Advanced topics in Computer Networks Lecture 2: ATM: an overview University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network Outline Why ATM ATM overview ATM standard Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network Why ATM? Problems with IP? Best effort, lack of reliability TCP on top of IP Being stateless Hop to hop or point to point. Lack of QoS. How important is QoS? It is essential in some applications including VoIP Video on demand. Teleconferencing. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network Supporting QoS in IP QoS parameter Bounded delay and jitter. Min rate IP should identify flow and keep state in nodes. Contradictory to original design goal, flexibility and scalability. Can we have both? We try. This is the main challenge of Internet. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM overview ATM = Asynchronous Transfer Mode. ATM = Data Network + Phone Network Combination of Internet method of communication (packet switching) and phone companies’ method (circuit switching) Asynchronous- The way bandwidth is allocated. The time slot is not allocated by temporal position like Synchronous transfer mode, STM, (circuit switch). Transfer mode= Multiplexing and switching. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM overview (cont) Connection oriented- Need signaling like circuit switch. Establish connection. Tear down connection. Create a virtual connection or circuit, then order is preserved in carrying data. Problems? Overhead for connection and tearing down. Loosing connectivity in case of failure in nodes. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM vs. STM STM or TDM Peak rate allocation Fast and simple logic. Not good for bursty traffic like data. ATM- uses statistical multiplexing. Giving bandwidth for label. Has to process cells . Congestion is possible. Variable cell delay (Queuing). But, network controls routing, errors, flow, copying and priorities. Overcome to congestion and delay is fundamental to ATM. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM vs. Phone Network Current phone networks are synchronous (periodic). STM ATM = Asynchronous Transfer Mode Phone networks use circuit switching. ATM networks use “Packet” Switching In phone networks, all rates are multiple of 8 kbps. With ATM service, you can get any rate. You can vary your rate with time. With current phone networks, all high speed circuits are manually setup. ATM allows dialing any speed. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network Interfaces User to Network Interface (UNI): Public UNI, Private UNI Network to Node Interface (NNI): Private NNI (P-NNI) Public NNI = Inter-Switching System Interface (ISSI) Intra-LATA ISSI (Regional Bell Operating Co) Inter-LATA ISSI (Inter-exchange Carriers) Broadband Inter-Carrier Interface (B-ICI) Data Exchange Interface (DXI) Between packet routers and ATM Digital Service Units (DSU) Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM standard Standard is different from concept. It deals with implementation. ATM Forum is a standard body of developer, service provider and other interested bodies. What standard does is to decide: Size of cell Header format Signaling Interfaces Traffic control Network management Testing Internetworking Univ. of Tehran Adv. topics in Computer Network

ATM Cell format GFC=Generic Flow Control (Was used in UNI but not in NNI) The field is used for VPI in NNI VPI/VCI=0/0 Þ Idle cell; 0/n Þ Signaling HEC: 1 + x + x 2 + x 8 4 7 GFC/VPI VPI VCI PT CLP Header Error Checksum(HEC) Data U.S ANSI proposed 64 bytes data with 5 byte header Euro. ETSI proposed 32 bytes with 4 bytes header Finally, 48 bytes with 5 bytes header. Univ. of Tehran Adv. topics in Computer Network

Connection Identifiers VPI/VCI are local and swapped in each switch. VCI is allocated in connection setup time. VPI is usually used by management. By VPI, a set of VCI are treated as the same. VPI/VCI=0/0 Þ Idle cell; 0/n Þ Signaling Each cell contains a 24/28-bit connection identifier First 8/12 bits: Virtual Path, Last 16 bits: Virtual Channel VP service allows new VC's w/o orders to carriers Univ. of Tehran Adv. topics in Computer Network

Header Error Check (HEC) Protects header only Optional Correction mode: Correct one bit errors if no earlier errors Discard cells with bad HEC Used for cell delineation in SONET Recalculated on each hop CRC generator is 1 + x + x 2 + x 8 Single bit error (correction) Multibit error (discard) Correction mode Correction mode Detection mode No Error Error No error detected Univ. of Tehran Adv. topics in Computer Network

ATM protocol stack AAL = ATM Adaptation Layer Peer to peer connection PHY PHY PHY Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network Protocol layers The ATM Adaptation Layer How to break application messages to cells The ATM Layer Transmission/Switching/Reception Congestion Control/Buffer management Cell header generation/removal at source/destination Reset connection identifiers for the next hop (at switch) Cell address translation Sequential delivery Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM network ATM layer: Transporting cells Cell encapsulation and decapsulation. Insertion and extraction of idle cells Cell header processing Cell queuing Cell coping GFC management in UNI Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM network (cont) Connection Admission Ctrl Connection admission or rejection VPI/VCI assignment and removal Exchanging and processing signaling cells Traffic Ctrl: Maintain agreed QoS Usage parameter ctrl Selective cell discarding Implicit service priorities Explicitly forward Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network ATM network (cont) ATM layer management Configuring the network and switch fabric Monitoring the network operation Detecting troubles ATM switch Doing actual cell switching. Univ. of Tehran Adv. topics in Computer Network

Adv. topics in Computer Network History of ATM 1980: Narrowband ISDN adopted Early 80's: Research on Fast Packets Mid 80's: B-ISDN Study Group formed 1986 ATM approach chosen for B-ISDN June 1989: 48+5 chosen (64+5 vs 32+4). October 1991: ATM Forum founded July 1992: UNI V2 released by ATM Forum 1993: UNI V3 and DXI V1 1994: B-ICI V1 Univ. of Tehran Adv. topics in Computer Network