371 -1 -0291 An Introduction to Computer Networks

371 -1 -0291: An Introduction to Computer Networks v v v Homepage http: //help. cse. bgu. ac. il/cse/Courses/list. asp Handout #15: ATM - Asynchronous Transfer Mode Additional Reading Text book: Chap. 3. 3, 6. 5. 4 1

Before ATM POTS Computer Network TV Network (Plain Old Telephone Service) (LAN, MAN, WAN) (CATV, Satellite, etc. ) VOICE DATA VIDEO Circuit Switching Delay Guarantee Qo. S Packet Switching No Delay Guarantee No Qo. S (best effort) Circuit Switching Delay Guarantee Qo. S Limited Speed Data No Video Not suitable for Voice and Video No data or voice Lect-12: ATM Computer Networks 2

Background Motivation: v To carry multiple types of traffic (with different Qualities of Service - Qo. S) over a single network. Problem: v The Internet carries data efficiently, but carries real-time traffic poorly. v The telephone network carries voice clearly, but data inefficiently. ATM was first proposed in ~1985 as a way to integrate data communication with telephony. Lect-12: ATM Computer Networks 3

Multi-Service Network ATM Objective: Service and Vendor Integration into one transport network bulk data video Multi-service Network voice Lect-12: ATM interactive data Computer Networks 4

Key Attributes of ATM Virtual Circuits (VC): • All “packets” (cells) in a virtual circuit follow the same path through the network. Which means: • Resources may be reserved along the path. • Packet-switching is used at each ATM switch. Fixed-length Cells: • ATM cells are 53 bytes long: 48 bytes of data (payload), and 5 bytes of header. • Cells are fixed-length to simplify switch design, and to allow fine-grain control of delay through a switch. • Short cells • reduce packetization average delay for voice • reduce delay variance (jitter) • near cut-through behavior Lect-12: ATM Computer Networks 5

Why delay is reduced ? A voice packet would otherwise wait behind a large data packet Lect-12: ATM Computer Networks 6

Key Attributes of ATM (cont. ) Asynchronous (statistical multiplexing): • Cells from different VCs are statistically and async multiplexed on the same link Quality of Service (Qo. S): • Five service classes • Each class is specified by a set of service parameters High Speed Access: • Consolidate traffic coming from many LAN’s Lect-12: ATM Computer Networks 7

ATM: A 3 -Layer Model ATM terminal native appl data ATM switch Lect-12: ATM Computer Networks 8

ATM Layering Example v End devices have three layers v ATM switches has only the bottom two layers Lect-12: ATM Computer Networks 9

ATM Topology – High View NNI Lect-12: ATM Computer Networks 10

ATM Interfaces v User-Network Interface (UNI) Ø v public and private Network-Network Interface (NNI) Lect-12: ATM Computer Networks 11

Virtual Circuits v v v ATM is connection oriented An ATM connection is called virtual channel/connection/circuit (VC) A VC must be established between two end points before data can be transmitted v Tens of thousands of VCs per fiber v Different VC, different bandwidth, Qo. S v User data and signaling on different VCs Lect-12: ATM Computer Networks 12

Virtual Paths v Virtual Private Network over public ATM Ø Ø many individual VCs between two sites switches need to switch VCs individually even if they start and end at the same points VCs starting and ending at the same end points can be bundled in Virtual Paths (VP) v Switches may switch only VPs, not aware of individual VCs inside the VP v Lect-12: ATM Computer Networks 13

Virtual Paths & Virtual Channels VP VC VC VP Lect-12: ATM VP VC VC VP Physical Channel VC VC Computer Networks 14

Virtual Path Connection (VPC) Lect-12: ATM Computer Networks 15

Virtual Channel Connection (VCC) Lect-12: ATM Computer Networks 16

VP and VC Labels v VPs and VCs are labeled with integer identifiers Ø Ø v v Virtual Path Identifier (VPI) Virtual Channel Identifier (VCI) VPI/VCIs are assigned hop by hop Same end-to-end ATM connection may have different VPI/VCI in different hop Hop-by-hop label assignment allows reuse of the same number at different hops The network maps VPI/VCIs at adjacent hops to create an end-to-end virtual circuit Lect-12: ATM Computer Networks 17

