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Teracom Tutorial: Packets and Frames

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Teracom Tutorial: Packets and Frames
2.27  Packets and Frames
Packets are for networks
Routing to network address = destination
Frames are lower level idea
Error control, framing
Frame address identifies station on link
Packets inside frames
It is important to understand how packets and frames are related. A packet is a block of user data, such as a piece of an e-mail message, with a network address pasted onto the front. Packets are used on networks. The network equipment looks at the network address and uses this information to make a route decision.
A frame is a lower-level idea. Frames are used on cabling systems. The cabling system can have multiple stations connected on the cable. A frame has framing to mark the beginning and end, sender and receiver addresses to indicate the stations on the cable, low-level control information, and a payload.
The main purpose of doing frames is to determine whether the payload has been transmitted without any errors over the cable. If an error is detected, the frame will have to be retransmitted somehow.
The main purpose of packets is to append a network address to your data. The network address is used by network equipment to make route decisions.
When we transmit a packet into and across the network, we want to make sure that it is transmitted with no errors… so we insert the packet in a frame. The thing which frames are carrying these days is not so much a screen of dumb terminal data, but more often, a packet. Packets inside frames.
Notice that there are two addresses: the network address and the link address. The basic idea is that the network address is the destination. This is globally meaningful, and doesn't change. The link address on the frame is locally meaningful on a particular cabling system, and will change as the data moves from one cabling system to another. This is the basic idea. There are exceptions to the basic idea.
In the old days, datacom service providers assigned network addresses and their equipment used the packet address as a basis of making routing decisions. These were called packet-switched network services. This is slow, because every intermediate router at the phone company would have to bring in the frame, perform the CRC check on the frame, look at the frame address, extract the packet, look at the network address on the packet, make a routing decision, change the frame address, recalculate the FCS and send off the frame. This takes a long time, and so the performance was poor.
Some newer datacom services from telecommunication services providers systems have started to speed things up by not using the packet address for routing across the datacom service provider's network, but cheating using the frame address field to indicate how to get to the destination across the datacom service provider's network. Hence, we have a transition from packet-switched services to Frame Relay services from the phone company. Note that the users still make up packets which are being carried inside the frames.
Source: Teracom Course 101, Telecom, Datacom and Networking for Non-Engineers,
Module 2: Understanding Data Communications, slide 2.27
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