Notes On Chapter Fourteen -- Connection-Oriented Networking And ATM

• 14.1 Introduction
  
  
  • This chapter is about Asynchronous Transfer Mode (ATM) -- a technology for both LAN's and WAN's that does connection-oriented packet switching.
    
    
• 14.2 A Single, Global Network
  
  
  • Telecom companies invented ATM as a way to "grow instead of die" -- cope with decreasing profit margins by diversifying into data and video networking. They aimed to provide:
    
    
    • Universal Service (everybody, everywhere)
      
      
    • Support For All Users (voice, video, data)
      
      
    • Single, Unified Infrastructure (same technology for all services on any network, be it LAN or WAN)
      
      
    • Service Guarantees (the same delivery guarantees as other voice, video and data networks )
      
      
    • Support For Low-Cost Devices (e.g. cheap ATM phones that rely on the network to perform well)
      
      
• 14.3 ISDN And ATM
  
  
  • Integrated Services Digital Network (ISDN) was actually the first technology developed aimed at achieving the goals above. By the time it emerged it was obsolescent. ATM is the follow-on to ISDN which has tried to satisfy the goals.
    
    
• 14.4 ATM Design And Cells
  
  
  • Packet "jitter" is variance in the delay experienced by packets traveling in a network. Varying amounts of traffic congestion in packet-switched networks causes varying delay -- jitter. Jitter degrades voice and video.
    
    
  • In data networks, larger packets are considered "good" because the amount of header overhead tends to be low. However when sending sound or video, the time it takes to fill a large packet with samples introduces a delay between the last sample in packet N and the first sample in packet N+1. Delay in voice or data streams creates "static." Also, it is very difficult to implement echo cancellation for voice communication when there is high delay.
    
    
  • So that ATM would be able to support voice, video, and data, designers of ATM standards chose a packet size of 53 octets. In ATM parlance the packet is called a cell.
    
    
  • Critics doubt that one technology can be adequate for voice, video, and data.
    
    
• 14.5 Connection-Oriented Service
  
  
  • Asynchronous Transfer Mode is connection-oriented.
    
    
  • Host X requests a connection and host Y agrees to accept the connection.
    
    
  • The network hardware establishes a data-path/connection and returns a connection identifier to X and Y. (However, the connection identifier returned to X is usually not the same as the one returned to Y.)
    
    
  • The hosts place the connection identifier in the packets and the ATM switches use it as a basis for routing packets.
    
    
• 14.6 VPI/VCI
  
  
  • ATM uses virtual circuits.
    
    
  • The connection identifier is a 24-bit quantity. The first 8 bits (the VPI) identify the virtual path, and the last 16 bits (the VCI) identify the particular virtual channel (aka circuit) using the path.
    
    
• 14.7 Labels and Label Switching
  
  
  • When an ATM switch forwards a packet it extracts the 24-bit VPI/VCI connection identifier, uses it as an "index" into a forwarding table, reads off a port number, sends the packet out the corresponding switch port, and also replaces the VPI/VCI in the packet with a new VPI/VCI.
    
    
  • Each communicating host sees the connection identifier by which it knows the connection, and not the other host's connection identifier.
    
    
• 14.8 An Example Trip Through An ATM Network
  
  
  • The figure refers to a VC from S to R. S refers to the VC as #3 and R calls it #6. When switch A receives a cell from S containing VPI/VCI #3, it looks up #3 in its forwarding table. The table tells switch A to change the VPI/VCI to #4 and route the cell out interface 2, which it does. Switch B receives the cell with VPI/VCI equal to #4. Switch B uses #4 to do its lookup, and the cell goes out interface 2 with VPI/VCI set to #1. In switch C, it is routed out interface 2 with VPI/VCI #6, which R recognizes as the identifier of the channel between S and R.
    
    
• 14.9 Permanent Virtual Circuits
  
  
  • The virtue of the label-switching idea is that a path can be configured one hop at a time from ATM switch to ATM switch without any need for a global value of the VPI/VCI. At each hop it is only necessary to pick a VPI/VCI that is not in use on the local switch.
    
    
  • ATM networks offer a service called permanent virtual channel (PVC). A PVC works like a dedicated point-to-point leased line, but it is actually a path permanently "programmed" into the network. Network administrators form the path by first designating a set of ATM switches to be the "nodes" of the path and then manually configuring the switches so they will route the connection appropriately along the path. (This configuring of the switches is called provisioning in ATM parlance.)
    
