1 ATM: What it is, and what it isn't Carey Williamson University of Calgary

2 ATM: What it is n Asynchronous Transfer Mode n A low-layer networking technology based on fast packet-switching of small fixed size packets called cells n ATM provides a single transport mechanism for integrated services traffic: data, voice, video, image, graphics... n All statistically multiplexed at ATM layer

3 ATM: What it isn’t n Synchronous Transfer Mode (STM) n STM relies on pre-assigned “slots” for each user within a frame, and global timing information to mark frame boundaries n Example: T1 transmission (1.544 Mbps)

4 T1 Transmission 125 microseconds (8000 cycles/sec) Framing bit 8 bit sample for call i An example of one frame from T1 digital transmission scheme Room for 24 calls, with 8 bits from each 24 x 8 bits = 192 bits + 1 framing bit = 193 bits 193 bits/frame X 8000 frames/sec = Mbps

5 T1 Transmission (Cont’d) Frame 1Frame 2Frame 3 Call i uses slot i in each frame 8 bits/slot X 1 slot/frame X 8000 frames/sec = bits/sec = 64 kbps Calls can be allocated k slots per frame to give allocated bandwidth that is k x 64 kbps Idle slots are wasted

6 Synchronous Transfer Mode n STM relies on positional association: slots are identified by their relative position from the start of the frame (global timing info) n Each user knows which slot(s) to use n All slots are the same size (e.g., 8 bits) n Bandwidth allocated in multiples of slots n Efficient for Constant Bit Rate traffic n Inefficient for Variable Bit Rate traffic

7 Asynchronous Transfer Mode n ATM does not use a priori assignment of slots to users n Slots are assigned “on demand” on an as needed basis n Users can use whichever slots are empty

8 ATM Transmission Bandwidth allocation in ATM may reserve a certain percentage of the total slots for a given call, but which slots you get is determined at time of transmission All slots are the same size (53 bytes = 1 ATM cell) Slotted transmission scheme Any user can use any empty slot No notion of specific slots assigned to specific users Can support arbitrary bit rates

9 Asynchronous Transfer Mode n No global timing relationship between slots (i.e., cells) of different users (asynchronous) n Efficient for Variable Bit Rate traffic n Implication: the cell in each slot has to be completely self-identifying (i.e., overhead)

10 Advantages of ATM n Better for bursty traffic (i.e., VBR) n Statistical multiplexing gain n Better network utilization n Same mechanism works for all traffic types n Simple and fast hardware switching

11 Characteristics of ATM n Point to point technology n Connection-oriented: an end-to-end connection (called a virtual channel) must be set up using a signalling protocol before any data cells can be sent on that VC n “Bandwidth on demand” n Statistical multiplexing n Integrated services

12 ATM Cell n 53 bytes n 5 byte header n 48 byte payload (data) n Virtual Path Identifier (VPI) n Virtual Channel Identifier (VCI) n Simple control fields

13 ATM Cell Format GFCVPI Payload (48 bytes) VPIVCI HEC PTCLPVCI ATM UNI Cell Specification RES

14 Why ATM will win n ATM is a scalable technology -scalable in bandwidth -scalable in distance -scalable in deployment

15 ATM: Scalable Bandwidth n ATM is not tied to any particular bit rate or physical layer network technology n ATM is simply the abstract concept of fast packet switching with small fixed size cells n Can do low speed ATM (e.g., 1.5 Mbps) n Can do high speed ATM (e.g., 155 Mbps) n Primary interest: high speed ATM networks

16 ATM: Scalable Distance n ATM can be used for LANs n ATM can be used for MANs n ATM can be used for WANs n Initial market: ATM LANs, enterprise area networks, LAN backbones n Future: wide area network backbone, ATM to the desktop, wireless ATM

17 ATM: Scalable Deployment n Emphasis on interoperability, compatibility n Incremental evolutionary path to ATM n Ethernet => switched Ethernet => ATM hub n Start with one switch, N ports, plus NICs n Add more ports as needed n Add more switches as needed n Hierarchical cascading structure

18 Why ATM will win n ATM is a scalable technology -scalable in bandwidth -scalable in distance -scalable in deployment n Global standards (ATM Forum, ITU) n Billions of dollars invested