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# Signal Routing
### **Network Addresses**
In networks with routers, nodes need a way to address each other. Even if a router forwards signals to all its links like a hub, nodes must determine if they are the intended recipients of messages.
This is achieved by assigning a unique identifier, called a **network address**, to each node. **Each message includes the recipient's identifier, allowing routers to know where to forward communications.** This system works best when addresses are assigned based on the network's geographical structure rather than randomly.
Nodes with addresses according to the router they’re connected to
### **Routing Tables**
Routing is the process of selecting a path between two nodes across a network. Routers handle this task using a routing table, which tells the router where to forward communication for each node. For example, in the routing table below, router A forwards messages for node A2 on link 2, regardless of where it receives them from.
**Example Routing Table for Router A:**
| Destination | Link | Cost |
| ----------- | ---- | ---- |
| A1 | 1 | 4 |
| A2 | 2 | 2 |
| B? | 3 | 5 |
| B? | 4 | 8 |
| C? | 3 | 9 |
| C? | 4 | 6 |
The router chooses routes based on cost, which could be based on network delay or transmission cost. In this case, router A forwards communications for nodes starting with C on link 4 because it has a lower cost.
### **Forwarding Tables**
A forwarding table is a simplified version of the routing table. It only contains the optimal routes, which makes it smaller and faster for lookup, important for low-latency routing.
**Example Forwarding Table for Router A:**
| Destination | Link |
| ----------- | ---- |
| A1 | 1 |
| A2 | 2 |
| B? | 3 |
| C? | 4 |
### **Routing Protocols**
Routers can fail, and new nodes are added to networks often. To keep routing tables current, routers use routing protocols to communicate changes. For instance, if router A stops hearing from router C, it will update its table to send communications to C through router B.
### **Signal Relaying**
Signals can be relayed through a network using either circuit switching or packet switching.
### **Circuit Switching**
In a circuit-switched network, a dedicated communication channel is established for the duration of the call.
* **Example**: Early telephone networks required a switchboard operator to connect two telephones, creating a closed circuit.
* **Advantages**: Signal delay remains constant, and messages arrive in the same order.
* **Disadvantages**: Inefficient use of capacity, as other users cannot access the channel when it's idle (e.g., during pauses in conversation).
### **Packet Switching**
In a packet-switched network, data is divided into packets, which consist of a header and a payload.
* **Header**: Contains delivery information, such as sender and recipient addresses.
* Each router has a queue for incoming packets and forwards them based on its forwarding table.
* Packet-switching routers do not maintain state, meaning no channels are opened or closed at the routing level.
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