Question

Two advantages for full mesh over shared medium Two disadvantages for full mesh over shared medium.

Outline what a MAC protocol is and why it is used for shared medium case

Answer 1

What is Mesh Topology?

A mesh topology is a network topology in which all the network nodes are individually connected to most of the other nodes. There is not a concept of a central switch, hub or computer which acts as a central point of communication to pass on the messages.

Unlike other network topologies, it can be divided into two kinds:

• Fully connected mesh topology and,
• Partially connected mesh topology

A fully connected mesh topology has all the nodes connected to every other node. If you know the graph theory, then it is like a fully connected graph where all the nodes are connected to every other node.

On the other hand, a partially connected mesh topology does not have all the nodes connected to each other.

Advantages of mesh topology:

• Each connection can carry its own data load
• It is robust
• A fault is diagnosed easily
• Provides security and privacy

Disadvantages of mesh topology:

• Installation and configuration are difficult if the connectivity gets more
• Cabling cost is more and the most in case of a fully connected mesh topology
• Bulk wiring is required
• Medium Access Control (MAC) :: The Medium Access Control (MAC) protocol is used to provide the data link layer of the Ethernet LAN system. The MAC protocol encapsulates a SDU (payload data) by adding a 14 byte header (Protocol Control Information (PCI)) before the data and appending an integrity checksum, The checksum is a 4-byte (32-bit) Cyclic Redundancy Check (CRC) after the data. The entire frame is preceded by a small idle period (the minimum inter-frame gap, 9.6 microsecond (µS)) and a 8 byte preamble (including the start of frame delimiter).

• Preamble

  The purpose of the idle time before transmission starts is to allow a small time interval for the receiver electronics in each of the nodes to settle after completion of the previous frame. A node starts transmission by sending an 8 byte (64 bit) preamble sequence. This consists of 62 alternating 1's and 0's followed by the pattern 11. Strictly speaking the last byte which finished with the '11' is known as the "Start of Frame Delimiter". When encoded using Manchester encoding, at 10 Mbps, the 62 alternating bits produce a 10 MHz square wave (one complete cycle each bit period).

  

  The purpose of the preamble is to allow time for the receiver in each node to achieve lock of the receiver Digital Phase Lock Loop which is used to synchronise the receive data clock to the transmit data clock. At the point when the first bit of the preamble is received, each receiver may be in an arbitrary state (i.e. have an arbitrary phase for its local clock). During the course of the preamble it learns the correct phase, but in so doing it may miss (or gain) a number of bits. A special pattern (11), is therefore used to mark the last two bits of the preamble. When this is received, the Ethernet receive interface starts collecting the bits into bytes for processing by the MAC layer. It also confirms the polarity of the transition representing a '1' bit to the receiver (as a check in case this has been inverted).

  Header

  

  MAC encapsulation of a packet of data

  The header consists of three parts:

• A 6-byte destination address, which specifies either a single recipient node (unicast mode), a group of recipient nodes (multicast mode), or the set of all recipient nodes (broadcast mode).
• A 6-byte source address, which is set to the sender's globally unique node address. This may be used by the network layer protocol to identify the sender, but usually other mechanisms are used (e.g. arp). Its main function is to allow address learning which may be used to configure the filter tables in a bridge.
• A 2-byte type field, which provides a Service Access Point (SAP) to identify the type of protocol being carried (e.g. the values 0x0800 is used to identify the IP network protocol, other values are used to indicate other network layer protocols). In the case of IEEE 802.3 LLC, this may also be used to indicate the length of the data part. Th type field is also be used to indicate when a Tag field is added to a frame.

• CRC

  The final field in an Ethernet MAC frame is called a Cyclic Redundancy Check (sometimes also known as a Frame Check Sequence). A 32-bit CRC provides error detection in the case where line errors (or transmission collisions in Ethernet) result in corruption of the MAC frame. Any frame with an invalid CRC is discarded by the MAC receiver without further processing. The MAC protocol does not provide any indication that a frame has been discarded due to an invalid CRC.

