ATM Networks An Engineering Approach to Computer Networking An Engineering Approach to Computer Networking
Why ATM networks? Different information types require different qualities of service Different information types require different qualities of service ■ ■ from the network from the network ◆ stock quotes vs. USENET stock quotes vs. USENET ◆ Telephone networks support a single quality of service Telephone networks support a single quality of service ■ ■ ◆ and is expensive to boot and is expensive to boot ◆ Internet supports no quality of service Internet supports no quality of service ■ ■ ◆ but is flexible and cheap but is flexible and cheap ◆ ATM networks are meant to support a range of service qualities ATM networks are meant to support a range of service qualities ■ ■ at a reasonable cost at a reasonable cost ◆ potentially can subsume both the telephone network and the potentially can subsume both the telephone network and the ◆ Internet Internet
Design goals Providing end-to-end quality of service Providing end-to-end quality of service ■ ■ High bandwidth High bandwidth ■ ■ Scalability Scalability ■ ■ Manageability Manageability ■ ■ Cost-effective Cost-effective ■ ■
How far along are we? Basic architecture has been defined Basic architecture has been defined ■ ■ But delays have resulting in ceding desktop to IP But delays have resulting in ceding desktop to IP ■ ■ Also, little experience in traffic specification, multicast, and fault Also, little experience in traffic specification, multicast, and fault ■ ■ tolerance tolerance We may never see end-to-end ATM We may never see end-to-end ATM ■ ■ ◆ but its ideas continue to powerfully influence design of next- but its ideas continue to powerfully influence design of next- ◆ generation Internet generation Internet ◆ Internet technology + ATM philosophy Internet technology + ATM philosophy ◆ Note--two standardization bodies Note--two standardization bodies ■ ■ ◆ ATM Forum ATM Forum ◆ ◆ International Telecommunications Union-Telecommunications International Telecommunications Union-Telecommunications ◆ Standardization Sector (ITU-T) Standardization Sector (ITU-T)
Concepts 1. Virtual circuits 1. Virtual circuits 2. Fixed-size packets ( cells cells ) ) 2. Fixed-size packets ( 3. Small packet size 3. Small packet size 4. Statistical multiplexing 4. Statistical multiplexing 5. Integrated services 5. Integrated services Together Together can carry multiple multiple types of traffic can carry with end-to-end quality of service
1. Virtual circuits Some background first Some background first ■ ■ Telephone network operates in synchronous transmission mode synchronous transmission mode Telephone network operates in ■ ■ ◆ the destination of a sample depends on where it comes from, and the destination of a sample depends on where it comes from, and ◆ when it came when it came ◆ example--shared leased link example--shared leased link ◆ Problems with STM Problems with STM ■ ■ ◆ idle users consume bandwidth idle users consume bandwidth ◆ ◆ links are shared with a fixed cyclical schedule => links are shared with a fixed cyclical schedule => quantization quantization of of ◆ link capacity link capacity ✦ can’t ‘dial’ bandwidth can’t ‘dial’ bandwidth ✦
Virtual circuits (contd.) STM is easy to overcome STM is easy to overcome ■ ■ ◆ use use packets packets ◆ ◆ metadata metadata indicates destination =>arbitrary schedule and no wasted indicates destination =>arbitrary schedule and no wasted ◆ bandwidth bandwidth Two ways to use packets Two ways to use packets ■ ■ ◆ carry entire destination address in header carry entire destination address in header ◆ ◆ carry only an identifier carry only an identifier ◆ Data Sample ATM cell VCI Data Datagram Addr. Data
Virtual circuits (contd.) Ids save on header space Ids save on header space ■ ■ But need to be pre-established But need to be pre-established ■ ■ We also need to switch Ids at intermediate points (why?) We also need to switch Ids at intermediate points (why?) ■ ■ Need translation table translation table and and connection setup connection setup Need ■ ■
