5.2. New Technologies in Computer Networks

• This section is mostly concerned with the Wide-Area Networks (WANs).

5.2.1. Fiber Optic Networks

1. Basics of Fiber Optic Networks

• Light is used to transmit signals

• Signals travel along fiber channel, bouncing off walls

  

  Bits are transmitted as pulses, at about 69% the speed of light.

  

• Advantage over copper: more bits per inch (bits are smaller, not faster).

• Electrical signals can be converted to optical signal at a few gigabits/sec --> gigabit networks

Ultimate bandwidth of fiber

• Three colors of light can be used to signal (at 850, 1300, and 1500 nm, about 200 nm wide).

  Each has about 25 terahertz of capacity. So aggregate capacity could be 50-75 terabits.

2. SONET (Synchronous Optical Network)

• Important -- because telephone companies will switch to it, making parts cheaper --> designed to replace copper.

• OC = "Optical Carrier", STM = Synchronous Transport Module"

  -----------------------------------------------------
   Data Rate       SONET (US)     CCITT (International)    
  (Mbits/sec)      Designation         Designation       
  -----------------------------------------------------
      51.84           OC-1                 -            
     155.52           OC-3               STM-1          
     622.08           OC-12              STM-4          
    1224.16           OC-24              STM-8          
    2488.32           OC-48              STM-16          
  -----------------------------------------------------
  

• Telephone companies will probably only use STM-1, STM-4, and STM-16.

3. WDM (Wavelength Division Multiplexing networks)

• Problem: current equip. cannot handle electrical signals for many gigabits per second

  Solution: use multiple transmitters/receivers with different wavelengths.

• Aggregate bandwidth is in the terabits, one node only gets gigabit

• Biggest problem: getting transmitters and receivers to agree on frequencies.

5.2.2. ISDN (Integrated Services Digital Network)

• Main Features:
  • All-digital telephony system

  • Integrated services for ordinary data, voice, and sometimes video (e.g., in videoconference)

• B (Bearer) channel: 64 kbps for data
  • Mostly circuit switched, but also supports packet switching

  D (Delta) channel: 16 kbps

  • D-channel takes care of call set-up, call control (call forwarding, call waiting, etc.), and maintenance

  • Control and maintenance can be done in real-time in D-channel while B-channel is still transmitting data

• Basic Rate: two B-channel and one D-channel

  Primary Rate: 23 B-channels and one D-channel in U.S. and Japan; 30 B-channels and two D-channels in Europe

• Broadband ISDN (B-ISDN)
  • Add H-channels whose bandwidth ranges from 384 kbps to over 130 Mbps per channel (see Table 10.3, p. 372 of Steinmetz and Nahrstedt)

  • Since 1988, ATM is the basis of B-ISDN

5.2.3. ADSL (Asymmetric Digital Subscriber Line)

• This is a Copper Access Technology -- twisted-pair copper wires

  ADSL is not about line, it is about the two modems at each end.

• Old voice band modems use a bandwidth not exceeding 3.3 KHz. DSL modems uses twisted-pair bandwidth from 0 to 80 KHz.

• To the subscriber, many (say fifty) pairs of twisted-pair telephone wires will bundled in a single cable. Cables coming out of a central office have hundreds or even thousands of pairs bundled together.

Name         Meaning                 Data Rate           Mode   
_______________________________________________________________________

V.32     Voice Band Modems          1200 bps to         Duplex   
V.34                                28,800 bps


DSL      Digital Subscriber         160 kbps1           Duplex
          Line                             


HDSL     High Data Rate Digital     1.544 Mbps          Duplex   
          Subscriber Line           2.048 Mbps          Duplex  


ADSL     Asymmetric Digital         1.5 to 9 Mbps       Down2      
          Subscriber Line           16 to 640 kbps      Up        
                                                         

VDSL     Very high data rate        13 to 52 Mbps       Down    
          Digital Subscriber        1.5 to 2.3 Mbps     Up3	
          Line
_______________________________________________________________________

1.  160 Kbps divides into two ISDN B channels (64 kbps), one D channel 
    (16 kbps) and link administration.
2.  "Down" means downstream, from the network to the subscriber.
    "Up" means upstream.
3.  Future VDSL systems may have upstream rates equal to downstream, 
    but on much shorter lines.

• The downstream speed of the ADSL depends largely on distance:

       Up to 18,000 feet	1.544 Mbps (T1)
             16,000 feet        2.048 Mbps (E1)
             12,000 feet        6.312 Mbps (T2)
              9,000 feet	8.448 Mbps

5.2.4. ATM (Asynchronous Transfer Mode)

• Adopted by CCITT in 1988 as the basis for the development of B-ISDN. So often the terms ATM Networks and B-ISDN are used synonymously.

• The following table illustrates the ATM layers below the Applcation Layer (adapted from Fig. 1-31 [Tanenbaum 3rd ed.]):

 OSI    ATM     ATM  
Layer  Layer  Sublayer     Functionality
-------------------------------------------------------------------------------------
                CS        Interface (convergence), services to different applications
 3/4    AAL     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
                SAR       cell/packet segmentation and reassembly 
-------------------------------------------------------------------------------------
                          Flow control
 2/3    ATM               Cell header generation/extraction
                          Virtual circuit / virtual path management
                          Cell multiplexing/demultiplexing
-------------------------------------------------------------------------------------
                          Cell rate decoupling
                          Header error checking (checksum) 
  2             TC        Cell generation
                          Packing/unpacking cells
                          Frame generation
 - -  Physical  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                          Bit timing
  1             PMD
                          Physical network access
-------------------------------------------------------------------------------------
   
   AAL -- ATM Adaptation Layer
   CS  -- Convergence Sublayer
   SAR -- Segmentation And Reassembly sublayer
   TC  -- Transmission Convergence sublayer
   PMD -- Physical Medium Dependent sublayer
   

Cell networks

• Cell Networks send data packets in very small units (cells)

  

  Chips are available to segment and reassemble (SAR chips)

• Cell size -- most common 53 byte cells (5 bytes control, 48 bytes data)

Advantages of cell networks

• Efficiency: very little wasted bandwidth

• Lower latency. Serialization delay in a packet network:

  

• Serialization delay in a cell network:

  

• Makes network better for delivering voice, video, and data.

  Increases speed of store-and-forward networks (it takes about 1 microsecond to receive a 128 byte cell on a 1 Gbit network).

• Original ISDN provided for fixed 64 kbps channels. Fixed size channels are never the right size.

Cell Format

• 53 bytes -- 5 bytes header, 48 bytes payload

  

• Header has 2 formats -- one for inside the network (network-network-interface, or NNI) and one for entering/leaving network (user-network-interface, or UNI).

ATM NNI Cell header

• VPI = Virtual Path Identifier, VCI = Virtual Channel Identifier

  VCI + VPI = unique routing indicator for the cell.

  Like phone number, VPI is like area code (604), VCI is like phone number (555-1212)

  Routers can use hop-by-hop routing, may route only on VPI in the network backbone.

• Payload type:

  

  CLP = Cell Loss Priority. 1 - low priority, 0 - high priority (a hint for switches as to which cell to drop)

  CRC = Cyclic Redundancy Code -- used for header error detection and correction

ATM UNI Cell header

• The UNI header is similar to the NNI header, except it has 4 bits for Generic Flow Control (GFC) to control the amount of traffic entering the network.