Advanced Computer Networks: Lecture 16. This lecture will cover the following: example of a real network Asynchronous Transfer Mode (ATM); how switches are built and contention within switches; connection-oriented packet-switched network; virtual-circuit routing; encapsulation and segmentation for AAL 5;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 16 Where we are now … • Understand different ways to move through network (forwarding) – Read signs at each switch (datagram) – Follow a known path (virtual circuit) – Carry instructions (source routing) • Bridge approach to extending LAN concept Where we are now … • Next – Example of a real network (ATM) – How switches are built and contention within switches ATM (Asynchronous Transfer Mode) • Defined by ATM Forum (formed in Oct. 1991) – Telephone industry (link providers to build networks) – Data network industry • High speed switching technology: right thing at right place at right time ? ? ? ATM (Asynchronous Transfer Mode) • Common in WANs, can also be used in LANs – Competing technology with Ethernet, but areas of application only partially overlap • Connectionoriented packet switched network – Virtualcircuit routing • Typically implemented on SONET (other physical layers possible) ATM (Asynchronous Transfer Mode) • Signaling (connection setup) Protocol: Q.2931 – Discovering routes and allocating resources at switches • ATM address format – E.164 and NSAP (Network Service Access Point) – Different from MAC addresses ATM Signaling • Connection setup called signaling (standard Q.2931) • Route discovery, resource resv, QoS, • Send through network – Request setup circuit – Send setup frame on setup circuit • Establish locally – No intermediate switch involvement – Requires preestablished virtual path Cell Switching (ATM) • Fixed length (53 bytes) frames are called cells – 5byte (header + 1byte CRC8) + 48 byte payload • Standard defines 3 layers (5 sublayers) – Layers interface to physical media and to higher layers (e.g., encapsulating variablelength frames) Cell Switching (ATM) • 2level connection hierarchy – Virtual circuits – Virtual paths • Bundles of virtual circuits • Travel along common route • Reduces forwarding information 10 Queuing Behavior Examples • Shorter queues: two chunks (or frames) arrive simultaneously at time – 4kB: link is idle until all data arrive at time 327.68 us; 8 kB left to send – 53B: link nearly fully utilized (waits 4.24us); at time 327.68 microseconds, roughly 4kB left to send 18 Why 53byte Frames ? • Telephone community wish: carry voice effectively • Demands ATM to improves latency for audio data – Voice encoded at 64kbps: 8bit smpl at 8KHz – Need full cell’s worth of samples before sending cell • 1Kbyte cells > 125ms per cell (human detectable) 19 • 53 byte cells implies 6 ms of data Why 53byte Frames ? – Smaller latency implies no need for echo cancellers – Audio reconstruction • Expect low rate of cell loss; can interpolate loss (6 ms) • Compromise: 48 bytes = 32(Europe) + 64(US) / 2 20 ATM Cell Format • UserNetwork Interface (UNI) GFC – – – – – – 16 VPI VCI Type CLP HEC(CRC-8) 384 (48 bytes) payload Hosttoswitch format GFC: Generic Flow Control (still being defined) VCI/VPI: Virtual Circuit/Path Identifier Type: management, congestion control, AAL5 (later) CLP: Cell Loss Priority HEC: Header Error Check (CRC8) • NetworkNetwork Interface (NNI) – Switchtoswitch format – GFC becomes part of VPI field 21 Segmentation and Reassembly • Variablelength packets passed to ATM are often larger than 48 bytes – May not fit in the ATM payload • Fragmentation is required at the source – Highlevel message into lowlevel packets 22 Segmentation and Reassembly • Destination reassembles the fragments – Transforms fragments back into the message • ATM name this procedure as Segmentation and Reassembly (SAR) 23 Segmentation and Reassembly • ATM Adaptation Layer (AAL) – – – – – Application to ATM cell mapping AAL header contains information for reassembly AAL1, AAL2 for applications needing guaranteed rate AAL3/4 designed for variablelength packet data AAL5 is an alternative standard for packet data AAL AAL … … ATM ATM 24 ATM Layers • ATM Adaptation Layer (AAL) – Convergence Sublayer (CS) supports different application service models – Segmentation and Reassembly (SAR) supports variablelength frames • ATM Layer – Handles virtual circuits, cell header generation, flow control • Physical layer – Transmission Convergence (TC) handles error detection, framing – Physical medium dependent (PMD) sublayer handles encoding CS AAL SAR ATM TC PHY PMD 25 AAL 3/4 • Provides information to allow variable size packets to be sent in fixedsize ATM cells • Convergence Sublayer Protocol Data Unit (CSPDU) 8 CPI Btag – – – – 16 BAsize < 64 KB payload 0-24 Pad 8 Etag 16 Length CPI: Common Part Indicator (version field) Btag/Etag:beginning and ending tags (same) BAsize: hint on reassembly buffer space to allocate Length: size of whole PDU • Segmented into cells: header/trailer + 44byte data 26 ATM Cell Format for AAL 3/4 40 ATM header 10 type seq MID 352 (44 bytes) payload length 16 CRC-10 • Type (isstart? and isend? bits) – – – – BOM (10): beginning of message COM (00): continuation of message EOM (01): end of message SSM (11): singlesegment message • SEQ: sequence number (for cell loss/reordering) • MID: multiplexing ID (mux onto virtual circuits) • Length: number of bytes of PDU in this cell 27 Encapsulation and Segmentation for AAL3/4 < 64 KB bytes CS-PDU header 44 bytes ATM header 4-7 bytes CS-PDU trailer User data 44 bytes AAL header Cell payload 44 bytes AAL trailer