Virtual Channel Identifier (VCI) v Each VC is identified by a unique VCI on a given link v VCIs therefore have only local significance v An end-to-end ATM connection traversing several links will be identified by different VCIs at different links v VCI translation occurs at the link boundary v VCIs are assigned to a connection during call set-up Lect-12: ATM Computer Networks 18

Virtual Circuit Setup Virtual Circuits are established hop-by-hop from end-to-end. A VC Setup Ack Lect-12: ATM VC Setup Rqst VC Setup Ack B Ack Computer Networks 19

Virtual Circuit Identifiers VC Table at Port 3: 1 4 9 VC Table at Port 2: VC in VC out Port out 1 3 1 4 12 1 9 2 2 VC in VC out Port out 3 4 1 12 1 1 3 3 2 2 2 12 1 4 1 VC in VC out Port out 2 1 Lect-12: ATM 1 2 7 1 7 Computer Networks 20

Fixed-length cells 48 bytes 5 bytes PAYLOAD DATA VCI: HEC: HEC VCI 10 bits 24 bits Virtual Circuit Identifier: represents a flow between two user processes. Header Error Control: detects and corrects errors in the cell header. Lect-12: ATM Computer Networks 21

Cell Format v User-Network Interface (UNI) 4 8 16 3 1 GFC VPI VCI Type CLP Ø Ø Ø Ø v 8 HEC (CRC-10) 384 (48 bytes) Payload host-to-switch format GFC: Generic Flow Control • Arbitrate access to the link at local site VCI: Virtual Circuit Identifier VPI: Virtual Path Identifier (see below) Type: management, congestion control, AAL 5 (see below) CLP: Cell Loss Priority HEC: Header Error Check (CRC-8); detect & 1 -bit correction Network-Network Interface (NNI) Ø Ø switch-to-switch format GFC becomes part of VPI field Lect-12: ATM Computer Networks 22

VP and VC Switch v v v Two types of ATM switches VP switch does not look at VCIs, switching is based on VPIs only VCI does not change when passing through a VP switch; VPI may change VC switch looks at both VPI and VCI (as well as VPI) may change when passing through a VC switch Lect-12: ATM Computer Networks 23

VP Switch VPI=5 VCI=15 Lect-12: ATM VP Switch VPI=7 VCI=15 VP Switch VPI=9 VCI=15 Computer Networks VP Switch VPI=6 VCI=15 24

VC Switch VPI=5 VCI=15 Lect-12: ATM VP Switch VPI=7 VCI=15 VC Switch VPI=9 VCI=24 Computer Networks VP Switch VPI=6 VCI=24 25

Permanent Virtual Circuit VPI=0 VCI=51 VPI=0 VCI=39 No signaling required to setup a VC; the VC is pre-setup. Lect-12: ATM Computer Networks 26

Permanent Virtual Circuit v v VPIs and VCIs for the end-to-end connection are pre-configured in hardware Semi-Permanent PVC : VPIs and VCIs are preconfigured, but can be erased and reconfigured later PVC is good for Virtual Leased Line for VPN PVC does not scale well for an ATM LAN/WAN with many stations : too many PVCs required Lect-12: ATM Computer Networks 27

Switched Virtual Circuit Signaling messages VPI=0 VCI=5 Signaling required to setup a VC Lect-12: ATM Computer Networks 28

Switched Virtual Circuit User messages VPI=x 1 VCI=x 2 VPI=Y 1 VCI=Y 2 X 1, X 2, Y 1, Y 2 may take different values at successive call setups. Lect-12: ATM Computer Networks 29

Switched Virtual Circuit v Circuit on demand v Scalable solution to ATM connectivity v Different VPIs and VCIs at successive call setups for the same source-destination. Lect-12: ATM Computer Networks 30

Recall - Encapsulation v Outgoing: Ø Take chunk of data from layer above (e. g. , IP) Ø Add a header on the front (e. g. , Ethernet header) Ø May add something on the back (e. g. , CRC) Ø Pass to next lower layer v But… ATM can only handle 48 byte data chunks! v Therefore need data Fragmentation and Reassembly … at the link layer! v Question: where should this be done? (OS in software, device card in hardware/firmware? ) Lect-12: ATM Computer Networks 31

