Link Layer

To move  to packet  from one node( host  or packet swtich0 to  net node  in the  route the network  layer  relies  on the  services  of the link layer. Particularly  at each  node the  network  layer  passes the datagram  down to the  link layer which delivers the datagram to the next node along the route  the link layer  passes the datagram  up  to the  network  layer.

The services  provided  by the link layer  depend the specific  link  layer  protocol that  is employed over the link. For example  some  protocol provide  delivery on  a link  basis that  is from transmitting node  over link to receiving node. Note  that this  reliable delivery service is different from the   reliable delivery service of TCP, which  provides reliable delivery  from one end system to another. Example  of link  layers  include Ethernet and the point to point  protocol (PPP). As Datagram's  typically  need to  traverse several links  to travel from  sources to destination a datagram's  may be  handle by different  link  layer  protocol at different  links  along  its route. For example  a datagram may be handled  by Ethernet  on one link  and by  PPP on the next  link. The  network  layer  will  receive  a different  service from each of the different link layer  protocols. Link  layer  packets are known  as frames.