Features of virtual circuits All packets must follow the same path (why?) All packets must follow the same path (why?) ■ ■ Switches store per-VCI state Switches store per-VCI state ■ ■ ◆ can store QoS information can store QoS information ◆ Signaling => separation of data data and and control control Signaling => separation of ■ ■ Virtual circuits do not automatically guarantee reliability Virtual circuits do not automatically guarantee reliability ■ ■ Small Ids can be looked up quickly in hardware Small Ids can be looked up quickly in hardware ■ ■ ◆ harder to do this with IP addresses harder to do this with IP addresses ◆ Setup must precede data transfer Setup must precede data transfer ■ ■ ◆ delays short messages delays short messages ◆ Switched vs. Permanent virtual circuits Switched vs. Permanent virtual circuits ■ ■
More features Ways to reduce setup latency Ways to reduce setup latency ■ ■ ◆ preallocate preallocate a range of a range of VCIs VCIs along a path along a path ◆ ✦ Virtual Path Virtual Path ✦ ◆ send data cell along with setup packet send data cell along with setup packet ◆ ◆ dedicate a VCI to carry dedicate a VCI to carry datagrams datagrams, reassembled at each hop , reassembled at each hop ◆
2. Fixed-size packets Pros Pros ■ ■ ◆ Simpler buffer hardware Simpler buffer hardware ◆ ✦ packet arrival and departure requires us to manage fixed buffer packet arrival and departure requires us to manage fixed buffer ✦ sizes sizes ◆ Simpler line scheduling Simpler line scheduling ◆ ✦ each cell takes a constant chunk of bandwidth to transmit each cell takes a constant chunk of bandwidth to transmit ✦ ◆ Easier to build large parallel packet switches Easier to build large parallel packet switches ◆ Cons Cons ■ ■ ◆ overhead for sending small amounts of data overhead for sending small amounts of data ◆ ◆ segmentation and segmentation and reassembly reassembly cost cost ◆ ◆ last unfilled cell after segmentation wastes bandwidth last unfilled cell after segmentation wastes bandwidth ◆
3. Small packet size At 8KHz, each byte is 125 microseconds At 8KHz, each byte is 125 microseconds ■ ■ The smaller the cell, the less an endpoint has to wait to fill it The smaller the cell, the less an endpoint has to wait to fill it ■ ■ ◆ packetization packetization delay delay ◆ The smaller the packet, the larger the header overhead The smaller the packet, the larger the header overhead ■ ■ Standards body balanced the two to prescribe 48 bytes + 5 byte Standards body balanced the two to prescribe 48 bytes + 5 byte ■ ■ header = 53 bytes header = 53 bytes ◆ => maximal efficiency of 90.57% => maximal efficiency of 90.57% ◆
4. Statistical multiplexing Suppose cells arrive in bursts Suppose cells arrive in bursts ■ ■ ◆ each burst has 10 cells evenly spaced 1 second apart each burst has 10 cells evenly spaced 1 second apart ◆ ◆ gap between bursts = 100 seconds gap between bursts = 100 seconds ◆ What should be service rate of output line? What should be service rate of output line? ■ ■
Statistical multiplexing We can trade off worst-case delay against speed of output trunk We can trade off worst-case delay against speed of output trunk ■ ■ SMG = sum of peak input/output rate SMG = sum of peak input/output rate ■ ■ Whenever long term average rate differs from peak, we can Whenever long term average rate differs from peak, we can ■ ■ trade off service rate for delay trade off service rate for delay ◆ key to building packet-switched networks with QoS key to building packet-switched networks with QoS ◆
5. Integrated service Traditionally, voice, video, and data traffic on separate networks Traditionally, voice, video, and data traffic on separate networks ■ ■ Integration Integration ■ ■ ◆ easier to manage easier to manage ◆ ◆ innovative new services innovative new services ◆ How do ATM networks allow for integrated service? How do ATM networks allow for integrated service? ■ ■ ◆ lots of bandwidth: hardware-oriented switching lots of bandwidth: hardware-oriented switching ◆ ◆ support for different traffic types support for different traffic types ◆ ✦ signaling signaling ✦ ✦ admission control admission control ✦ ✦ easier scheduling easier scheduling ✦ ✦ resource reservation resource reservation ✦
Challenges Quality of service Quality of service ■ ■ ◆ defined, but not used! defined, but not used! ◆ ◆ still needs research still needs research ◆ Scaling Scaling ■ ■ ◆ little experience little experience ◆ Competition from other LAN technologies Competition from other LAN technologies ■ ■ ◆ Fast Ethernet Fast Ethernet ◆ ◆ FDDI FDDI ◆ Standardization Standardization ■ ■ ◆ political political ◆ ◆ slow slow ◆
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