  The link layer CRC therefore protects the frame from corruption while being transmitted over the physical mediuym (cable). A new CRC is added if the packet is forwarded by the router on another Ethernet link. While the packet is being processed by the router the packet data is not protected by the CRC. Router processing errors must be detected by network or transport-layer checksums.

  Inter Frame Gap

  After transmission of each frame, a transmitter must wait for a period of 9.6 microseconds (at 10 Mbps) to allow the signal to propagate through the receiver electronics at the destination. This period of time is known as the Inter-Frame Gap (IFG). While every transmitter must wait for this time between sending frames, receivers do not necessarily see a "silent" period of 9.6 microseconds. The way in which repeaters operate is such that they may reduce the IFG between the frames which they regenerate.

  Byte Order

  It is important to realise that nearly all serial communications systems transmit the least significant bit of each byte first at the physical layer. Ethernet supports broadcast, unicast, and multicast addresses. The appearance of a multicast address on the cable (in this case an IP multicast address, with group set to the bit pattern 0xxx xxxx xxxx xxxx xxxx xxxx) is therefore as shown below (bits transmitted from left to right):

      0                           23 IP Multicast Address Group  47
      |                           | <--------------------------->|
      1000 0000 0000 0000 0111 1010 xxxx xxx0 xxxx xxxx xxxx xxxx
      |                                     |
      Multicast Bit                         0 = Internet Multicast
                                            1 = Assigned for other uses

  However, when the same frame is stored in the memory of a computer, the bits are ordered such that the least significant bit of each byte is stored in the right most position (the bits are transmitted right-to-left within bytes, bytes transmitted left-to-right):

      0                           23                            47
      |                           |                             |
      0000 0001 0000 0000 0101 1110 0xxx xxxx xxxx xxxx xxxx xxxx
              |                     <--------------------------->
              Multicast Bit          IP Multicast Address Group

  CSMA /CD

  The Carrier Sense Multiple Access (CSMA) with Collision Detection (CD) protocol is used to control access to the shared Ethernet medium. A switched network (e.g. Fast Ethernet) may use a full duplex mode giving access to the full link speed when used between directly connected NICs, Switch to NIC cables, or Switch to Switch cables.

  Receiver Processing Algorithm

  

  Runt Frame

  Any frame which is received and which is less than 64 bytes is illegal, and is called a "runt". In most cases, such frames arise from a collision, and while they indicate an illegal reception, they may be observed on correctly functioning networks. A receiver must discard all runt frames.

  Giant Frame

  Any frame which is received and which is greater than the maximum frame size, is called a "giant". In theory, the jabber control circuit in the transceiver should prevent any node from generating such a frame, but certain failures in the physical layer may also give rise to over-sized Ethernet frames. Like runts, giants are discarded by an Ethernet receiver.

  Jumbo Frame

  Some modern Gigabit Ethernet NICs support frames that are larger than the traditional 1500 bytes specified by the IEEE. This new mode requires support by both ends of the link to support Jumbo Frames. Path MTU Discovery is required for a router to utilise this feature, since there is no other way for a router to determine that all systems on the end-to-end path will support these larger sized frames.

  A Misaligned Frame

  Any frame which does not contain an integral number of received bytes (bytes) is also illegal. A receiver has no way of knowing which bits are legal, and how to compute the CRC-32 of the frame. Such frames are therefore also discarded by the Ethernet receiver.

  Other Issues

  The Ethernet standard dictates a minimum size of frame, which requires at least 46 bytes of data to be present in every MAC frame. If the network layer wishes to send less than 46 bytes of data the MAC protocol adds sufficient number of zero bytes (0x00, is also known as null padding characters) to satisfy this requirement. The maximum size of data which may be carried in a MAC frame using Ethernet is 1500 bytes (this is known as the MTU in IP).

  A protocol known as the "Address Resolution Protocol" (arp) is used to identify the MAC source address of remote computers when IP is used over an Ethernet LAN.

  Exception to the Rule

  An extension to Ethernet, known as IEEE 802.1p allows for frames to carry a tag. The tag value adds an extra level of PCI to the Ethernet frame header. This increases the size of the total MAC frame when the tag is used. A side effect of this is that NICs and network devices designed to support this extension require a modification to the jabber detection circuit.