Chapter 5. Multimedia Network Communications

The network layers defined by the International Standards Organization for the Open Systems Interconnection (OSI) are:

1. Physical Layer

   • defines electrical and mechanical interfaces (e.g., fiber optics) to the network and determines the maximum bandwidth

     Needs for higher bandwidth for audio/video data

2. Data Link Layer

   • transmission of data frames, access protocol to physical layer, error correction, flow control and frame synchronization

     Retransmission as a method for error correction is not useful for multimedia (a late frame in audio/video is a lost frame). Error control at this layer will be dropped in e.g., ATM networks.

3. Network Layer

   • switches and routes packets, establishes logical association of remote stations. Providing services such as addressing, internetworking, error handling, congestion control, and sequencing of packets.

     Quality of Service (QoS) parameters defined for multimedia data transmission. Resource reservation based on QoS will guarantee small jitter and correct packet ordering.

4. Transport Layer

   • provides a process-to-process connection.

     QoS and synchronization of the multimedia data need to be enhanced.

5. Session Layer

   • coordinates interaction between user application processes on different hosts. Types of sessions: point-to-point, multicast (one to many), multidrop (many to one).

     TCP/IP and ATM networks do not explicitly have this layer, it is mostly folded into Transport Layer.

6. Presentation Layer

   • Manages abstract data structures, conversion of different data formats/codes

     In TCP/IP and ATM networks, this layer is merged into Application Layer.

7. Application Layer

   • contains various protocols, e.g., ftp, telnet, SMTP (e-mail), etc.

     Increasing varieties of applications, e.g., HTTP, MIME, video-on-demand

1. Broadcast Networks (e.g., Ethernet)

• Bandwidth ranges from 10 to 100 Mbps.

• To send a message, put address of recipient on message, send to everyone. Most will ignore.

• Multiple access to network solved by sensing when network is free (carrier sense) and detecting collisions (collision detection).

  --> CSMA/CD (Carrier Sense, Multiple Access/Collision Detection).

• Each station can be directly connected to a hub (so to make up a star, not a bus), each will then enjoy the full Ethernet bandwidth.

2. Token rings

• Data transmitted in one direction around the ring, and can be read by all stations.

• A special packet, called the token, circulates while ring is idle.

  To transmit, a station grabs the token, transmits, releases the token.

  Regulates access to networks by only allowing one unit to broadcast at a time.

• A multiple priority scheme is usually used for access control.

  A station can transmit a frame at a given priority if it can grab a token with an equal or lower priority. Otherwise, make reservation.

3. FDDI (Fiber Distributed Data Interface)

• A successor of the original Token Ring.

  It has a dual ring topology with primary ring for data transmission and secondary ring for fault tolerance (See Figure 10.3 from the Steinmetz and Nahrstedt book.)

• 100 Mbps over distances up to 100 km with up to 500 stations.

• Supports both synchronous and asynchronous modes.

  The synchronous mode enables bandwidth reservation and the asynchronous mode is similar to the Token Ring protocol.

• FDDI-2 supports an additional mode -- isochronous mode in which network is time sliced, each machine gets a fixed piece.
  --> circuit switched services (isochronous services) for delay-sensitive applications, e.g., audio and video; and packet switched services (synch and asynch) for others in the same network.

• WAN refers to networks across cities and countries.

  Between LAN and WAN, there is also the Metropolitan Area Network (MAN)

• T1 -- 1.544 Mbps (Time-Division Multiplexing (TDM) to support 24 digital voice or data channels, each at 64 kbps). It uses copper wires, one pair in each direction.

• T3 (e.g., the NSFNET) -- 44.736 Mbps.

• Internet is used as an example here. Section 5.2 addresses new generation networks.

1. Internet Layering

               Layer                  Example
	       -----                  -------

	 -----------------
	 |  Application  |	FTP, Telnet, SMTP, X-Windows, MIME, HTTP
	 -----------------
	 |   Transport   |	UDP, TCP, TP4, Routing
	 -----------------
	 |   Internet    |      IGMP, IP, CLNP
	 -----------------
	 |  Subnetwork   |	Ethernet, X.25, FDDI, Token Ring
	 -----------------
	 |   Data Link   |	HDLC, PPP, SLIP
	 -----------------
	 |   Physical    |      RS-232, V.35, Fiber Optic
	 -----------------

2. Network Layer -- IP (Internet Protocol)

Gateways and Routing

• LANs allow point-to-point message transmission on local networks.

• Problem: What if I want to send a message to a machine on a different network?
  

  Solution: Use machine G to forward the message. In other words, a program running on G takes the message from A and routes it to B. G is called a gateway, and the program is called a router.

Routing using the Internet Protocol

• Problem: A wants to send to B, but must send to G first. So how does G know to forward the message to B?

  Solution: Don't use B's Ethernet address. Use another name for B (it's IP address) and provide a translation table from IP addresses to Ethernet addresses (called a routing table).

• A common scheme: Internet Addresses are 32 bit numbers called Internet Protocol (IP) addresses. Usually specified using "dot" notation (e.g. 804d953f = 128.77.149.63).

3. Transport Layer -- UDP and TCP

(1) UDP (User Datagram Protocol)

• Connectionless -- IP packets address tells only what machine packet is bound for, not which user or process on the machine.

  UDP adds port number (16 bit identifier) to packet to indicate destination process

• It supports only multiplexing and checksum.

  Higher level protocols must then take care of retransmission, divide and reassemble packets, flow control, congestion avoidance, etc.

(2) TCP (Transmission Control Protocol)

• Connection-oriented -- provide reliable communication between pair of processes across the net.

• Simplest idea: send one segment, wait for acknowledgment (ACK).

  If ACK not received in a little while (the retransmission timeout, or RTO), resend segment.

• Maximum bandwidth is limited by round trip time (RTT):

  • Max BW = (segment size)/RTT