    
• 14.10 Switched Virtual Circuits
  
  
  • ATM also supports dynamic allocation-deallocation of virtual circuits. This facility is called Switched Virtual Channel (SVC).
    
    
  • Host X sends a message to a directly connected switch S, requesting a connection to host Y. S finds a path to Y and sends the request along the path. At each hop, pairs of switches cooperate to set the VPI/VCI's to be used, and they make their forwarding table entries for the connection. When the connection is complete a message travels back from Y to X notifying X that the connection is ready to use.
    
    
  • There are reserved virtual channels (similar to PVC's) between directly connected switches for sending the control messages required to do the "signaling" that builds the SVC's as just described.
    
    
• 14.11 Quality of Service (QoS)
  
  
  • ATM supports connections with quality of service (QoS) assurance.
    
    
  • For example one could establish an ATM connection that guarantees a minimum of 64Kbps throughput and a maximum of 500ms delay.
    
    
  • Some of the available QoS options are:
    
    
    • Constant Bit Rate (CBR) [good for uncompressed audio or video]
      
      
    • Variable Bit Rate (VBR) [good for compressed audio or video]
      
      
    • Available Bit Rate (ABR) [good for data]
      
      
• 14.12 The Motivation For Cells And Label Switching
  
  
  • Designers felt that the fixed-size cells and label-switching of ATM were necessary to support maximum data rates and QoS. They also designed ATM to operate at high bit rates such as OC-3 speed of 155 Mbps.
    
    
  • 14.12.1 Cells vs. Packets
    
    
    • For example: the QoS paradigm demands that all packets be short: What can be done if a high priority packet H arrives at an ATM switch S immediately after S has begun sending a long packet L? It is not practical to interrupt the sending of L. H has to be delayed for a long time.
      
      
    • With fixed-size cells, there are few problems with:
      • external fragmentation of memory,
      • designing hardware to detect the ends of cells, or
      • knowing how long it will take to transmit a cell
      
      
  • 14.12.2 Label Switching vs. Routing
    
    
    • Typically the routing and label-switching tasks of an ATM switch are "hardwired." The task is not performed by software executing in a CPU. The upshot is that this work gets done very quickly.
      
      
    • Such ATM switches can routinely attain aggregate throughputs of 2.4 Gbps.
      
      
• 14.13 ATM Data Transmission And AAL5
  
  
  • ATM Adaptation Layer 5 (AAL5) is a protocol that allows applications to send and receive data packets of variable size, up to 64K octets. The protocol is implemented with segmentation/reassembly.
    
    
• 14.14 Critique of ATM
  
  
  • Expense: ATM has to "do everything" so the switches and connections to computers are very expensive.
    
    
  • Connection Setup Latency: It takes time to set up an ATM connection. For short messages a connectionless service would be faster.
    
    
  • Cell Tax: The header is ten percent of the contents of each packet. If packets were longer, the percentage of header overhead would be less.
    
    
  • Specification Of Service Requirements: Applications using the network have to request QoS.
    
    
    • Requesting more than needed can result in waste of resources or unnecessary waiting for resources to become available.
      
      
    • Requesting less than what is needed leads to poor performance.
      
      
  • Lack Of Efficient Broadcast: On an ATM network a broadcast must be "simulated" by sending a copy of the message to each host.
    
    
  • Complexity Of QoS: It is difficult to configure ATM to enforce the limits of the QoS. Also there seems to be little purpose of the fine-grained levels of service other than to provide the owners of the network with a way to "meter" and charge for network usage.
    
    
  • Assumption Of Homogeneity: ATM does not tend to interoperate successfully with other network technologies, yet multiple technologies are bound to exist, especially in view of the expense of ATM.
    
    
  • Gigabit Ethernet is a a very viable alternative to an ATM LAN.
    
    
  • ATM often transports over SONET, but one can now send IP traffic directly over SONET. This would eliminate the cell tax.
    
    
• 14.15 Multiprotocol Label Switching (MPLS)
  
  
  • MPLS is designed to serve the Internet core - ATM-like label switches to provide VC's among IP routers.
    
    
• 14.16 Summary
  
  
  • "... the expense, complexity, and lack of interoperability with other technologies have prevented ATM from becoming more prevalent."