem uses a checksum to ensure data integrity The checksum is sent in the last cell A 32 bit (4 octet) CRC

AAL5 Logical Packets HD HDHDHD CRC ATM Cells D AAL5 Logical Packet CRC

ATM Switching Device Driver (Data Link) Application Adaptation Layer Cell Transport Signal Cell Transport Signal Device Driver (Data Link) Application Adaptation Layer Cell Transport Signal Switch AAL5

ATM Switching As can be seen in the diagram, ATM switches exist entirely in the hardware layer As a result, they are much faster than routers, which require software execution Routers must read packets from electronic signals into a memory buffer (which is slow) Routers then convert packets back into electronic signals onto a new network connection

TCP/IP Tunneling Using ATM As an Example (Chapter 18)

ATM Networking Drawbacks Small, finite sized cells provide faster transmission speeds However, 53 octet cells are incompatible with other technologies which are in widespread use ATM addressing also differs significantly from other forms of addressing For example the TCP/IP protocol suite is the most common network protocol system Most Internet applications are based on TCP/IP ATM networks are not broadcast networks Each cell only arrives at its intended destination Broadcasting & multicasting are not directly supported

TCP/IP Tunneling TCP/IP tunneling allows transmission of TCP/IP packets over ATM (and other non- TCP/IP) networks The use of TCP/IP over these networks allows applications which normally only run on TCP/IP networks to operate on all networks

TCP/IP Tunneling with AAL5 AAL5 provides a convenient means to send large IP datagrams over an ATM network The IP datagrams, including the header information can be inserted into an AAL5 logical packet The AAL5 logical packet will be sent, using ATM cells, to the destination The destination will extract the IP datagram and use it as if the datagram were sent across a normal TCP/IP network

IP Switching Researchers are interested to see if they can use switching technology to route IP packets Switching happens faster than routing Techniques for IP switching are hardware- oriented

LAN Emulation Many people are using ATM network hardware in order to achieve ATM speeds in a LAN network Most ATM hardware can be configured to simulate an emulated LAN (E-LAN) E-LANs use the same techniques as TCP/IP tunneling to forward packets such as Ethernet frames using AAL5

ATM Service Categories ATM provides 4 service categories: CBR (Constant Bit Rate): A constant bandwidth is reserved and guaranteed by the network UBR (Unspecified Bit Rate): Data is transmitted when bandwidth is available, and not when bandwidth is not available ABR (Available Bit Rate): The network will provide feedback about network congestion, under the assumption the node will adjust its transmission to meet the current availability of network bandwidth VBR (Variable Bit Rate): A minimum bandwidth is reserved and guaranteed by the network, although available bandwidth may increase/peak above this value RT (Real Time): Fluctuation of bandwidth is minimized  Used for non-guaranteed streaming audio/video NRT (Non Real Time): Any bandwidth available is used  Used for downloads