ATM

1
ATM

• Asynchronous Transfer Mode

2
ATM Networks

• Use optical fibre similar to that used for FDDI
  networks
• ATM runs on network hardware called SONET
• ATM cells (packets) are 53 octets long
• 5 bytes of header information
• 48 bytes of data
• ATM networks are packet-switched, but still
  create a (virtual) circuit through the network
• Before transfer can occur, the network must
  create a path (called a virtual circuit) between
  the two machines
• Once the virtual circuit (VC) has been
  established, packets can be transferred between
  the machines

3
SONET Hardware

• ATM/SONET network interface cards use
  photodetectors to read optical signals and
  photoreactors to generate them
• Instead of network hubs, ATM networks use
  switches to generate virtual circuits
• These are not the same as Ethernet switches
• ATM cells do not pass through routers or network
  nodes
• As a result, cells are transferred very quickly

4
Creating Virtual Circuits

• Creating a virtual circuit has been compared to
  making a telephone call
• A network node sends a request to the ATM switch
  specifying the destination
• The switch interacts with any other switches
  necessary to align themselves to form a complete
  path
• When communication is complete, the node sends a
  disconnect message to the switch
• The switch will then notify all switches involved
  to release the connection

5
ATM Switches

• In ATM, switches act as network hubs and routers
• They provide interconnection between network
  nodes
• They establish a path between source and
  destination
• The difference between switches and routers is
  that switches create this path once, not once for
  each message

6
Virtual Circuits

• Virtual circuits are also often called virtual
  paths
• For reduced bandwidth usage for path
  identification, these are numbered
• A virtual path identifier is used to indicate
  which virtual path is intended for a cell
• Within virtual circuits, a number of channels is
  available
• Nodes can share virtual paths
• Virtual paths can be used to communicate between
  more than one pair of nodes

7
ATM Messaging
N1
S5
S3
S6
S2
Connect N1
Connect N1
M1
S4
N2
S1
8
ATM Messaging
N1
S5
M8
S3
S6
S2
S4
N2
S1
9
ATM Cell Format Data Cells
GFC
VPI
VCI
PT
HEC
Data
CLP
8
16
4
3
1
8
48x8

• Generic flow control Not currently used
• Virtual path identifier An identifier for the
  virtual path/circuit
• Virtual channel identifier An identifier to
  identify which channel within the specified
  virtual path/circuit
• Payload Type 3 flag bits
• Cell loss priority Should the cell be discarded
  in the event of a congested switch?
• Header error check Cyclical redundancy check
  for the cell header

10
Cell Header Field Payload Type

• The payload type field contains three bits of
  flags, which are used for different purposes
• Lets focus on the bits for user data
• First bit User data or control data?
• Control cells could include call setup cells
• Second bit Is the sending switch congested?
• Third bit Is this the last cell in a logical
  packet?
• Logical packets will be discussed later when we
  talked about AAL5

11
ATM Efficiency

• ATM switches do not suffer from router-like
  slowdowns where cells would be buffered and
  processed
• Also hub-like duplicates are avoided
• The fixed size of ATM cells allows for hardware
  optimizations
• The time to transmit a 53 octet cell is constant
• Assembling cells can also be done more quickly by
  hardware

12
Virtual Circuits

• A Detailed Discussion

13
Virtual Circuits

• Virtual circuits can be created in 2 ways
• Permanent Virtual Circuits
• Switched Virtual Circuits

14
Permanent Virtual Circuits

• PVCs are circuits that are configured manually
  (through software)
• Thus they require more effort to create
• PVCs provide compatible service between any two
  ATM networks
• PVCs are used when connections between two
  computers will be required for long periods of
  time

15
Switched Virtual Circuits

• SVCs are created automatically (by the hardware)
  when they are needed
• SVCs are the VCs that were described in the
  previous class
• When an SVC is to be created
• Each switch communicates with the next switch in
  the path
• When an SVC has been created the switches closest
  to the source and destination send those nodes
  the VC identifier (VCI a 24 bit number)
• The VCI need not be the same value for each node

16
Virtual Circuit Creation

• Creation of virtual circuits (PVCs and SVCs)
  takes a long time
• While the VC is being created, the VC is not yet
  useable
• Creating a VC, sending one cell (packet), and
  closing the VC is very inefficient and should be
  avoided
• The cell transport layer handles circuit
  management
• If a VC already exists and is open, the cell
  transport layer uses the existing VC for
  transmission
• If no VC is available to the node, a VC is
  created, and the VCs identifier is added to the
  switches circuit table

17
ATM Service Model

• Most ATM layers are implemented in hardware
• This means that ATM network device drivers are
  typically high level, where technologies such as
  TCP/IP tunneling can be implemented easily
  (discussed later)

18
ATM Service Model
Application
Software
Device Driver (Data Link)
Adaptation Layer
Hardware
Cell Transport
Signal
Optical Fibre
19
Adaptation Layer