ATM Adaptation Layer (AAL): General encapsulation picture PDU = “protocol data unit” -- network-speak User data AAL PDU Segmentation and Reassembly ATM cell Lect-12: ATM Computer Networks 32

Segmentation and Reassembly v ATM Adaptation Layer (AAL) Ø AAL 1 and 2 are designed for applications that need guaranteed bit rate (e. g. , voice, video) (const or var) Ø AAL 3/4 designed for packet data (connection (-less)) Ø AAL 5 is an alternative “simple” std. for packet data AAL … … ATM Lect-12: ATM Computer Networks 33

AAL 3/4 Main job: convert var length packet to fixedlength cells v “Convergence Sublayer Protocol Data Unit” (CS-PDU) v Ø CPI: common part indicator (version field) Ø Btag/Etag: beginning and ending tag Ø BAsize: hint on amount of buffer space to allocate Ø Length: size of whole PDU Lect-12: ATM Computer Networks 34

Special Cell Format for AAL 3/4 Ø Type • BOM: beginning of message • COM: continuation of message • EOM end of message Ø Ø Ø SEQ: sequence of number MID: message id Length: number of bytes of PDU in this cell Lect-12: ATM Computer Networks 35

Efficiency AAL 3/4 makes it even worse v Best possible is 1 - 9/53 = 83% v Often much worse due to CS-PDU header and partial last cell v Lect-12: ATM Computer Networks 36

(AAL 3/4) AAL 5: “Simple” v CS-PDU Format Ø Ø Length: size of PDU (data only) Ø v pad so trailer always falls at end of ATM cell CRC-32 (detects missing or miss-ordered cells) Cell Format Ø Ø v end-of-PDU bit in Type field of ATM header This is the key Stronger protection Lect-12: ATM Computer Networks 37

LAN Emulation Many IP protocols assume some form of broadcast/multicast on a LAN v ATM link layer means you must either v Ø Ø v Modify those protocols Fake the *cast LAN Emulation (LANE) is expensive and complex Lect-12: ATM Computer Networks 38

ATM Service Classes v Constant Bit rate (CBR) Ø Ø v Variable Bit rate – real-time (VBR-rt) Ø Ø Ø v Ø Similar to Integrated Services Controlled Load. No hard-bound on end-to-end delay. Available Bit rate (ABR) Ø v Leaky-bucket regulated source. Leaky-bucket Spec (specification of the transmitted and received traffic - (s, r) values). Network guarantees end-to-end delay. Variable Bit rate – non-real-time (VBR-nrt) Ø v Assumes source sends at constant rate. Used for emulation of circuit-switching. Includes Congestion-control mechanisms Unspecified Bit rate (UBR) Ø Whatever is left-over: “best-effort” Lect-12: ATM Computer Networks 39

The adoption of ATM was designed to replace the Internet by offering end-to-end Qo. S, highperformance switches, and integration of the voice and data networks. v It didn’t happen as planned. v Ø Ø Ø Complex standards. Momentum and simplicity of IP and the Internet. Signaling to create Virtual Circuits complex and not robust. But a large fraction of the core of the Internet today is ATM… Lect-12: ATM Computer Networks 40

How ATM (and other technologies) are used in the Internet today Logically, it is useful to think of the Internet as a population of end-hosts interconnected by routers. v Logically (at the Network Layer) this is true. v But there is plenty of additional equipment at the Link Layer and the Physical Layer. v Lect-12: ATM Computer Networks 41

The Network Layer View of the Internet End hosts Routers Lect-12: ATM Computer Networks 42

End-users connected via LANs Ethernet switch (e. g. Ethernet) Lect-12: ATM Computer Networks 43

End-users connected via Modems or DSL Your local Central Office Lect-12: ATM Computer Networks 44

ATM is frequently used as a Link Layer ATM switch Packet Switched (IP routers) Lect-12: ATM Circuit Switched (SONET) Computer Networks 45

ATM and IP traffic are carried over the SONET (telephony) circuit switched network SONET “Physical Layer” Switched Packet Switched Circuit switches carry circuits between routers and ATM switches. (IP routers) (SONET) Lect-12: ATM Computer Networks 46