feat: complete section 1.4

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@@ -86,7 +86,50 @@ The Internet protocol stack consists of five layers, each with specific responsi
 
 ### 1.4 Network Delays and Traffic Intensity (5%)
 
-[To be completed]
+#### Four Types of Network Delays:
+
+**1. Processing Delay (d_proc):**
+- **Definition:** Time required for a router to examine the packet header and determine where to direct the packet
+- **Where it occurs:** At routers and switches when they process packet headers
+- **Typical values:** Microseconds, depends on router hardware capabilities
+
+**2. Queuing Delay (d_queue):**
+- **Definition:** Time a packet waits in router queues before being transmitted
+- **Where it occurs:** In router output buffers when packets wait for transmission
+- **Characteristics:** Highly variable, depends on network traffic intensity and congestion
+
+**3. Transmission Delay (d_trans):**
+- **Definition:** Time required to push all packet bits onto the transmission link
+- **Where it occurs:** At the sending router when transmitting packet bits
+- **Formula:** d_trans = L/R (where L = packet length in bits, R = link transmission rate in bps)
+
+**4. Propagation Delay (d_prop):**
+- **Definition:** Time for a signal to propagate from sender to receiver over physical medium
+- **Where it occurs:** Along the physical transmission medium (cables, fiber, wireless)
+- **Formula:** d_prop = d/s (where d = physical distance, s = propagation speed ≈ 2×10^8 m/s)
+
+#### Traffic Intensity:
+
+**Definition:** Traffic intensity (ρ) = La/R, where:
+- L = average packet length (bits)
+- a = average packet arrival rate (packets/sec)
+- R = transmission rate (bps)
+
+**Why Traffic Intensity ≤ 1:**
+
+Traffic intensity represents the fraction of time the link is busy transmitting packets. When designing networks:
+
+- **ρ < 1:** Link can handle the offered traffic load. Average queuing delay remains finite.
+- **ρ = 1:** Link operates at capacity. Queuing delays become very large and unstable.
+- **ρ > 1:** Traffic exceeds link capacity. Queues grow without bound, leading to packet loss and network instability.
+
+**Design Principle:** Networks must be designed with ρ < 1 to ensure:
+- Stable operation with bounded delays
+- Ability to handle traffic variations and bursts
+- Acceptable quality of service for applications
+- Prevention of congestion collapse
+
+A safety margin (typically ρ ≤ 0.7-0.8) is often used to account for traffic variability and ensure good performance.
 
 ### 1.5 Web Caching (5%)