• The adaptation layer provides an abstraction
  which hides hardware concerns from the
  application
• e.g. The 53 octet limit for cell sizes
• ATM networks use ATM Adaptation Layer 5 (AAL5)
  technology to provide this abstraction
• AAL5 provides a high level interface, allowing
  packets between 1 and 65,535 octets of data
• AAL5 also handles data integrity maintenance
  (using a CRC check inside the packets)

20
AAL5

• As stated previously, AAL5 provides a high-level
  abstraction to the software
• AAL5 layers appear to accept larger chunks of
  data, which is segmented into 48 octet cells and
  managed transparently
• This process is known as Segmentation and
  Reassembly (SAR)
• AAL5 also ensures data integrity by using a
  checksum (CRC)
• AAL5 does not deal with cell delivery (as the
  cell transport layer does), so AAL5 is not
  necessary inside switches
• AAL5 is considered an end-to-end layer
• Whereas cell transport is considered a
  machine-to-machine layer

21
Segmentation

• Segmentation is the process of turning a chunk of
  data into a group of ATM cells
• For example, the chunk of data might be a packet
  from another type of network
• e.g. An Ethernet frame
• Since ATM cells travel on the same virtual
  circuit, they do not arrive out of order
• Thus sequencing information is not necessary
• Each sequence of 48 octets is sent in its own cell

22
Reassembly

• Reassembly is the process of recombining ATM
  cells into the original data chunk
• As the destination node receives cells, it
  removes the 48 octets and appends it to the end
  of a buffer
• One of the configuration parameters, called
  Payload Type is used to indicate the final cell

23
Data Integrity

• As mentioned before, the AAL5 subsystem uses a
  checksum to ensure data integrity
• The checksum is sent in the last cell
• A 32 bit (4 octet) CRC

24
AAL5 Logical Packets
ATM Cells
D
CRC
AAL5 Logical Packet
25
ATM Switching
Application
Application
Device Driver (Data Link)
Device Driver (Data Link)
Adaptation Layer
Adaptation Layer
AAL5
Cell Transport
Cell Transport
Cell Transport
Signal
Signal
Signal
Switch
26
ATM Switching

• As can be seen in the diagram, ATM switches exist
  entirely in the hardware layer
• As a result, they are much faster than routers,
  which require software execution
• Routers must read packets from electronic signals
  into a memory buffer (which is slow)
• Routers then convert packets back into electronic
  signals onto a new network connection

27
TCP/IP Tunneling

• Using ATM As an Example
• (Chapter 18)

28
ATM Networking Drawbacks

• Small, finite sized cells provide faster
  transmission speeds
• However, 53 octet cells are incompatible with
  other technologies which are in widespread use
• ATM addressing also differs significantly from
  other forms of addressing
• For example the TCP/IP protocol suite is the most
  common network protocol system
• Most Internet applications are based on TCP/IP
• ATM networks are not broadcast networks
• Each cell only arrives at its intended
  destination
• Broadcasting multicasting are not directly
  supported

29
TCP/IP Tunneling

• TCP/IP tunneling allows transmission of TCP/IP
  packets over ATM (and other non-TCP/IP) networks
• The use of TCP/IP over these networks allows
  applications which normally only run on TCP/IP
  networks to operate on all networks

30
TCP/IP Tunneling with AAL5

• AAL5 provides a convenient means to send large IP
  datagrams over an ATM network
• The IP datagrams, including the header
  information can be inserted into an AAL5 logical
  packet
• The AAL5 logical packet will be sent, using ATM
  cells, to the destination
• The destination will extract the IP datagram and
  use it as if the datagram were sent across a
  normal TCP/IP network

31
IP Switching

• Researchers are interested to see if they can use
  switching technology to route IP packets
• Switching happens faster than routing
• Techniques for IP switching are hardware-oriented

32
LAN Emulation

• Many people are using ATM network hardware in
  order to achieve ATM speeds in a LAN network
• Most ATM hardware can be configured to simulate
  an emulated LAN (E-LAN)
• E-LANs use the same techniques as TCP/IP
  tunneling to forward packets such as Ethernet
  frames using AAL5

33
ATM Service Categories

• ATM provides 4 service categories
• CBR (Constant Bit Rate) A constant bandwidth is
  reserved and guaranteed by the network
• UBR (Unspecified Bit Rate) Data is transmitted
  when bandwidth is available, and not when
  bandwidth is not available
• ABR (Available Bit Rate) The network will
  provide feedback about network congestion, under
  the assumption the node will adjust its
  transmission to meet the current availability of
  network bandwidth
• VBR (Variable Bit Rate) A minimum bandwidth is
  reserved and guaranteed by the network, although
  available bandwidth may increase/peak above this
  value
• RT (Real Time) Fluctuation of bandwidth is
  minimized
• Used for non-guaranteed streaming audio/video
• NRT (Non Real Time) Any bandwidth available is
  used
• Used for downloads